Solar Photovoltaic vs. Solar Thermal — Understanding the Differences
The transition to renewable energy is gaining momentum as concerns about climate change and energy security escalate, and solar power is leading the way. Solar photovoltaic (PV) and solar thermal are both leading sustainable solutions. Read this guide to learn the differences and decide which best suits your purposes.
Solar PV vs. Solar Thermal — What’s the Difference?
Quick Answer: Solar PV and solar thermal both harness energy from the sun but for different purposes. Photovoltaic (PV) systems convert sunlight directly into electricity, while thermal systems produce thermal energy for residential heating systems such as hot water or space heaters.
The differences also come down to how they capture energy from sunlight. PV systems generate electricity when photovoltaic panels capture solar energy and convert it into DC electricity. Thermal systems capture the sun’s heat through thermal panels that absorb the sun’s thermal energy and transmit it to a heat-transfer fluid.
In this article, you’ll learn:
- The differences between solar photovoltaics and thermal energy systems;
- How a photovoltaic panel converts sunlight into electricity;
- The different types of solar thermal systems, including flat-plate collectors and evacuated-tube collectors;
- Which system is best for your energy needs.
Solar Photovoltaic
Solar photovoltaic (PV) technology is a renewable energy system that converts sunlight into electricity via solar panels. A PV panel contains photovoltaic cells, also called solar cells, which convert light photons (light) into voltage (electricity). This phenomenon is known as the photovoltaic effect.
How Does Solar Photovoltaic Work?
Photovoltaic panels consist of semiconductor materials (usually silicon). When sunlight strikes the surface of a PV panel, the semiconductor absorbs energy from the photons. That reaction releases electrons from their atomic bonds. It creates a flow of electrons, resulting in an electric current.
The generated electric current is in the form of a direct current (DC). An inverter converts the DC power into alternating current (AC) to make this electricity usable for most household appliances and the electrical grid.
Components of Solar Photovoltaic (PV) System
PV systems have various interconnected components that work together to provide electricity to your home. These components include:
- Photovoltaic Panels
- Charge Controller
- Solar Battery Bank
- Inverter
- Utility Meter
- Electric Grid
Off-grid systems only use the first four components, as they do not utilize utility meters or electric grids.
The solar panels are your system’s first (and most important!) component. They interface directly with the sun’s rays, converting the photons into electricity.
An inverter converts direct current (DC) electricity into alternating current (AC) electricity. The inverter is crucial since PV panels produce DC electricity, while most household appliances and electrical systems operate on AC. Common types of inverters include string inverters, microinverters, and hybrid inverters.
The charge controller comes next in a PV system. This device sits between the photovoltaic panels and batteries to regulate the electricity that passes between them. The charge controller prevents overcharging and transmits an electrical current to the battery bank.
A battery bank stores electricity for later use. Also called a solar battery, it is handy for cloudy days or wintertime when your PV array produces less power.
Utility meters are an essential part of any grid-tied system. These devices measure the flow of electricity between the electric grid and your home’s solar system. A utility meter will track the electricity produced and consumed by a home.
The electric grid is the final component of a grid-tied system. The power produced from a residential solar array is sent through a utility meter and out into the electric grid. When a home draws power, it also pulls from the electric grid (unless the system has an energy storage system like a battery).
Net metering programs allow homeowners to receive payment for any excess energy produced during a billing period.
Advantages
- Whole-Home Power — provides electricity to an entire house, including appliances, lights, water heating, HVAC, and more.
- Renewable Energy — PV systems are a renewable energy source, reducing the user’s reliance on fossil fuels and utility companies.
- Lower Electricity Bills — photovoltaics can drastically reduce or eliminate your monthly electricity bill.
Disadvantages
- Upfront Cost — upfront costs for a PV system can be in the tens of thousands. However, this cost is easily recoupable over the system’s lifetime.
- Aesthetics — some home and business owners may find a solar array visually unappealing.
