Techniques to Maximize Solar Panel Power Output
Two recent articles, Energy Harvesting With Low Power Solar Panels and Solar Battery Charger Maintains High Efficiency at Low Light, discuss how to efficiently harvest energy with low power solar panels. Both of these articles mention a concept known as maximum power, which in the context of solar panels is the ability to extract as much power as possible from the solar panel without collapsing the panel voltage. When discussing solar panels and power, terms such as Maximum Power Point Tracking (MPPT) and Maximum Power Point Control (MPPC) are often used. Let’s look into the definition and meaning of these terms in more detail.
As can be seen in Figure 1, the output current of a solar panel varies nonlinearly with the panel voltage. Under short-circuit conditions the output power is zero since the output voltage is zero. Under open-circuit conditions the output power is zero since the output current is zero. Most solar panel manufacturers will specify the panel voltage at maximum power (VMP). This voltage is typically around 70 – 80% of the panel’s open circuit voltage (VOC).In Figure 1 the maximum power is just under 140W with VMP just under 32V and IMP just under 4.5A.
Ideally, any system using a solar panel would operate that panel at its maximum power output. This is particularly true of a solar powered battery charger, where the goal, presumably, is to capture and store as much solar energy as possible in as little time as possible. Put another way, since we cannot predict the availability or intensity of solar power, we need to harness as much energy as possible while energy is available.
There are many different ways to try to operate a solar panel at its maximum power point. One of the simplest is to connect a battery to the solar panel through a diode. This technique is described here in the article Energy Harvesting With Low Power Solar Panels. It relies on matching the maximum power output voltage of the panel to the relatively narrow voltage range of the battery. When available power levels are very low (approximately less than a few tens of milliwatts), this may be the best approach.
The opposite end of the spectrum is an approach that implements a complete Maximum Power Point Tracking (MPPT) algorithm. There are a variety of MPPT algorithms, but most will have some ability to sweep the entire operating range of the solar panel to find where maximum power is produced. The LT8490 and LTC4015 are examples of integrated circuits that perform this function. The advantage of a full MPPT algorithm is that it can differentiate a local power peak from a global power maximum. In multi-cell solar panels, it is possible to have more than one power peak during partial shading conditions (see Figure 2). Typically, a full MPPT algorithm is required to find the true maximum power operating point. It does so by periodically sweeping the entire output range of the solar panel and remembering the operating conditions where maximum power was achieved. When the sweep is complete, the circuitry forces the panel to return to its maximum power point. In between these periodic sweeps, the MPPT algorithm will continuously dither the operating point to ensure that it operates at the peak.
An intermediate approach is something that Linear Technology calls Maximum Power Point Control (MPPC). This technique takes advantage of the fact that the maximum power voltage (VMP) of a solar panel does not, typically, vary much as the amount of incident light changes (see Solar Battery Charger Maintains High Efficiency in Low Light for more information). Therefore, a simple circuit can force the panel to operate at a fixed voltage and approximate maximum power operation. A voltage divider is used to measure the panel voltage and if the input voltage falls below the programmed level, the load on the panel is reduced until it can maintain the programmed voltage level. Products with this functionality include the LTC3105, LTC3129, LT3652(HV), LTC4000-1, and LTC4020. Note that the LT3652 and LT3652HV datasheet refer to MPPT rather than MPPC, but this is largely because Linear Technology had not come up with the MPPC terminology when the LT3652 product was released.
A final note about MPPC and the LTC3105 – the LTC3105 is a boost converter that can start up at the exceedingly low voltage of 0.25V. This makes the LTC3105 particularly well suited for boosting the output voltage of a “1S” solar panel (i.e. a solar panel whose output voltage is that of a single photovoltaic cell, even if the panel has many photovoltaic cells in parallel). With a 1S solar panel, there will be only one maximum power point – it is not possible to have multiple power peaks. In this scenario, differentiating between multiple maxima is not necessary.
