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Decoding Solar Panel Output: Voltages, Acronyms, and Jargon. Single solar cell

Decoding Solar Panel Output: Voltages, Acronyms, and Jargon. Single solar cell

    Decoding Solar Panel Output: Voltages, Acronyms, and Jargon

    For those that are new to solar power and photovoltaics (PV), unlocking the mysteries behind the jargon and acronyms is one of the most difficult early tasks. Solar panels have many different voltage figures associated with them. There is a good amount to learn when it comes to solar panel output.

    Types of solar panel voltage:

    • Voltage at Open Circuit (VOC)
    • Voltage at Maximum Power (VMP or VPM)
    • Nominal Voltage
    • Temperature Corrected VOC
    • Temperature Coefficient of Voltage
    • Measuring Voltage and Solar Panel Testing

    Voltage at Open Circuit (VOC)

    What is the open circuit voltage of a solar panel? Voltage at open circuit is the voltage that is read with a voltmeter or multimeter when the module is not connected to any load. You would expect to see this number listed on a PV module’s specification sheet and sticker. This voltage is used when testing modules fresh out of the box and used later when doing temperature-corrected VOC calculations in system design. You can reference the chart below to find typical VOC values for different types of crystalline PV modules.

    Nominal Voltage VOC – typical VMP – typical # of cells in series
    12 21 17 36
    18 30 24 48
    18 33 26 54
    20 36 29 60
    24 42 35 72

    Voltage at Maximum Power (VMP or VPM)

    What is the Max Power Voltage of a solar panel? Voltage at maximum power is the voltage that occurs when the module is connected to a load and is operating at its peak performance output under standard test conditions (STC). You would expect to see this number listed on a modules specification sheet and sticker. VMP is at the place of the bend on an I-V curve; where the greatest power output of the module is. It is important to note that this voltage is not easily measured, and is also not related to system performance per se. It is not uncommon for a load or a battery bank to draw down the VMP of a module or array to a few volts lower than VMP while the system is in operation. The rated wattage of a PV module can be confirmed in calculations by multiplying the VMP of the module by the current at max power (IMP). The result should give you [email protected] or power at the maximum power point, the same as the module’s nameplate wattage. The VMP of a module generally works out to be 0.5 volts per cell connected in series within the module. You can reference the chart to find typical VMP values for different types of crystalline modules.

    Nominal Voltage

    What is the voltage of a solar panel? Nominal voltage is the voltage that is used as a classification method, as a carry-over from the days when battery systems were the only things going. You would NOT expect to see this number listed on a PV module’s specification sheet and sticker. This nomenclature worked really well because most systems had 12V or 24V battery banks. When you had a 12V battery to charge you would use a 12V module, end of story. The same held true with 24V systems. Because charging was the only game in town, the needs of the batteries dictated how many cells inside the PV should be wired in series and or parallel, so that under most weather conditions the solar modules would work to charge the battery(s). If you reference the chart, you can see that 12V modules generally had 36 cells wired in series, which over the years was found to be the optimum number for reliable charging of 12V batteries. It stands to reason that a 24V system would see the numbers double, and it holds true in the chart. Everything worked really well in this off grid solar system as the and evolved along the same nomenclature so that when you had a 12V battery and you wanted solar power, you knew you had to get a “12V” module and a “12V” controller. Even though the voltage from the solar module could be at 17VDC, and the charge controller would be charging at 14V, while the inverter was running happily at 13VDC input, the whole system was made up of 12V “nominal” components so that it would all work together. This worked well for a good while until maximum power point technology (MPPT) became available and started popping up. This meant that not all PV was necessarily charging batteries and that as MPPT technology evolved, even when PV was used in charging batteries, you were no longer required to use the same nominal voltage as your battery bank. String inverters changed the game for modules, as they were no longer forced in their design to be beholden to the voltage needs of deep cycle batteries. This shift allowed manufacturers to make modules based on physical size, wattage characteristics, and use other materials that produced module voltages completely unrelated to batteries. The first and most popular change occurred in what are now generally called 18V “nominal” modules. There are no 18V battery banks for RE systems. The modules acquired this name because their cell count and functional voltage ratings put them right in between the two existing categories of 12V and 24V “nominal” PV modules. Many modules followed with 48 to 60 cells, that produced voltages that were not a direct match for 12V or 24V nominal system components. To avoid bad system design and confusion, the 18V moniker was adopted by many in the industry but ultimately may have created more confusion among novices that did not understand the relationship between cells in series, VOC, VMP, and nominal voltage. With this understanding, things get a lot easier, and the chart should help to unlock some of the mystery.

    Temperature-Corrected VOC

    The temperature-corrected VOC value is required to ensure that when cold temperatures raise the VOC of an array, other connected equipment like MPPT controllers or grid tie inverters are not damaged. This calculation is done in one of two ways. The first way involves using the chart in NEC 690.7. The second way involves doing calculations with the Temperature Coefficient of Voltage and the coldest local temperature.

    Temperature Coefficient of Voltage

    What is a solar panel temperature coefficient? The temperature coefficient of a solar panel is the value represents the change in voltage based on temperature. Generally, it is used to calculate Cold Temp/Higher Voltage situations for array and component selection in cooler climates. This value may be presented as a percentage change from STC voltages per degree or as a voltage value change per degree temp change. This information was not easily found in the past, but is now more commonly seen on spec pages and sometimes module stickers.

    Measuring Voltage and Solar Panel Testing

    How do I measure voltage on a solar panel? Voltages can be read on a solar panel with the use of a voltmeter or multimeter. What you’ll see below is an example of a voltmeter measuring VOC with a junction box. This would be the view from the back of the PV module. Using a multimeter is the best way to measure solar panel output.

    When researching solar panel output, it can be overwhelming to understand the different voltage figures and acronyms used. For those new to solar power and photovoltaics (PV), decoding the terminology can be a challenge. In this blog post, we will break down the basics of solar panel output, including voltage, acronyms, and jargon, to help you get up to speed.

    What are solar amps and watts?

    Solar amps and watts are two measurements of the amount of electrical energy that a solar panel produces. Solar amps (A) measure the rate of electric current produced by a photovoltaic cell, while solar watts (W) measure the amount of power delivered to an electrical load. Both solar amps and watts are related to the efficiency rating of residential solar panels. The higher the efficiency rating, the higher the number of solar amps and watts produced.

    There are many types of 60-cell solar panels on the market for home solar applications, each with varying efficiency ratings and amp/watt outputs. High efficiency panels are capable of producing more solar watts than low-efficiency panels, although they tend to cost more upfront. By choosing the right panel, homeowners can ensure that their solar array is producing enough power to meet their electricity needs.

    decoding, solar, panel, output, voltages, acronyms

    Why do solar panels have so many voltages associated with them?

    Solar panels have a variety of voltage figures associated with them due to the different types of solar panels, their placement in a solar panel system, and their power production. The most common type of rooftop solar panel uses a direct current (DC) and produces a low voltage. This low voltage is typically between 20 and 40 volts, depending on the specific type of panel. To increase the voltage output, multiple solar panels can be wired together in a series or parallel connection, or both, depending on the specific solar energy system.