- Space Requirements — whole-home PV systems require around 480 sq ft (45 M2 on average.)
Solar Thermal
Solar thermal panels perform a similar function to PV panels by converting sunlight into usable energy. However, thermal panels differ in that they use a heat-transfer fluid — either water or air — to capture the energy, as opposed to the semiconductors of PV panels.
Thermal systems are an efficient and environmentally friendly method for residential or commercial heating. They reduce the user’s dependency on fossil fuels and lower greenhouse gas emissions.
How Does Solar Thermal Work?
Depending on the intended usage, there are a few different types of thermal systems. In all solar thermal systems, a heat-transfer fluid (water or air) collects energy from the sun. The hot fluid is then used directly in the space for heating, or it can produce steam for mechanical energy.
Most residential systems use flat-plate collectors. The thermal panel consists of a dark, flat surface encased in a thermally-insulated box. The dark color of the panel allows more energy absorption.
Another common type of thermal system is the evacuated tube collector. This type of panel features a series of glass tubes containing a vacuum, which reduces energy loss.
Either of these panel types can work for hot water heating, space heating, or electricity generation.
For home heating, the heat-transfer fluid can circulate through pipes in a floor through radiant heating. A radiant floor system radiates the heat from the liquid into the room.
Solar thermal systems can also operate on a commercial scale for energy production. The heat-transfer fluid produces steam that, when passed through a turbine, powers a generator that produces electricity.
Components Used in a Solar Thermal System
While individual systems will vary, a few components are common to most thermal systems.
Solar thermal collectors are the “panels” in a thermal system. They are usually installed on a home’s roof and convert the sun’s energy into heat.
The heat transfer fluid flows through a thermal collector and transfers the heat to the rest of the system.
The pump station distributes the heat transfer fluid throughout the system.
A controller monitors and regulates the transfer process. It controls the other system components, ensuring safe and reliable operation.
A hot water tank will likely be integrated into the design if the thermal system is for heating household water. For radiant heating systems, pipes installed in the floor allow the heat transfer fluid to flow throughout the home.
Advantages
- Efficiency — thermal systems are an efficient way to convert sunlight into heat for your home.
- Renewable Energy — a thermal system utilizes renewable energy to reduce environmental impact.
- Reduced Heating Bills —a thermal system may reduce your monthly water, gas, or electricity bill.
Disadvantages
- Limited Use — thermal systems are not practical for whole home electricity generation. A photovoltaic system is more efficient for this purpose.
- Incompatibility — radiant heating systems are part of the construction process, installed before pouring the concrete foundation for a home. It is often impractical to retrofit a home with radiant heating.
- Intermittent Energy Generation — a thermal system will only function while sunlight is available, so your energy production may decrease depending on the weather, season, or time of day.
Conclusion
Solar PV and solar thermal both utilize renewable energy. PV systems harness sunlight to generate electricity to use throughout your home, while solar thermal systems use sunlight to heat water or residential spaces. Either system can be liberating, freeing you from monthly electric bills and reliance on fossil fuels.
A solar thermal system may work for you if you just need to heat your home. Otherwise, photovoltaic systems are much more versatile — you can heat your home and water while also powering your home’s electrical system.
If you’re ready to install a PV system for your home, check out EcoFlow’s innovative solar solutions.
Frequently Asked Questions
No, solar PV systems and solar thermal systems are not the same. PV systems convert sunlight into electricity using photovoltaic cells, while thermal systems capture the sun’s heat using a heat-transfer fluid. Both harness solar energy but serve different purposes and use different technologies.
Yes, thermal systems can work in colder climates with less sunlight. While their efficiency may decrease in cold conditions, they can still provide heat by absorbing diffused sunlight. Evacuated tube collectors are particularly effective in cold climates due to their vacuum insulation, which minimizes heat loss.