In summary, many different ways of operating a solar panel at its maximum output operating condition exist. The panel can be connected to a battery (through a diode) whose voltage range is close to the maximum power voltage of the panel. A full MPPT algorithm, including periodic global sweeps to find the global maximum and a continuous dither to remain at that maximum (an example is the LT8490), can be used. Other products implement an input voltage regulation technique (MPPC) to operate a solar panel at a fixed operating voltage including the LTC3105, LTC3129, LT3652(HV), LTC4000-1 and the LTC4020. In the coming months, Linear Technology will introduce yet another technique for operating a solar panel at its maximum power point. Stay tuned!
Trevor Barcelo has over 15 years of experience at Linear Technology as an analog IC design engineer, design manager and product line manager. He began his career at Linear Technology’s headquarters in Milpitas, CA by designing the LTC1733 Lithium-ion battery charger. After moving to the company’s Boston Design Center, he continued designing battery chargers and USB power managers including the LTC4053, LTC4066 and LTC4089. He holds five patents related to power management. He currently defines battery charging, power management and wireless power products while managing a team of design engineers developing those products.
Trevor received an M.S. in Electrical Engineering from Stanford University and a B.A. in Physics from Harvard University.
Solar Panel Efficiency in 2023
Solar panel efficiency measures how well solar panels are able to convert sunlight into usable electricity.
Thanks to advances in technology, solar panel efficiency has steadily improved over time. As a result of this increase in the current solar panel efficiency, you can get the same amount of power with fewer panels on your roof, or more power with the same number of solar panels.
This article will explain what solar panel efficiency means, how the average efficiency of solar panels affects power production, the role high-efficiency solar panels play in the clean energy transformation, and more.
What Determines Solar Panel Efficiency?
Your solar panel’s ability to produce energy by converting the sunlight it receives to usable electricity depends upon five crucial factors: materials, wiring, reflection, age, and temperature.
Impact of Materials on Solar Panel Efficiency
Solar panel manufacturers use different substances to create different types of solar panels, including:
- Polycrystalline silicon
- Monocrystalline silicon
- Cadmium telluride
- Multi-junction solar cells
Each solar panel material helps determine how much sunlight will be converted to electricity. Most manufacturers today use monocrystalline silicon solar cell technology for their panels because of their superior efficiency. Monocrystalline photovoltaic (PV) cells are more efficient than other panel types because they are made from a single crystal of silicon, which means electrons can move more easily through the cell.
In addition to existing solar panel materials like silicon, solar companies are exploring other materials that could deliver even more efficient panel technology, including both organic and more recyclable options. (We have a deeper discussion of these developments below.)
Impact of Wiring on Solar Panel Efficiency
Solar panels can be wired in series and in parallel, and the different wiring configurations have an impact on how your solar panel system will function and how much power it can produce. There are advantages and disadvantages to each option, so it’s one of the decisions a solar panel company will make when designing the ideal solar power system for your home.
Wiring also helps determine the right inverter for your solar arrays, whether it’s a string inverter, power optimizer, microinverter, hybrid inverter, or something else entirely. The type of inverter that is used–especially in terms of how power is collected from the panels and sent to your house, battery storage, and electricity grid–can impact your solar panel system’s overall efficiency.
Within the panels themselves, wiring and “busbars” (the metal connecting solar cells in the solar panel that actually captures and transfers electricity) have an effect on efficiency, with more efficient panels using different configurations and different technologies to improve this process.
Impact of Reflection on Solar Panel Efficiency
The amount of light reflected away from a solar cell’s surface impacts solar panel efficiency. Solar panel efficiency depends on the amount of light they can absorb and convert into electricity.
If light reflects off the surface of the panel, it can’t become electricity, which lowers the efficiency of that panel. Solar power panels with textured surfaces and anti-reflection coatings can help minimize the amount of light that gets reflected away.
Impact of Age on Solar Panel Efficiency
The average lifespan of solar panels is about 25 to 30 years. Throughout this period, your solar system should generate all the electricity you need to power your home, unless the panels get blocked by too much shade, dirt, or other debris.