    When solar panels are connected in a series, the voltages are added together. This means that connecting two 20-volt solar panels in series would yield a total voltage output of 40 volts. Connecting three panels in series would result in a 60-volt output, and so on. This method is often used when the total voltage needs to be higher than what a single panel can provide.

    In contrast, when solar panels are connected in parallel, the wattage is added together. This means that connecting two 10-watt solar panels in parallel would yield a total wattage output of 20 watts. Connecting three panels in parallel would result in a 30-watt output, and so on. This method is often used when the total wattage needs to be higher than what a single panel can provide.

    The voltage output of a solar panel also depends on its power production, which is measured by the manufacturer at Standard Test Conditions (STC).

    What does STC mean?

    STC is defined as an irradiance of 1,000 W/m2 and cell temperature of 25 degrees Celsius. Because real-world conditions are rarely equal to STC, the actual power output of a solar panel may differ from its rated output. This is why it’s important to understand the various voltages associated with your particular solar energy system to ensure it meets your needs. To determine solar panels rated output, you need to know two figures: the solar panel wattage (measured in watts) and solar panel efficiency (measured in percent). Solar installation involves connecting solar panels to a photovoltaic system that can use or store the generated electricity. The efficiency rating of solar panels varies depending on factors such as environment, angle, and geographic location, but typically ranges between 15–20%. Knowing what wattage solar panels generate helps determine their overall performance in terms of power production for any given solar installation project. Understanding the various voltages associated with solar energy systems can be challenging for those new to the technology but once you’ve grasped this knowledge, you’ll have the knowledge you need to make informed decisions about your own solar energy installation.

    How many size should my solar panel be?

    When choosing a solar panel size, you must consider your energy needs and the hours of sunlight available in your area. The size of the solar panel will determine how much electricity it can produce, measured in kilowatt hours (kWh). Your energy needs will determine the type of solar panel that you need.

    If you’re looking to produce a specific amount of electricity, the total number of solar panels that you need will depend on their wattage rating. Generally, the higher the wattage rating, the more electricity it will generate. You can calculate how many solar panels you need to meet your energy requirements by dividing your kWh requirement by the wattage of each panel.

    For example, if you have an energy requirement of 10 kWh per day and you are using solar panels with a rating of 250 watts, then you would need 40 solar panels.

    When choosing the size of your solar panel, make sure to consider the hours of sunlight available in your area as well. The more sunlight available, the fewer solar panels you’ll need to meet your energy requirements.

    In summary, the size of the solar panel that you need depends on your energy needs and the hours of sunlight available in your area. You can calculate how many panels you need to meet your energy requirements by dividing your kWh requirement by the wattage of each panel.

    How Many Volts Does a Solar Panel Produce?

    Photovoltaic cells are used in solar power panels, which can include 32, 36, 48, 60, 72, or 96 cells in their design. Typically, a solar panel with 32 cells can output 14.72 volts (each cell producing about 0.46 volt of electricity). These compartments are set up in either a rectangle or square frame. As the number of cells increases, so do the size and weight of solar panels. Commercial electric power generation uses solar power panels with greater cell configurations. But What is solar panel output voltage ac or dc? Read the article to learn about how many volts does a solar panel produce and other facts related to it.

    How Many Volts Does a Solar Panel Produce?

    So, how many volts does a solar panel produce? Although there are currently cells available with a size of 158 mm 158 mm, the most common solar cell used according to industry standards has a size of 156 mm 156 mm and produces 0.5 Volts under the STC (Standard Test Conditions). The total number of volts produced by a panel will be determined by summing these. Typically, we employ panels with 36, 60, and 72 cells. As we previously discussed, one cell generates 0.5 V as Vmax (maximum voltage produced).

    • 36 cells 0.5 V = 18 V (Vmax)
    • 30 V is equal to 60 cells multiplied by 0.5 V. (Vmax)
    • 36 V is equal to 72 cells multiplied by 0.5 V. (Vmax)

    Cut-cell panels, which can have up to 120 and 144 cells, are popular today.

    What is Solar Panel Output Voltage Ac or Dc?

    Before learning how many volts does a solar panel produce, you must learn what is solar panel output voltage ac or dc. Power is produced using Direct Current (DC) solar panels. Alternating Current (AC) powers most homes. A solar panel’s DC power output is converted by an inverter into AC power, keeping the AC voltage at 110 volts and a clean 60 cycles (Hertz) per second.

    Households primarily use AC while solar panels provide DC. Thus, inverters transform solar energy into a form that may be used in the homes of your customers. Direct current (DC) and low voltage are used by the most popular kind of rooftop solar panel. Based on the particular type of panel, this low voltage ranges between 20 and 40 volts. Although many homeowners prefer the concept of producing their own electricity, installing solar panels involves much more than simply hammering photovoltaic panels onto your roof. In actuality, the cost of going solar is just 25 to 30 percent accounted for by solar panels. In reality, creating a complete system that complies with current electrical code and is safe and reliable requires careful design, technical know-how, and expensive electrical equipment.

    Inverters must be properly matched to the output voltage of the panels because they are rated in terms of watts (or battery if so used). A tiny percentage of power is lost by inverters as heat. This can reduce their efficiency and take up a few watts you might rather put to better use elsewhere.

    How Many Volts Does a Solar Panel Produce Per Hour?

    Now, you have learned about how many volts does a solar panel produce, but how many volts does a solar panel produce in an hour? The majority of solar panels generate between 170 and 350 watts per hour. However, it also relies on the amount of direct sunlight and the climate. Per solar panel, it ranges from 0.17 to 0.35 kWh on average. However, according to research, 230 to 275 watts of power can be produced by a conventional solar power panel. Hence, a solar panel produces volts anywhere between 228.67 volts to 466 volts per hour. 4

    How Many Volts Does a Solar Panel Produce Per Day?

    You have learned how many volts does a solar panel produce per hour, but how many volts does a solar panel produce per day? Though there are numerous factors that can determine how much electricity a solar panel can create, in the United States, you can anticipate that a single solar panel will typically yield about 2 kWh each day.

    How Many Volts Does a 300w Solar Panel Produce?

    So, how many volts does a 300w solar panel produce? The amount of electricity produced by a solar panel depends on the panel size, the efficiency of the solar cells inside the panel, and the amount of sunlight the panel gets. A 300-watt (0.3kW) solar panel in full sunlight actively generates power for one hour, it will generate 300 watt-hours (0.3kWh) of electricity. A 300-watt panel produces 240 volts, which equals 1.25 Amps.

    How Many Volts Does a 200w Solar Panel Produce?

    You have learned about 300w solar panels, but it would have come to your mind how many volts does a 200w solar panel produce. It is possible for 200w solar panels to produce voltage at a variety of levels. For 200-watt panels, there are two different voltage outputs: 18V and 28V. The voltage output of 200-watt panels is typically 18V. This generates approximately 11 amps each hour. Alternatively, 200 Watt 28 V panels produce about 7 amps of power every hour.