Solar photovoltaic systems typically have a lifespan of 25-30 years, with panel efficiency gradually decreasing over time. Thermal systems can last around 20-25 years. Both systems require periodic maintenance to ensure optimal performance, and some individual components may need replacement within the lifetime of a system.
Photovoltaics are versatile technology that can generate electricity for various residential uses, including lighting, appliances, and heating. Solar thermal is a more specialized technology best suited for water and space heating. Combining both technologies can give you the best of both worlds and maximize energy savings.
EcoFlow is a portable power and renewable energy solutions company. Since its founding in 2017, EcoFlow has provided peace-of-mind power to customers in over 85 markets through its DELTA and RIVER product lines of portable power stations and eco-friendly accessories.
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.
| 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.
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.
Electrical Characteristics of PV Modules
A solar module can be seen as a black box that with two connectors, producing a current, I, at a voltage, U.
For the purpose of the electrical characteristics of a solar cell, the inside of that black box can be described by an electric cicuit with only 4components:
- Current Source: This is the source of the photo current, and it is: with the cell area, A, the intensity of incoming light, H, and the response factor ξ in units of A / W.
- Diode: This non-linear element reflects the dependance on the Band gap and losses to recombination. It is characterised by the reverse current, I0, which measures the leakage of electrons and re-combining and by a quality factor, q, with values between 1. 2, an empirical factor.
- Shunt Resistor Rp: represents losses incurred by conductors.
- Serial Resistor Rs: also reprents losses incurred by non-ideal conductors.
The relationship between I and U of a single cell is then expressed by: with thermal voltage
with temperature, T (in Kelvin), Boltzmann constant k = 1.38e.23 and the elementary charge e = 1.602e.19. Using this formula, we can calculate maximum power points, but also behaviour under different temperatures.
In a module, a number of cells are put in series into a string, and a number of strings in parallel. Putting cells in series adds to the voltage, whereas putting cells in parallel adds to the current, so that:
Current-Voltage Curve: I-U Characteristics
The I-U equation can only be solved iteratively. The graph to the right shows a typical curve.
Sensitivity: The curve is highly sensitive to changes in:
| Conversion Efficiency | Shift to higher currents |
| Temperature | Higher termperature results in lower open-circuit voltage, and higher short-circuit current. Overall result is a shift of the mpp to a lower power. |
| Reverse saturation current | Higher leakage results in flatter curve. |
| Serial resistance | Higher losses result in lower voltage |
This is the conversion efficiency that is observed when the module is subjected to light with intensity 1kW/m 2 under standard conditions is called nominal efficiency. The peak power of the module is related to the module area A and nominal efficiency ηnomby:
The nominal efficiency can be obtained from the manufacturer’s data sheet.
Should the conditions differ from the standard testing condition, the nominal module efficiency must be multiplied by a relative module efficiency, ηrel. This factor is dependant on changes in temperature, intensity of the incoming light and ratio of diffuse radiation to direct radiation. The instantaneous power supplied by the module is:
Pmodule= H/H0 or: Ppeak=H0 A ηnomηrel with intensity of incoming light H.
Values for the relative efficiency can be obtained from manufacturer’s data sheet.
Relative Module Efficiency
Here is a typical curve for relative efficiency over the intensity of the incoming light for different temperatures. Naturally, at 25°C and 1,000W/m 2 the relative efficiency is 1.0, as these are the standard test conditions.
Nevertheless, the conversion efficiency is nearly constant over a wide range of intensities, only dropping sharply below 10% of the standard 1,000W.
Changes in temperature cause the curve to shift upwards (if colder) or downwards (if warmer). Silicon is more sensitive to temperature changes than many of the thin-fim materials.
Maximum Power Tracking
With light varying its intensity throughout the day, the maximum power point moves to different voltages and currents. That means, if, for instance, the voltage is forced to be constant, the solar module will most likely not operate in its power optimum.