However, during that 25- to 30-year timeframe, your panel efficiency will slowly decrease over time, until they eventually reach a point where the solar panels don’t create enough electricity for all of your home’s needs. This degradation rate is factored into the initial design of a solar power system, but it still impacts your solar panel’s efficiency.
Impact of Temperature on Solar Panel Efficiency
The climate of your area impacts solar panel efficiency, as the energy levels of electrons are determined by their level of excitement. Contrary to what you might think, solar panels are more efficient at lower temperatures. Because the electrons on the thin layer of silicon are calmer and less excited, they can move with greater ease and increased numbers through the transmission lines to generate solar power.
When it is hot, the electrons are more excited and moving in different directions. This makes it more challenging for them to move effectively through the transmission lines, leading to lower solar energy production levels during a hot day.
In other words, you will experience higher voltage and enhanced generation efficiency during a cold sunny day compared to a hot sunny day.
How To Calculate Solar Panel Efficiency
The easiest way to calculate the efficiency of your solar panels is with this formula:
Efficiency (%) = (Pmax ÷ Area) ÷ (1000) x 100%
- Pmax = max solar panel power (in Watts)
- Area = length x width of the solar panel (in m2)
- 1000 = Standard Test Condition (STC) irradiance
Let’s break it down a bit for deeper comprehension.
Find your panel’s max power capacity
You can find this information labeled as Pmax or maximum power on the spec sheet for your solar panels.
Get your panel’s physical dimensions
The standard panel dimensions are 65 inches by 39 inches, but you can also find and verify this information for your own panels–specifically, length and width–on the specification sheet.
Calculate the power unit area of your panel
To get the power unit area of your module, divide its Pmax into its area.
Factor in the STC
Standard Test Condition (STC) represents the ideal environment used by solar manufacturers when testing average solar panel output. These conditions include a cell temperature of 25 °C and air mass of 1.5, and solar irradiance of 1000 W/m2. That value of 1000 W/m2 is what’s used in the efficiency equation.
Calculate solar panel efficiency
Using some sample numbers, we can walk through the math. Assuming your solar panels are 2 m2 in area, produce 400 watts, and receive 1,000 W/m2 of sunlight, the efficiency of your solar panels is 20%.
Efficiency (%) = (400 ÷ 2) ÷ (1000) x 100%
Solar Panel Efficiency in 2023
The average solar panel efficiency in 2023 ranges from 15% to 20%. At the high end, the most efficient solar panels available for public use achieve 22% efficiency.
While the majority of solar panels available in the United States today are below 20% efficiency, we can anticipate that the efficiency of solar energy systems will continue to improve as the solar industry pursues advanced photovoltaic technology.
History of Solar Panel Efficiency
The first major breakthrough in solar power was made by Alexandre-Edmond Becquerel in 1839. He discovered the photovoltaic effect, the root of the modern solar cell. Since then, solar innovation and improvement have remained a FOCUS for the scientific community and the solar industry as they look to improve solar panel efficiency and increase energy output over time.
Below is a brief synopsis of major advances in solar energy efficiency over the last three decades, courtesy of new solar panel technology.
- 1992: The University of South Florida manufactures a 15.89% efficient thin-film cell
- 2012: Solar Frontier achieves 17.8% efficiency
- 2015: First Solar CdTe thin film technology reaches 18.6% efficiency
- 2015: SolarCity hits 22.04% efficiency
- 2015: Panasonic’s 72-cell prototype achieves 22.5% efficiency
- 2015: SunPower attains 22.8% efficiency with its X22 panel
- 2016: The Swiss Center for Electronics and Microtechnology and the National Renewable Energy Laboratory achieve 29.8% efficiency
- 2016: The University of South Wales researchers attain 34.5% efficiency
- 2017: George Washington University and Naval Research attain a 44.5% solar cell efficiency
- 2018: Research into perovskite reveals a theoretical upper limit of 66% efficiency
To be clear, a majority of these high efficiency ratings of the most efficient solar panels are achieved in a laboratory setting, and aren’t yet possible at a commercial scale due to high production costs.