    How Many Volts Does a 500w Solar Panel Produce?

    About more than a decade ago, only 200-300-watt solar panels were considered standard-size solar panels. After many years developers developed 500-watt solar panels. These panels aren’t yet optimal for residential use. They are more suitable for commercial and industrial setups. There isn’t much info on how many volts does a solar panel produce but many sources claim that a 500-watt solar panel typically produces 20–25 amps at 12 volts. It can charge for 5 to 6 hours if you have adequate sunlight.

    How Many Volts Does a 750w Solar Panel Produce?

    A 750w solar panel supply perfectly produces 220 volts executing 3.18 volts. If your inverter has 750 watts of electricity, you should check to determine if it runs on 12 volts, 14 volts, 24 volts, or 28 volts.

    The voltage of the inverter is typically higher than 12 volts in inverters with a power of 750 watts. However, since 12 volts is the lowest value, we will still include it in the calculation. Therefore, the inverter’s amps at 100% efficiency will be equal to 62.5 amps (750 watts / 12 volts). Since there is the least chance for the inverter to be of 100% efficiency, we will consider 80% efficiency. Then the amp range would be around 62.5 amps / 0.8 = 78.13 amps.

    How Many Volts Does a 100w Solar Panel Produce?

    The voltage that solar panels produce when they produce electricity varies according to the number of cells and the amount of sunlight that they receive. Typically, a 100-watt solar panel produces about 18 volts of maximum power voltage.

    The solar panel should be situated where the majority of the day’s sunlight falls in the noon sky for maximum output. Peak sunlight is what is needed for it to be as effective as possible. Most solar panels, however, don’t always receive these favorable conditions and frequently produce less than 100 watts when there is little sunlight.

    How Many 12v Batteries are Needed to Power a House?

    When estimating how much electricity your solar panel can generate, it’s critical to take your batteries’ wattage into account. One watt equals one joule per second in the energy unit of wattage. You must be aware of how much energy your home consumes on a daily basis in order to determine how many batteries you require. For a typical American home, that often means that you need at least eight to ten batteries.

    To supply sufficient backup power in the event that the primary power source fails, you’ll need a number of batteries. For instance, a single lithium-ion battery can power your lights during a power outage, but a solar-plus-storage system requires a larger battery bank.

    You should have enough batteries to power your whole house. Get a separate backup load panel to power your most important appliances if you don’t already have one. The cost of this alternative, however, will increase by 1,000 to 2,000 for you.

    How Many Solar Panels Do You Need To Charge A 100Ah?

    How many solar panels are required to charge a 100Ah battery depends on both the battery’s capacity and the amount of sunlight that is available. A 100-watt solar panel will typically charge a 100 Ah battery. A 12V battery is intended to work with a 100-watt solar panel.

    At least two 100-watt solar panels are required for a 100 Ah lead-acid deep-cycle battery. You’ll require three 100-watt panels if you’re utilizing a lithium-ion battery. Three 100-watt panels working together may charge a 100Ah battery in three hours as opposed to one panel charging a 100Ah battery in roughly five hours. The typical solar panel has 100 watts of power. Larger solar panels with a higher wattage have a lower output than smaller ones. However, keep in mind that optimum operating circumstances for solar panels are uncommon. For instance, a solar panel with a 100W output will only supply 85 watts of power in actual use.

    Olivia is committed to green energy and works to help ensure our planet’s long-term habitability. She takes part in environmental conservation by recycling and avoiding single-use plastic.

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    The Shockley Queisser Efficiency Limit

    It was first calculated by William Shockley and Hans Queisser in 1961. A solar cell’s energy conversion efficiency is the percentage of power converted from sunlight to electrical energy under standard test conditions (STC). The STC conditions approximate solar noon at the spring and autumn equinoxes in the continental United States with the surface of the solar cell aimed directly at the sun.

    The modern SQ Limit calculation is a maximum efficiency of 33% for any type of single junction solar cell. The original calculation by Shockley and Queisser was 30% for a silicon solar cell. Current solar cell production efficiencies vary by the Band gap of the semiconductor material as shown on the left. See Junctions Band Gaps page.

    The best modern production silicon cell efficiency is 24% at the cell level and 20% at the module level as reported by SunPower in March of, 2012. In a laboratory, the record solar cell efficiency is held by the University Of New South Wales in Sydney, Australia at 25%.

    There are a number of assumptions associated with the SQ Limit that restrict its general applicability to all types of solar cells. Although there are numerous programs underway to find ways around the SQ Limit, it is still applicable to 99.9% of the solar cells on the market today. Top

    The Critical SQ Limit Assumptions:

    • One semiconductor material (excluding doping materials) per solar cell.
    • One p/n junction per solar cell.
    • The sunlight is not concentrated. a one sun source.
    • All energy is converted to heat from photons greater than the Band gap.

    Where Does The 67% Of Energy Loss Go?

    • 47% of the solar energy gets converted to heat.
    • 18% of the photons pass through the solar cell.
    • 02% of energy is lost from local recombination of newly created holes and electrons.
    • 33% of the sun’s energy is theoretically converted to electricity.
    • 100% total sun’s energy.

    If the theoretical limit for silicon cells is about 30%, what happens to the other 6% that is lost from the best production cell efficiency of 24%? Some sunlight is always reflected off the surface of the cell even though the surface is usually texturized and coated with an anti-reflective coating. In addition there are some losses at the junction of the silicon cell with the electrical contacts that carry the current to the load. Finally, there are some losses due to manufacturing impurities in the silicon. Top

    What Electro-Magnetic Waves Are Absorbed By A Solar Cell?

    Shown to the left is the complete spectrum of electro-magnetic radiation. The long radio waves at the right are the weakest. The most powerful rays (gamma rays) are very short and to the left.

    For a semiconductor electron to move into an external load circuit, its energy level must be increased from its normal valence level (tightly bound to one atom) to its higher energy conduction level (free to move around). The amount of energy to boost it to the higher level is called the Band gap energy. See Junctions Band Gaps page.

    decoding, solar, panel, output, voltages, acronyms

    Only photons with at least the Band gap energy will be able to free electrons to create a current. Sunlight photons with less than the Band gap energy will simply pass through the solar cell. Put in terms of radiation, all the photons in the visible spectrum are strong enough to causeelectrons to jump the Band gap.

    Some infrared, all microwave, and all radio waves do not have enough energy and pass right through the solar cell.

    In the sunlight energy distribution chart to the left, only the mustard colored photons can be absorbed and create electricity in a crystalline silicon cell. Absorption of electromagnetic radiation is the process by which the energy of a photon from the sun is transformed into other forms of energy for example electricity or heat.

    The red colored wavelenghts do not have enough energy and the yellow ones have too much energy. The yellow wavelengths are absorbed and generate electricity, but a lot of their energy is lost. That is because photons with excess Band gap energy generate a free electron and a hole, but their extra energy gets dissipated as heat.