In practice, a Maximum Power Point Tracker (MPP), is inserted between the solar module and the load (its output) in order to ensure optimum operation. Such MPP tracker is an adjustable DC- to DC transformer, which contains a high frequency switch providing a matching between the load and the solar module.
Manufacturers quote 2 efficiency factors:
- Maximum Efficiency Factor. this is the measure of how close the device gets to the true maximum power point. Typical values are around 97%.
- Euorpean Efficiency Factor. in reality, the tracking efficiency is not constant across the whole voltage range. However, due to variations in solar irradiance, the mpp tracker will often have to work in a non-optimal range. Hence, the European efficiency factor is an average efficiency factor that one would expect in the middle of Europe. Typical values are around 95%
Most MPP trackers are step-down trackers, driving a high-voltage load from a low-voltage solar module. For instance: MPP input voltage range 200. 400V transformed into a maximum voltage of 550V.
How Do Solar Panels Work? (2023)
You know solar power can save you money, but do you know how it works? Ensure you’re an informed buyer by reading our guide.
Tamara Jude is a writer specializing in solar energy and home improvement content. She has a background in journalism and an enthusiasm for research, with more than six years of experience producing and writing content. In her spare time, she enjoys traveling, attending concerts and playing video games.
Angela Bunt is an accomplished editor with more than a decade of experience writing, producing and editing content. She has a breadth of knowledge spanning the home, travel, music and health industries, and she is a proud New Hampshire homeowner. In her spare time, Angela enjoys live music, watching the Real Housewives and hanging out with her dog, Jim.
Most homeowners begin their search for solar panels by shopping for the best solar companies or the best equipment. However, it’s equally important to know how solar panels work so you can understand the small details of the process. We at the Guides Home Team have created this guide to give you a better understanding about the system you’re about to invest in.
- What Are Solar Panels?
- The Photovoltaic Effect
- Solar Panel Materials
- Solar Panel Efficiency
- How Do Solar Panels Work?
- The Bottom Line
Offers a range of financing options 24/7 customer service line Panel insurance protects against theft and damage
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What Are Solar Panels?
A solar panel, or photovoltaic (PV) panel, converts the sun’s energy into electricity. They’re most commonly composed of glass, wires and silicon and mounted on your rooftop or on the ground. Solar installations require a large number of panels to properly power your home. The configuration of solar panels, known as a solar array, depends on your home’s energy needs and available roof space. Solar panels work best on cloudless days with ample sunshine exposure. Performance can be hindered by shadows covering the panels, cloudy days or night. The panels still work, but their ability to capture the sunlight is diminished.
The Photovoltaic Effect
The photovoltaic effect is key to how solar technology generates energy. This process begins when particles of sunlight, known as photons, knock electrons away from atoms. Electricity is created once these electrons are set into motion. Solar panels capture these electrical charges and convert them into electricity to power your home. Each panel contains several interconnected photovoltaic cells (PV cells) made to capture sunlight. These PV cells are small, about 6 inches long by 6 inches wide. A solar panel contains between 32 and 48 PV cells but varies based on the panel’s size and intended output. Each cell uses nonmetal semiconductor material to absorb sunlight. Traditional solar photovoltaic panels use silicon, one of Earth’s most abundant elements. However, silicon isn’t a great conductor on its own. Solar manufacturers use a process called “doping” to enhance its conductivity. This involves adding impurities that create bonds to the silicon to improve the electrical charge. Boron is added to create a positive charge while phosphorus creates a negative charge. Two layers of silicon are placed on top of each other, each treated with one of these elements. When brought together, they create an electrical field. Metal plates are added to the sides of the silicon solar cells to help push the electron through the wiring and into your home.
Solar Panel Materials
Most solar panel systems use crystalline silicon panels, some of the best solar p an els for residential installations. PV panels are available in two options: monocrystalline and polycrystalline. Thin-film solar panels are a cheaper alternative to conventional panels and use thin layers of PV materials. Their manufacturing process and composition is different from traditional panels. Monocrystalline (mono) panels use a pure form of single-crystalline silicon. This process is more expensive and time-consuming but results in higher power production. Polycrystalline panels are made with multiple silicon fragments, making manufacturing more cost-effective. However, these fragments create imperfections, lowering the panels’ efficiency. Thin-film panels are the cheapest to manufacture because they use thin PV materials instead of silicon, but they have the lowest energy production.
Solar Panel Efficiency
Efficiency measures how much sunlight a panel can absorb and convert into electricity. For example, solar panels with an efficiency rating of 16% can absorb 16% of the sun’s rays and convert them into energy. According to the National Renewable Energy Laboratory (NREL), most solar panels have efficiency ratings between 16% and 22%, with an average of 19.2%. The pure silicon used in mono panels has no surface flaws, so these panels have the highest efficiency ratings, typically between 17% and 23%. Poly panels have a lower rating between 13% and 16%. Thin-film panels have the lowest efficiency ratings, with ranges between 7% and 11%.
The Bottom Line: How Do Solar Panels Work?
Your solar system’s productivity begins with the panels. By choosing the right panels, components and accessories, you can maximize your money-saving potential. Knowing how solar panels work not only improves your knowledge as a consumer but makes you more informed in the buying process. To ensure you get the best solar panels available, compare at least three solar companies in your research. Use our tool below to begin your solar buying journey.
How to Design and Install a Solar PV System?
Today our modern world needs energy for various day to day applications such as industrial manufacturing, heating, transport, agricultural, lightning applications, etc. Most of our energy need is usually satisfied by non-renewable sources of energy such as coal, crude oil, natural gas, etc. But the utilization of such resources has caused a heavy impact on our environment.
Also, this form of energy resource is not uniformly distributed on the earth. There is an uncertainty of market such as in the case of crude oil as it depends on production and extraction from its reserves. Due to the limited availability of non-renewable sources, the demand for renewable sources has grown in recent years.
Solar energy has been at the center of attention when it comes to renewable energy sources. It is readily available in an abundant form and has the potential to meet our entire planet’s energy requirement. The solar standalone PV system as shown in fig 1 is one of the approaches when it comes to fulfilling our energy demand independent of the utility. Hence in the following, we will see briefly the planning, designing, and installation of a standalone PV system for electricity generation.
- : A Complete Guide About Solar Panel Installation. Step by Step Procedure with Calculation Diagrams
Planning of a Standalone PV system
Site assessment, surveying solar energy resource assessment:
Since the output generated by the PV system varies significantly depending on the time and geographical location it becomes of utmost importance to have an appropriate selection of the site for the standalone PV installation. Thus, the following points must be considered for the assessment and selection of locations for installation.
- Minimum Shade: It must be made sure that the selected site either at rooftop or ground should not have shades or should not have any structure that intercepts the solar radiation falling on the panels to be installed. Also, make sure that there won’t be any structural construction soon surrounding the installation that might cause the problem of shading.
- Surface Area: The surface area of the site at which the PV installation is intended should be known, to have an estimation of the size and number of panels required to generate the required power output for the load. This also helps to plan the installation of inverter, converts, and battery banks.
- Rooftop: In the case of the rooftop installation the type of roof and its structure must be known. In the case of tilt roofs, the angle of tilt must be known and necessary mounting must be used to make the panels have more incidents of solar radiation i.e. ideally the radiation angle must be perpendicular to the PV panel and practically as close as to 90 degrees.
- Routes: Possible routes for the cables from an inverter, battery bank, charge controller, and PV array must be planned in a way that would have minimum utilization of cables and lower voltage drop in cables. The designer should choose between the efficiency and the cost of the system.