    X-rays and Gamma rays have just too much energy to be absorbed at all. The mustard area is basically a picture of the SQ Limit applied to silicon as Shockley and Queisser calculated it in 1961. Top

    Strategies To Exceed The SQ Limit:

    Basically the strategies to obtain better efficiencies than the SQ Limit predicts are to work-around one or more of the critical assumptions listed above (and shown again below).

    1) One semiconductor material (excluding doping materials) per solar cell.

    Use more than one semiconductor material in a cell.

    2) One P/N junction per solar cell.

    Use more than one junction in a cell. tandem cells.

    decoding, solar, panel, output, voltages, acronyms

    3) The sunlight is not concentrated. a one sun source.

    Sunlight can be concentrated about 500 times using inexpensive lenses.

    4) All energy is converted to heat from photons greater than the Band gap.

    Combine a PV semiconductor with a heat based technology to harvest both forms of energy and/or

    Use quantum dots to harvest some of the excess photon energy for electricity.

    Strategies 1) and 2) Multi-junction Solar Cells. Tandem Cells

    The earliest and most frequent work around to the SQ Limit has been the use of multiple p/n junctions, each one tuned to a different frequency of the solar spectrum. Since sunlight will only react strongly with Band gaps roughly the same width as their wavelength, the top layers are made very thin so they are almost transparent to longer wavelengths. This allows the junctions to be stacked, with the layers capturing the shortest wavelengths on top, and the longer wavelength photons passing through them to the lower layers.

    The example of a multi-junction cell on the left has a top cell of gallium indium phosphide, then a tunnel diode junction, and a bottom cell of gallium arsenide. The tunnel junction allows the electrons to flow between the cells and keeps the electric fields of the two cells separate. Most of today’s research in multi-junction cells focuses on gallium arsenide as one of the component cells as it has a very desirable Band gap. Performing a calculation using the SQ methodology; a two-layer cell can reach a maximum theoretical efficiency of 42% and three-layer cells 49%.

    The record for a multi-junction cell is held by the University Of New South Wales (UNSW) in Sydney, Australia at 43% using a five cell tandem approach. However, the UNSW tandem cell is very expensive. In addition to the cost issue, there are other constraints that make the tandem cells complex. For example, all the layers must be lattice compatible with one another in their crystalline structure and the currents from each individual cell must match the other cells. Multi-junction cells are commercially used in only special applications because their expense currently outweighs any efficiency improvement. At the moment they are used in space where weight is most important and in concentrated PV systems where the sunlight is focused on a very small cell area requiring only small amounts of semiconductors per cell. Top

    Strategy 3) Concentrate The Sunlight

    Concentrated PhotoVoltaics (CPV), in which sunlight is focused onto a small solar cell by lenses to generate more power per unit of surface area, was an early favorite to increase solar efficiency. CPV’s main attraction is that it can leverage modest one sun cell electricity production to a much larger scale production using relatively simple and inexpensive optical concentration.

    Instead of a typical 6 inch by 6 inch solar cell, a 7 inch by 7 inch square plastic Fresnel (pronounced Fray-NELL) lens incorporating circular facets, is used to FOCUS the sunlight as shown on the left. A tiny, 39% efficient multi-junction solar cell is mounted at the focal point which converts the sun’s energy into electricity. Future cell efficiencies are expected to approach 50%. The Fresnel lens concentrates the sun’s energy about 500 times its normal intensity. A number of Fresnel lenses are manufactured as a single plastic piece. The tiny solar cells are mounted on a supporting plate at locations corresponding to the FOCUS point of each Fresnel lens. Hundreds of lenses make up a solar array mounted on a sun tracking heliostat. With a high module efficiency of 31%, CPV systems take up less land than traditional PV systems, use no water, and are ideal for desert type areas. See the Amonix discussion.

    Despite the concentration advantages, CPV has been slow to gain market share. While the tiny solar cells use less of the expensive semiconductor materials, cost is a factor as a two-axis sun tracking heliostat is necessary to accurately keep the FOCUS point on the solar cell as the sun travels east to west each day and north and south each season. CPV does not do well in cloudy climates as diffuse sunlight does not concentrate well. In addition, the large heliostats are not well suited for the small installations that have been the mainstream of the recent PV market. Today, CPV costs are very competitive and CPV is benefiting from growing demand for large utility size solar plants, especially in the desert areas of California, Arizona, Spain, and Australia. Top

    Strategy 4a) Combine a PV cell with heat based technology (PETE)

    The Stanford University Photon Enhanced Thermionic Emission (PETE) prototype uses concentrated sunlight as its source of energy and in a two step process uses both the sun’s photon energy and its heat. A thermionic converter consists of two electrodes separated by a vacuum, see the figure to the left. When the cathode is heated to a high temperature, electrons become excited, jump across the thin vacuum to the relatively cold anode, and drive a current through an external circuit back to the cathode. In the Stanford prototype, the cathode emitter is a semiconductor material rather than a metal electrode. First, the highly concentrated sunlight photons partially excite the electrons in the cathode semiconductor so that in step two, the remaining heat energy necessary for emission is lower than that for a standard thermionic converter (see the thermionic/PETE discussion on the Solar In-depth page). The surface of the cathode on the vacuum side is texturized to increase emissions. PETE converts about 25% of the sunlight’s energy into electricity at 200°C and higher efficiencies at higher temperatures, i.e. 45% at 1000°C. Because of the high temperatures this type of solar system would probably only be used by utilities to generate grid electricity. A lot of work needs to be done to get from today’s laboratory set up to a production product in the field. A competitive product is probably 8 to 10 years away. Top

    Strategy 4b) Quantum Dots Absorb Excess Photon Energy

    In a regular solar cell, each photon collision generates a particle pair consisting of one free hole and one free electron. Quantum Dots are extremely small nanocrystals (the names are used somewhat interchangeably) interspersed in a larger semiconducting material. Quantum Dots (QDs) range between 1 and 20 nanometers in size (one nanometer is one billionth of a meter). See the two pictures from MIT on the left.

    Semiconductors at this size have different physical properties than their big brothers. When photons with energy greater than the Band gap energy collide with a Quantum Dot several hot hole/electron pairs can be created as opposed to one pair and heat. Although silicon can be used as a nanocrystal, lead selenide (PbSE) also a semiconductor, is being used more frequently as the material of choice.

    Another characteristic of a Quantum Dot is that different sizes capture different wavelengths of light. Small dots capture small wavelengths and larger dots bigger wavelengths. Some researchers have figured out how to stack the dots from small to large to capture more photon energy similar to how tandem cells do (see strategy one/two above).

    Once a hot electron is created inside a Quantum Dot, it stretches its lifetime as much as a 1000 times before it cools. The electrons like to stay inside the QD. One of the challenges was to figure out how to extract the hot electrons from the QDs. No solar cells produced prior to December, 2011 have quantum efficiencies greater than 100 percent.

    Quantum efficiency (not to be confused with solar cell efficiency) per the National Renewable Energy Laboratory (NREL) located in Boulder, Colorado, is the “ratio of collected charge carriers (electrons or electron holes) to incident photons”. In layman terms. its the ratio of the number of electrons produced in a solar cell to the number of the sun’s photons hitting the cell.