To estimate the output power the solar energy assessment of the selected site is of foremost significance. Insolation is defined as the measure of the sun’s energy received in a specified area over a period of time. You can find this data using a pyranometer, however, it is not necessary as you can find the insolation data at your nearest meteorological station. While assessing the solar energy the data can be measured in two ways as follows:
- Kilowatt-hours per square meter per day (KWh/m 2 /day): It is a quantity of energy measured in kilowatt-hours, falling on square meter per day.
- Daily Peak Sun Hours (PSH): Number of hours in a day during which irradiance averages to 1000 W/m 2.
Peak sun hours are most commonly used as they simplify the calculations. Do not get confused with the “Mean Sunshine Hours” and “Peak Sun Hours” which you would collect from the meteorological station. The “Mean sunshine hours” indicates the number of hours the sunshine’s were as the “Peak sun hours” is the actual amount of energy received in KWh/m 2 /day. Amongst all months over a period of year use the lowest mean daily insolation value as it will make sure that the system will operate in a more reliable way when the sun is least due to unsuitable weather conditions.
Calculation of Energy Demand
The size of the standalone PV system depends on the load demand. The load and its operating time vary for different appliances, therefore special care must be taken during energy demand calculations. The energy consumption of the load can be determined by multiplying the power rating (W) of the load by its number of hours of operation. Thus, the unit can be written as watt × hour or simply Wh.
Energy demand Watt-hour = Power rating in Watt × Duration of operation in hours.
Thus, the daily total energy demand in Wh is calculated by adding the individual load demand of each appliance per day.
Total energy demand Watt-hour = ∑ (Power rating in Watt × Duration of operation in hours).
A system should be designed for the worst-case scenario i.e. for the day when the energy demand is highest. A system designed for the highest demand will ensure that the system is reliable. If the system meets the peak load demand it will meet the lowest demand. But designing the system for the highest demand will increase the overall cost of the system. On the other hand, the system will be fully utilized only during the peak load demand. So, we have to choose between cost and reliability of the system.
Inverter Converter (Charge Controller) Ratings
For choosing the proper inverter both the input and output voltage and current rating should be specified. The inverter’s output voltage is specified by the system load, it should be able to handle the load current and the current taken from the battery bank. Based on the total connected load to the system the inverter power rating can be specified.
Let’s consider 2.5 kVA in our case, hence an inverter with power handling capacity having a size of 20-30% higher than the power running the load should be chosen from the market. In the case of motor load, it should be 3-5 times higher than the power demand of such an appliance. In the case of the converter, the charge controller is rated in current and voltage. Its current rating is calculated by using the short-circuit current rating of the PV module. The value of voltage is the same as the nominal voltage of batteries.
Converter and Charge Controller Sizing
The charge controller rating should be 125% of the photovoltaic panel short circuit current. In other words, It should be 25% greater than the short circuit current of solar panel.
Size of solar charge controller in amperes = Short-circuit current of PV × 1.25 (Safety factor).
For example, we need a 6 numbers each of 160W solar panels for our system. Following are the related date of PV panel.
Suppose the PV module specification are as follow.
The required rating of solar charge controller is = (4 panels x 10 A) x 1.25 = 50 A
Now, a 50A charge controller is needed for the 12V DC system configuration.
Note: This formula is not applicable on MPPT Solar chargers. Please refer to the user manual or check the nameplate data rating for proper sizing.
Inverter Sizing
The size of Inverter should be 25% bigger than the total load due to losses and efficiency problem in the inverter. In other words, It should be rated 125% than the total load required in watts. For example, if the required wattage is 2400W, than the size of inverter should be:
So we need a 3kW of inverter in case of 2400W load.
Daily Energy Supplied to Inverter
Let us consider in our case the daily energy consumption by the load is 2700 Wh. Note that the inverter has its efficiency, thus the energy supplied to the inverter should be more than the energy used by the load, so the losses in the inverter can be compensated. Assuming 90% efficiency in our case, the total energy supplied by the battery to the inverter would be given as;
Energy supplied by the battery to the inverter input = 2700 / 0.90 = 3000 Wh/per day.