    Researchers from the NREL have reported quantum efficiencies of 114 percent in solar cells “excited” from photons from the high-energy region of the solar spectrum. That is from the near ultraviolet through the visible light spectrum. 350 to 700 nanometers. See the sunlight spectrum chart above.

    Energy is always conserved. The extra electrons come from the extra energy left over after the initial photon-electron collision. Light waves below 700 nanometers do not have enough energy to dislodge more than one electron-hole pair.

    NREL achieved this result with a layered quantum dot experimental cell composed of a surface of anti-reflective glass, a thin layer of semiconductor zinc oxide “textured” at the nano level, a QD layer of lead selenide doped with ethanedithol (a bonding agent) andhydrazine (a deposition stabilizer), and a thin layer of gold for the collector electrode.

    This process, which creates more than one electron-hole pair from a single photon, is called multiple exciton generation (MEG) by NREL.

    Shown at the left is an electron wave function in a Quantum Dot (i.e. the probability of an electron being in any specific location at any given time. purple is low probability and white is high probability).

    The practical upper limit for thin film solar cells is thought to be about 20%. The upper limit using Quantum Dots is thought to be about 30%. It should be emphasized that the research into Quantum Dots is at a very basic stage of demonstrating scientific principles. No one at this time has actually made a pre-production Quantum Dot solar cell. Production solar cells using Quantum Dots are thought to be about 10 years into the future.

    Solar cells

    by Chris Woodford. Last updated: January 22, 2022.

    W hy do we waste time drilling for oil and shoveling coal when there’s a gigantic power station in the sky up above us, sending out clean, non-stop energy for free? The Sun, a seething ball of nuclear power, has enough fuel onboard to drive our Solar System for another five billion years—and solar panels can turn this energy into an endless, convenient supply of electricity.

    Solar power might seem strange or futuristic, but it’s already quite commonplace. You might have a solar-powered quartz watch on your wrist or a solar-powered calculator. Many people have solar-powered lights in their garden. Spaceships and satellites usually have solar panels on them too. The American space agency NASA has even developed a solar-powered plane! As global warming continues to threaten our environment, there seems little doubt that solar power will become an even more important form of renewable energy in future. But how exactly does it work?

    Photo: NASA’s solar-powered Pathfinder airplane. The upper wing surface is covered with lightweight solar panels that power the plane’s propellers. Picture courtesy of NASA Armstrong Flight Research Center.


    • How much energy can we get from the Sun?
    • What are solar cells?
    • How are solar cells made?
    • How do solar cells work?
    • How efficient are solar cells?
    • Types of photovoltaic solar cells
    • How much power can we make with solar cells?
    • Power to the people
    • Why hasn’t solar power caught on yet?
    • A brief history of solar cells
    • Find out more

    How much energy can we get from the Sun?

    Solar power is amazing. On average, every square meter of Earth’s surface receives 163 watts of solar energy (a figure we’ll explain in more detail in a moment). [1] In other words, you could stand a really powerful (150 watt) table lamp on every square meter of Earth’s surface and light up the whole planet with the Sun’s energy! Or, to put it another way, if we covered just one percent of the Sahara desert with solar panels, we could generate enough electricity to power the whole world. [2] That’s the good thing about solar power: there’s an awful lot of it—much more than we could ever use.

    Photo: The amount of energy we can capture from sunlight is at a minimum at sunrise and sunset and a maximum at midday, when the Sun is directly overhead.

    But there’s a downside too. The energy the Sun sends out arrives on Earth as a mixture of light and heat. Both of these are incredibly important—the light makes plants grow, providing us with food, while the heat keeps us warm enough to survive—but we can’t use either the Sun’s light or heat directly to run a television or a car. We have to find some way of converting solar energy into other forms of energy we can use more easily, such as electricity. And that’s exactly what solar cells do.

    What are solar cells?

    A solar cell is an electronic device that catches sunlight and turns it directly into electricity. It’s about the size of an adult’s palm, octagonal in shape, and colored bluish black. Solar cells are often bundled together to make larger units called solar modules. themselves coupled into even bigger units known as solar panels (the black- or blue-tinted slabs you see on people’s homes—typically with several hundred individual solar cells per roof) or chopped into chips (to provide power for small gadgets like calculators and digital watches).

    Photo: The roof of this house is covered with 16 solar panels, each made up of a grid of 10×6 = 60 small solar cells. On a good day, it probably generates about 4 kilowatts of electricity.

    Just like the cells in a battery, the cells in a solar panel are designed to generate electricity; but where a battery’s cells make electricity from chemicals, a solar panel’s cells generate power by capturing sunlight instead. They are sometimes called photovoltaic (PV) cells because they use sunlight (photo comes from the Greek word for light) to make electricity (the word voltaic is a reference to Italian electricity pioneer Alessandro Volta, 1745–1827).

    We can think of light as being made of tiny particles called photons, so a beam of sunlight is like a bright yellow fire hose shooting trillions upon trillions of photons our way. Stick a solar cell in its path and it catches these energetic photons and converts them into a flow of electrons—an electric current. Each cell generates a few volts of electricity, so a solar panel’s job is to combine the energy produced by many cells to make a useful amount of electric current and voltage. Virtually all of today’s solar cells are made from slices of silicon (one of the most common chemical elements on Earth, found in sand), although as we’ll see shortly, a variety of other materials can be used as well (or instead). When sunlight shines on a solar cell, the energy it carries blasts electrons out of the silicon. These can be forced to flow around an electric circuit and power anything that runs on electricity. That’s a pretty simplified explanation! Now let’s take a closer look.

    How are solar cells made?

    Silicon is the stuff from which the transistors (tiny switches) in microchips are made—and solar cells work in a similar way. Silicon is a type of material called a semiconductor. Some materials, notably metals, allow electricity to flow through them very easily; they are called conductors. Other materials, such as plastics and wood, don’t really let electricity flow through them at all; they are called insulators. Semiconductors like silicon are neither conductors nor insulators: they don’t normally conduct electricity, but under certain circumstances we can make them do so.

    When we place a layer of n-type silicon on a layer of p-type silicon, a barrier is created at the junction of the two materials (the all-important border where the two kinds of silicon meet up). No electrons can cross the barrier so, even if we connect this silicon sandwich to a flashlight, no current will flow: the bulb will not light up. But if we shine light onto the sandwich, something remarkable happens. We can think of the light as a stream of energetic light particles called photons. As photons enter our sandwich, they give up their energy to the atoms in the silicon. The incoming energy knocks electrons out of the lower, p-type layer so they jump across the barrier to the n-type layer above and flow out around the circuit. The more light that shines, the more electrons jump up and the more current flows.

    This is what we mean by photovoltaic—light making voltage—and it’s one kind of what scientists call the photoelectric effect.

    How do solar cells work?

    Artwork: How a simple, single-junction solar cell works.