System Voltage
The inverter input voltage is referred to as the system voltage. It is also the overall battery pack voltage. This system voltage is decided by the selected individual battery voltage, line current, maximum allowable voltage drop, and power loss in the cable. Usually, the voltage of the batteries is 12 V so will be the system voltage. But if we need higher voltage it should be multiples of 12 V. i.e. 12 V, 24 V, 36 V, and so on.
By decreasing the current, power loss and voltage drop in the cable can be reduced, this can be done by increasing the system voltage. This will increase the number of batteries in the series. Therefore, one must choose between power loss and system voltage. Now for our case let us consider the system voltage of 24 V.
Sizing of the Batteries
While sizing the battery some parameters are needed to be considered as follows:
- Depth of Discharge (DOD) of the battery.
- Voltage and ampere-hour (Ah) capacity of the battery.
- The number of days of autonomy (It is the number of days required to power up the whole system (backup power) without solar panels in case of full shading or rainy days. We will cover this part in our upcoming article) to get the needed Ah capacity of batteries.
Let us consider we have batteries of 12 V, 100 Ah with DOD of 70%. Thus, the usable capacity of the is 100 Ah × 0.70 = 70 Ah. Therefore, the charged capacity that is required is determined as follows;
Required charge capacity = energy supplied by the battery to the inverter input/system voltage
Required charge capacity = 3000 Wh/ 24 V = 125 Ah
From this, the number of batteries required can be calculated as;
No. of batteries required = Required charge capacity / (100 × 0.7)
No. of batteries required = 125 Ah / (100 × 0.7) = 1.78 (round off 2 batteries)
Thus, 2 batteries of 12 V, 100 Ah are required. But due to round off 140 Ah instead of 125 Ah is required.
Required charge capacity = 2 × 100Ah × 0.7 = 140 Ah
Therefore, two 12 V, 100 Ah batteries in parallel to meet the above charge capacity. But as the individual battery is of 12 V, 100 Ah only and the system voltage requirement is of 24 V we need to connect two batteries in series to get the system voltage of 24 V as shown in figure 2 below:
So, in total there will be four batteries of 12 V, 100 Ah. Two connected in series and two connected in parallel.
Also, the required capacity of batteries can be found by the following formula.
Sizing of the PV Array
Different sizes of PV modules available in the market produce a different level of output power. One of the most common way to determine the sizing of the PV array is to use the lowest mean daily insolation (Solar irradiance) in peak sun hours as follows;
The total size of PV array (W) = (Energy demand per day of a load (Wh) / TPH) × 1.25
Nmodules = Total size of the PV array (W) / Rating of selected panels in peak-watts.
Suppose, in our case the load is 3000 Wh/per day. To know the needed total WPeak of a solar panel capacity, we use PFG factor i.e.
Total WPeak of PV panel capacity = 3000 / 3.2 (PFG)
Now, the required number of PV panels are = 931 / 160W = 5.8.
This way, we need 6 numbers of solar panels each rated for 160W. You can find the exact number of solar panels by dividing the WPeak by other rating i.e. 100W, 120W 150W etc based on the availability.
Note: The value of PFG (Panel Generation Factor) is varying (due to climate and temperature changes) in different regions e.g, PFG in USA = 3.22, EU = 293, Thailand = 3.43 etc.
over, the additional losses should be considered to find the exact panel generation factor (PGF). These losses (in %) occur due to :
- Sunlight not striking the solar panel straight on (5%)
- Not receiving energy at the maximum power point (excluded in case of MPPT charge controller). (10%)
- Dirt on solar panels (5%)
- PV panels aging and below specification (10%)
- Temperature above 25°C (15%)
Sizing of the Cables
The sizing of the cables depends on many factors such as maximum current carrying capacity. It should have a minimum voltage drop and have minimum resistive losses. As the cables would be placed in the outdoor environment it should be water-resistant and ultraviolet.