    A solar cell is a sandwich of n-type silicon (blue) and p-type silicon (red). It generates electricity by using sunlight to make electrons hop across the junction between the different flavors of silicon:

    • When sunlight shines on the cell, photons (light particles) bombard the upper surface.
    • The photons (yellow blobs) carry their energy down through the cell.
    • The photons give up their energy to electrons (green blobs) in the lower, p-type layer.
    • The electrons use this energy to jump across the barrier into the upper, n-type layer and escape out into the circuit.
    • Flowing around the circuit, the electrons make the lamp light up.

    Now for more detail.

    That’s a basic introduction to solar cells—and if that’s all you wanted, you can stop here. The rest of this article goes into more detail about different types of solar cells, how people are putting solar power to practical use, and why solar energy is taking such a long time to catch on.

    How efficient are solar cells?

    A basic rule of physics called the law of conservation of energy says that we can’t magically create energy or make it vanish into thin air; all we can do is convert it from one form to another. That means a solar cell can’t produce any more electrical energy than it receives each second as light. In practice, as we’ll see shortly, most cells convert about 10–20 percent of the energy they receive into electricity. A typical, single-junction silicon solar cell has a theoretical maximum efficiency of about 30 percent, known as the Shockley-Queisser limit. That’s essentially because sunlight contains a broad mixture of photons of different wavelengths and energies and any single-junction solar cell will be optimized to catch photons only within a certain frequency Band, wasting the rest. Some of the photons striking a solar cell don’t have enough energy to knock out electrons, so they’re effectively wasted, while some have too much energy, and the excess is also wasted. The very best, cutting-edge laboratory cells can manage just under 50 percent efficiency in absolutely perfect conditions using multiple junctions to catch photons of different energies.

    Chart: Efficiencies of solar cells compared: The very first solar cell scraped in at a mere 6 percent efficiency; the most efficient one that’s been produced to date managed 47.1 percent in laboratory conditions. Most cells are first-generation types that can manage about 15 percent in theory and probably 8 percent in practice.

    Real-world domestic solar panels might achieve an efficiency of about 15 percent, give a percentage point here or there, and that’s unlikely to get much better. First-generation, single-junction solar cells aren’t going to approach the 30 percent efficiency of the Shockley-Queisser limit, never mind the lab record of 47.1 percent. All kinds of pesky real-world factors will eat into the nominal efficiency, including the construction of the panels, how they are positioned and angled, whether they’re ever in shadow, how clean you keep them, how hot they get (increasing temperatures tend to lower their efficiency), and whether they’re ventilated (allowing air to circulate underneath) to keep them cool.

    Types of photovoltaic solar cells

    Most of the solar cells you’ll see on people’s roofs today are essentially just silicon sandwiches, specially treated (doped) to make them better electrical conductors. Scientists refer to these classic solar cells as first-generation, largely to differentiate them from two different, more modern technologies known as second- and third-generation. So what’s the difference?


    Photo: A colorful collection of first-generation solar cells. Picture courtesy of NASA Glenn Research Center (NASA-GRC).

    Over 90 percent of the world’s solar cells are made from wafers of crystalline silicon (abbreviated c-Si), sliced from large ingots, which are grown in super-clean laboratories in a process that can take up to a month to complete. [3] The ingots either take the form of single crystals (monocrystalline or mono-Si) or contain multiple crystals (polycrystalline, multi-Si or poly c-Si).

    First-generation solar cells work like we’ve shown in the box up above: they use a single, simple junction between n-type and p-type silicon layers, which are sliced from separate ingots. So an n-type ingot would be made by heating chunks of silicon with small amounts of phosphorus, antimony, or arsenic as the dopant, while a p-type ingot would use boron as the dopant. Slices of n-type and p-type silicon are then fused to make the junction. A few more bells and whistles are added (like an antireflective coating, which improves light absorption and gives photovoltaic cells their characteristic blue color, protective glass on front and a plastic backing, and metal connections so the cell can be wired into a circuit), but a simple p-n junction is the essence of most solar cells. It’s pretty much how all photovoltaic silicon solar cells have worked since 1954, which was when scientists at Bell Labs pioneered the technology: shining sunlight on silicon extracted from sand, they generated electricity.


    Photo: A thin-film, second-generation solar panel. The power-generating film is made from amorphous silicon, fastened to a thin, flexible, and relatively inexpensive plastic backing (the substrate). Photo by Warren Gretz courtesy of NREL (image ID #6321083).

    Classic solar cells are relatively thin wafers—usually a fraction of a millimeter deep (about 200 micrometers, 200μm, or so). But they’re absolute slabs compared to second-generation cells, popularly known as thin-film solar cells (TPSC) or thin-film photovoltaics (TFPV), which are about 100 times thinner again (several micrometers or millionths of a meter deep). Although most are still made from silicon (a different form known as amorphous silicon, a-Si, in which atoms are arranged randomly instead of precisely ordered in a regular crystalline structure), some are made from other materials, notably cadmium-telluride (Cd-Te) and copper indium gallium diselenide (CIGS). [4]

    Because they’re extremely thin, light, and flexible, second-generation solar cells can be laminated onto Windows, skylights, roof tiles, and all kinds of substrates (backing materials) including metals, glass, and polymers (plastics). What second-generation cells gain in flexibility, they sacrifice in efficiency: classic, first-generation solar cells still outperform them. So while a top-notch first-generation cell might achieve an efficiency of 15–20 percent, amorphous silicon struggles to get above 7 percent, the best thin-film Cd-Te cells only manage about 11 percent, and CIGS cells do no better than 7–12 percent. [5] That’s one reason why, despite their practical advantages, second-generation cells have so far made relatively little impact on the solar market.


    Photo: Third-generation plastic solar cells produced by researchers at the National Renewable Energy Laboratory. Photo by Jack Dempsey courtesy of NREL (image ID #6322357).

    The latest technologies combine the best features of first and second generation cells. Like first-generation cells, they promise relatively high efficiencies (30 percent or more). Like second-generation cells, they’re more likely to be made from materials other than simple silicon, such as amorphous silicon, organic polymers (making organic photovoltaics, OPVs), perovskite crystals, and feature multiple junctions (made from multiple layers of different semiconducting materials). Ideally, that would make them cheaper, more efficient, and more practical than either first- or second-generation cells. [6] Currently, the world record efficiency for third-generation solar is 28 percent, achieved by a perovskite-silicon tandem solar cell in December 2018.

    How much power can we make with solar cells?

    The total solar energy that reaches the Earth’s surface could meet existing global energy needs 10,000 times over.

    European Photovoltaic Industry Association/Greenpeace, 2011.

    In theory, a huge amount. Let’s forget solar cells for the moment and just consider pure sunlight. Up to 1000 watts of raw solar power hits each square meter of Earth pointing directly at the Sun (that’s the theoretical power of direct midday sunlight on a cloudless day—with the solar rays firing perpendicular to Earth’s surface and giving maximum illumination or insolation, as it’s technically known).

    In practice, after we’ve corrected for the tilt of the planet and the time of day, the best we’re likely to get is maybe 100–250 watts per square meter in typical northern latitudes (even on a cloudless day). That translates into about 2–6 kWh per day (depending on whether you’re in a northern region like Canada or Scotland or somewhere more obliging such as Arizona or Mexico). [11] Multiplying up for a whole year’s production gives us somewhere between 700 and 2500 kWh per square meter (700–2500 units of electricity). Hotter regions clearly have much greater solar potential: the Middle East, for example, receives around 50–100 percent more useful solar energy each year than Europe.

    Unfortunately, typical solar cells are only about 15 percent efficient, so we can only capture a fraction of this theoretical energy: perhaps 4–10 watts per square meter. [7] That’s why solar panels need to be so big: the amount of power you can make is obviously directly related to how much area you can afford to cover with cells. A single solar cell (roughly the size of a compact disc) can generate about 3–4.5 watts; a typical solar module made from an array of about 40 cells (5 rows of 8 cells) could make about 100–300 watts; several solar panels, each made from about 3–4 modules, could therefore generate an absolute maximum of several kilowatts (probably just enough to meet a home’s peak power needs).

    What about solar farms?

    But suppose we want to make really large amounts of solar power. To generate as much electricity as a hefty wind turbine (with a peak power output of maybe two or three megawatts), you need about 500–1000 solar roofs. And to compete with a large coal or nuclear power plant (rated in the gigawatts, which means thousand megawatts or billions of watts), you’d need 1000 times as many again—the equivalent of about 2000 wind turbines or perhaps a million solar roofs. (Those comparsions assume our solar and wind are producing maximum output.) Even if solar cells are clean and efficient sources of power, one thing they can’t really claim to be at the moment is efficient uses of land. Even those huge solar farms now springing up all over the place produce only modest amounts of power (typically about 20 megawatts, or about 1 percent as much as a large, 2 gigawatt coal or nuclear plant). The UK renewable company Ecotricity has estimated that it takes about 22,000 panels laid across a 12-hectare (30-acre) site to generate 4.2 megawatts of power, roughly as much as two large wind turbines and enough to power 1,200 homes. [8]

    Photo: The vast 91-hectare (225-acre) Alamosa Solar Generating Project in Colorado generates up to 30 megawatts of solar power using three cunning tricks. First, there are huge numbers of photovoltaic panels (500 of them, each capable of making 60kW). Each panel is mounted on a separate, rotating assembly so it can track the Sun through the sky. And each has multiple Fresnel lenses mounted on top to concentrate the Sun’s rays onto its solar cells. Photo by Dennis Schroeder courtesy of NREL (image ID #10895528).

    Power to the people

    Photo: A micro-wind turbine and a solar panel work together to power a bank of batteries that keep this highway construction warning sign lit up day and night. The solar panel is mounted, facing up to the sky, on the flat yellow lid you can see just on top of the display.

    Some people are concerned that solar farms will gobble up land we need for real farming and food production. Worrying about land-take misses a crucial point if we’re talking about putting solar panels on domestic roofs. Environmentalists would argue that the real point of solar power is not to create large, centralized solar power stations (so powerful utilities can go on selling electricity to powerless people at a high profit), but to displace dirty, inefficient, centralized power plants by allowing people to make power themselves at the very place where they use it. That eliminates the inefficiency of fossil fuel power generation, the air pollution and carbon dioxide emissions they make, and also does away with the inefficiency of transmitting power from the point of generation to the point of use through overhead or underground power lines. Even if you have to cover your entire roof with solar panels (or laminate thin-film solar cells on all your Windows), if you could meet your entire electricity needs (or even a large fraction of them), it wouldn’t matter: your roof is just wasted space anyway. According to a 2011 report [PDF] by the European Photovoltaic Industry Association and Greenpeace, there’s no real need to cover valuable farmland with solar panels: around 40 percent of all roofs and 15 percent of building facades in EU countries would be suitable for PV panels, which would amount to roughly 40 percent of the total electricity demand by 2020.

    It’s important not to forget that solar shifts power generation to the point of power consumption —and that has big practical advantages. Solar-powered wristwatches and calculators theoretically need no batteries (in practice, they do have battery backups) and many of us would relish solar-powered smartphones that never needed charging. Road and railroad signs are now sometimes solar powered; flashing emergency maintenance signs often have solar panels fitted so they can be deployed in even the remotest of locations. In developing countries, rich in sunlight but poor in electrical infrastructure, solar panels are powering water pumps, phone boxes, and fridges in hospitals and health clinics.

    Why hasn’t solar power caught on yet?

    The answer to that is a mixture of economic, political, and technological factors. From the economic viewpoint, in most countries, electricity generated by solar panels is still more expensive than electricity made by burning dirty, polluting fossil fuels. The world has a huge investment in fossil fuel infrastructure and, though powerful oil companies have dabbled in solar power offshoots, they seem much more interested in prolonging the lifespan of existing oil and gas reserves with technologies such as fracking (hydraulic fracturing). Politically, oil, gas, and coal companies are enormously powerful and influential and resist the kind of environmental regulations that favor renewable technologies like solar and wind power. Technologically, as we’ve already seen, solar cells are a permanent work in progress and much of the world’s solar investment is still based on first-generation technology. Who knows, perhaps it will take several more decades before recent scientific advances make the business case for solar really compelling?

    One problem with arguments of this kind is that they weigh up only basic economic and technological factors and fail to consider the hidden environmental costs of things like oil spills, air pollution, land destruction from coal mining, or climate change—and especially the future costs, which are difficult or impossible to predict. It’s perfectly possible that growing awareness of those problems will hasten the switch away from fossil fuels, even if there are no further technological advances; in other words, the time may come when we can no longer afford to postpone universal adoption of renewable energy. Ultimately, all these factors are interrelated. With compelling political leadership, the world could commit itself to a solar revolution tomorrow: politics could force technological improvements that change the economics of solar power.

    And economics alone could be enough. The pace of technology, innovations in manufacturing, and economies of scale continue to drive down the cost of solar cells and panels. Look what’s happened over the last decade or so. Between 2008 and 2009 alone, according to the BBC’s environment analyst Roger Harrabin, fell by about 30 percent, and China’s increasing dominance of solar manufacturing has continued to drive them down ever since. Between 2010 and 2016, the cost of large-scale photovoltaics fell by about 10–15 percent per year, according to the US Energy Information Administration; overall, the price of switching to solar has plummeted by around 90 percent in the last decade, further cementing China’s grip on the market. Six of the world’s top ten solar manufacturers are now Chinese; in 2016, around two thirds of new US solar capacity came from China, Malaysia, and South Korea.

    Photo: Solar cells aren’t the only way to make power from sunlight—or even, necessarily, the best way. We can also use solar thermal power (absorbing heat from sunlight to heat the water in your home), passive solar (designing a building to absorb sunlight), and solar collectors (shown here). In this version, 16 mirrors collect sunlight and concentrate it onto a Stirling engine (the gray box on the right), which is an extremely efficient power producer. Photo by Warren Gretz courtesy of NREL (image ID #6323238).