The cable must behave minimum voltage drop typically less than 2% as there is an issue of voltage drop in low voltage system. Under sizing of the cables will result in energy loss and sometimes can even lead to accidents. whereas the oversizing is not economically affordable. The cross-sectional area of the cable is given as;
- ρ is the resistivity of the conducting wire material (ohm-meters).
- L is the length of cable.
- VD is the maximum permissible voltage drop.
- IM is the maximum current carried by the cable.
Lets have a solved example for the above example.
Example:
Suppose we have the following electrical load in watts where we need a 12V, 120W solar panel system design and installation.
- An LED lamp of 40W for 12 Hours per day.
- A refrigerator of 80W for 8 Hours per day.
- A DC Fan of 60W for 6 Hours per day.
Now let’s find the number of solar panels, rating and sizing of charge controller, inverter and batteries etc.
Finding the Total Load
Total Load in Wh / day
= (40W x 12 hours) (80W x 8 hours) (60W x 6 hours)
= 1480 Wh / per day
The required wattage by Solar Panels System
= 1480 Wh x 1.3 … (1.3 is the factor used for energy lost in the system)
= 1924 Wh/day
Finding the Size and No. of Solar Panels
WPeak Capacity of Solar Panel
= 1924 Wh /3.2
Required No of Solar Panels
No of Solar Panels = 5 Solar Panel Modules
This way, the 5 solar panels each of 120W will capable to power up our load requirements.
Find the Rating and Size of Inverter
As there is only AC loads in our system for specific time (i.e. no additional direct DC load connected to the batteries) and our total required wattage is:
Now, the rating of inverter should be 25% greater than the total load due to losses in the inverter.
Inverter Rating Size = 225 W
Find the Size, Rating No of Batteries
Our load wattage and operational time in hours
= (40W x 12 hours) (80W x 8 hours) (60W x 6 hours)
Nominal Voltage of Deep Cycle Battery = 12V
Required Days of Autonomy (Power by batteries without solar panel power) = 2 days.
[(40W x 12 hours) (80W x 8 hours) (60W x 6 hours) / (0.85 x 0.6 x 12V)] x 2 days
The required capacity of batteries in Ampere-hour = 483.6 Ah
This way, we need a 12V 500Ah battery capacity for 2 days of autonomy.
In this case, we may use 4 number of batteries each of 12 V, 125Ah connected in parallel.
If the available battery capacity is 175Ah, 12 V, we may use 3 number of batteries. You can get the exact number of batteries by dividing the required capacity of batteries in Ampere-hour by the available battery Ah rating.
Required Number of batteries = Required capacity of batteries in Ampere-hour / Available battery Ah rating
Find The Rating and Size of Solar Charge Controller
The charge controller should be 125% (or 25% greater) than the solar panel short circuit current.
Size of solar charge controller in Amp = Short circuit current of PV × 1.25
The required rating of solar charge controller is = (5 panels x 8.8 A) x 1.25 = 44 A
So you can use the next nearest rated charge controller which is 45A.
Note that this method can’t be used to find the exact size of MPPT solar chargers. Please refer to the user manual provided by the manufacturer or see the nameplate rating printed on it.
Finding the Cable, CB, Switches Plug Ampacity
Use the following tools and explanatory posts with charts to find the exact amperage rating of wire and cables, switches plugs and circuit breakers.
The standalone PV system is an excellent way to utilize the readily available eco-friendly energy of the sun. Its design and installation are convenient and reliable for small, medium, and large-scale energy requirements. Such a system makes the availability of electricity almost anywhere in the world, especially in remote areas. It makes the energy consumer independent of the utility and other sources of energy such as coal, natural gas, etc.
Such a system can have no negative impact on our environment and can provide energy for long periods after its installation. The above systematic design and installation provide useful guidelines for our need for clean and sustainable energy in the modern world.