    Catching up fast?

    The tipping point for solar is expected to arrive when it can achieve something called grid parity, which means that solar-generated electricity you make yourself becomes as cheap as power you buy from the grid. Many European countries expected to achieve that milestone by 2020. Solar has certainly posted very impressive rates of growth in recent years, but it’s important to remember that it still represents only a fraction of total world energy. In the UK, for example, the solar industry boasted of a milestone achievement in 2014 when it almost doubled the total installed capacity of solar panels from roughly 2.8 GW to 5 GW. But that still represents only a couple of large power stations and, at maximum output, a mere 8 percent of the UK’s total electricity demand of roughly 60 GW (factoring in things like cloudiness would reduce it to some fraction of 8 percent).

    According to the US Energy Information Administration, in the United States, where photovoltaic technology was invented, in 2020, solar represented only 3 percent of the country’s total electricity generation. That’s about 2.3 times more than in 2017 (when solar was 1.3 percent), 3.3 times more than in 2016 (when the figure was 0.9 percent) and about 7.5 times as much as in 2014 (when solar stood at just 0.4 percent). [9] Even so, it’s still less than a third as much as coal and 26 times less than all fossil fuels. [10] Even a doubling in US solar would see it producing not much more than half as much electricity as coal does today (10 × 3 = 6 percent, compared to 10 percent for coal in 2020). It’s telling to note that two of the world’s major annual energy reviews, the BP Statistical Review of World Energy and the International Energy Agency’s Key World Energy Statistics, barely mention solar power at all, except as a footnote.

    Chart: Solar power is making more of our electricity every year, but still nowhere near as much as coal (which is in steep decline). This chart compares the percentage of electricity generated in the United States by solar power (green line) and coal (red line). The position is better than this in some countries and worse in others. Drawn by using historic and current data from US Energy Information Administration (historic data from that page is available from the Wayback Machine).

    Will that change anytime soon? It just might. According to a 2016 paper by researchers from Oxford University, the cost of solar is now falling so fast that it’s on course to provide 20 percent of the world’s energy needs by 2027, which would be a step change from where we are today, and a far faster rate of growth than anyone has previously forecast. modestly, the US EIA predicts that solar will be providing 20 percent of all US electricity by 2050. Can the pace of growth possibly continue? Could solar really make a difference to climate change before it’s too late? Watch this space!

    Find out more

    On this website

    • Climate change and global warming
    • Electronics
    • Energy
    • Passive-solar energy
    • Photoelectric cells
    • Pyranometers (devices that measure sunlight)
    • Renewable energy
    • Wind turbines

    Books for older readers

  • The Switch: How solar, storage and new tech means cheap power for all by Chris Goodall. Profile, 2016. An accessible economic argument demonstrating that year-on-year reductions in the cost of solar will soon make the switch away from fossil fuels inevitable.
  • Physics of Solar Cells: From Basic Principles to Advanced Concepts by Peter Würfel. Wiley, 2016. Another academic book about solar semiconductor physics.
  • Solar Energy: The Physics and Engineering of Photovoltaic Conversion, Technologies and Systems by Arno Smets at al. UIT Cambridge, 2016. A detailed but accessible introduction to solar science and technology.
  • Solar power in Sustainable Energy Without the Hot Air by David MacKay. UIT Cambridge, 2009. This excellent book compares different ways of making energy without fossil fuels.
  • Physics for Future Presidents by Richard Muller. W.W.Norton, 2008. Broadly similar to David MacKay’s book, though with less math and more politics and not just about energy. Part 2 is, however, devoted to energy issues and Chapter 6 covers solar power.
  • Books for younger readers

    • Solar Power: Capturing the Sun’s Energy by Laurie Brearley. Scholastic, 2018. A 48-page introduction billed as suitable for grades 3–5, ages 8–10.
    • Eyewitness Energy by Jack Challoner. New York/London, England: Dorling Kindersley, 2012: Explains the basic concepts of energy and the history of how people have harnessed it. Grades 4–6; ages 9–12.
    • Energy by Chris Woodford. New York/London, England: Dorling Kindersley, 2007: My own colorful little book about energy in the modern world. Ages 9–12.
    • Power and Energy by Chris Woodford. New York: Facts on File, 2004. Another of my books, this is a 100-page introduction to humankind’s efforts to harness energy. Suitable for grades 4–6; ages 9–12.


  • Solar generation was 3% of U.S. electricity in 2020, but we project it will be 20% by 2050: Today in Energy, US Energy Information Administration, November 16, 2021. Solar electric power is increasing rapidly, but from a very low base.
  • UK firm’s solar power breakthrough could make world’s most efficient panels by 2021 by Jillian Ambrose, The Guardian, August 15, 2020. A breathrough in the sue of perovskite-on-silicon cells promises a step-change in solar efficiency.
  • Power From Commercial Perovskite Solar Cells Is Coming Soon by Jean Kumagai, IEEE Spectrum, January 4, 2019. A closer look at perovskite solar technology now being developed in Oxford, England.
  • The Dawn of Solar Windows by Andy Extance, IEEE Spectrum, January 24, 2018. Can a window generate solar power efficiently and still remain transparent? Yes—and here’s how researchers think it could be done.
  • When Solar Panels Became Job Killers by Keith Bradsher. The New York Times, April 8, 2017. How China’s solar panel manufacturers are conquering the world.
  • Tesla Ventures Into Solar Power Storage for Home and Business by Diane Cardwell. The New York Times. May 1, 2015. How the pioneering electric car maker plans to revolutionize home energy storage as well.
  • Solar Energy Isn’t Always as Green as You Think by Dustin Mulvaney, IEEE Spectrum, August 26, 2014. Photovoltaics might sound environmentally friendly, but they’re sometimes produced by processes that harm workers and the environment.
  • Can Solar Power Go Truly Transparent? by Dave Levitan, IEEE Spectrum, August 25, 2014. Could we convert transparent Windows into effective solar power producers?
  • Perovskites: the future of solar power? by Bernie Bulkin, The Guardian, March 7, 2014. Most solar cells are currently manufactured using silicon semiconductors, but perovskites (minerals based on calcium titanium trioxide) could ultimately offer greater efficiency.
  • Hot summer bestows solar power bounty on Britain by John Vidal, The Guardian, July 26, 2013. Even a dull northern area like the UK has great solar potential. Around half a million British buildings now have solar panels installed.
  • On other websites

    • Solar Research: Lots of information on the latest from the US DOE’s National Renewable Energy Laboratory—home of cutting-edge research into sustainable power.
    • Measuring solar insolation: Simple maps from NASA compare the amount of sunlight received by different regions of our planet.


    • Energy 101: Solar PV: The US Department of Energy’s quick introduction explains how solar panels work and summarizes their advantages.
    • How solar farms could work: The CSEM company of Switzerland have animated the idea of a solar farm that could work in the oceans or the desert.
    • Suncatchers and Sterling Engines: Tessera Solar explain how solar power plants can use Stirling engines to convert thermal energy into electricity.


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