The National Renewable Energy Laboratory has been tracking the increase in research-cell efficiencies for a variety of solar panel technologies, and it shows the continued stair-step of improvements over time:
Solar panel efficiency over time has come a long way, thanks to the relentless efforts of solar panel manufacturers and scientists. These continual improvements in solar panel efficiency are great for the environment because they reduce our reliance on electricity created from fossil fuels and send even more energy to the grid. The clean energy they produce helps reduce our reliance on fossil fuels and also helps us combat greenhouse gas emissions.
Why are Modern Solar Panels Efficient?
There are two main reasons why modern solar panels are more efficient: advances in research and the materials used.
Solar Power Research
The desire to improve solar power efficiency drives the development of new solar panel technology, as these significant breakthroughs to improve solar energy systems continue in solar research centers across the globe.
Leading research facilities like the National Renewable Energy Laboratory and The Swiss Center for Electronics and Microtechnology use their resources, technologies, and expertise to conduct experimental studies that help develop the high-output solar panels of the future.
Improvements To Materials
Silicon has been the preferred semiconductor material for generating solar energy, and manufacturers have used silicon for decades because solar cells fabricated from it are low-cost, high-efficiency, and long-living.
Solar panel researchers continue to find new ways to increase the effectiveness of silicon, including the creation of ultra-thin crystalline layers, enhanced production processes that remove silicone dust from solar cells, and next-generation growth methods. These modern techniques have resulted in the dramatic solar efficiency improvements we discussed earlier in the article.
Beyond silicone, ongoing experimentation with more efficient materials like perovskite, and the incorporation of various solar technologies in the design process have combined to create the potential for even more highly efficient panels in the future.
Comparing Cost and Value to Rate Efficient Solar Panels
The average current solar panel efficiency ranges from 15% to 20%, but if you wish to install more efficient panels, you should be ready to pay more.
When designing a solar power system, consider the cost-benefit tradeoff between the higher price of more efficient panels and the amount of additional energy they create before you make that investment. It might be more economical to just install a few more lower-efficiency panels, compared to upgrading all panels to a higher efficiency to produce the same amount of power.
Because most customers buy panels that are 15% to 20% efficient, most manufacturers still produce a majority of their panels in that normal efficiency range. However, some circumstances may warrant the installation of high-efficiency solar panels, such as limited roof space or a complicated roof layout.
The space on your roof can impact the number of panels that can be installed. If that space can’t fit the number of lower-efficiency panels required to generate enough power for your needs, solar installers may use more efficient panels at an extra cost.
The design of your roof also determines the number of residential solar panels your home needs, and how they can be positioned. For example, if the area of your roof with more space isn’t angled to receive much sunlight, the area with less space will be considered. This could result in a custom layout for your solar panels which could increase your budget.
If you have a complex roof layout, Palmetto can work with you to determine the solar panel system design that will generate the electricity you need.
The Future of Solar Panel Efficiency
Various research centers are working to increase solar panel efficiency by experimenting with new materials such as organic photovoltaics, concentration photovoltaics, and quantum dots. Manufacturers are also incorporating exciting new technologies that drive the industry forward, including:
- Building-integrated PV panels
- Perovskite solar cells
- Floating solar farms (floatovoltaics)
- Solar skins
- Solar fabric
Floating solar farms are being deployed worldwide, and studies show that in utility-scale settings they can produce more electricity compared to ground-mounted or rooftop installations, thanks to the cooling effects of the water that boost their efficiency. The application of building-integrated PV and solar skins also demonstrates that the future of solar power efficiency is looking bright.
How To Improve the Efficiency of Your Solar Panels
Solar panels are designed to run efficiently without the homeowner needing to do anything. However, you can help ensure your panels are reaching their maximum efficiency by engaging in two simple chores:
- Keep shade off your solar panels
- Clean your panels regularly (Learn more about Solar Panel Cleaning)
You can also help detect possible issues with the efficiency of your solar panel system by tracking its performance through a mobile app like the Palmetto App and enrolling in a routine maintenance service like Palmetto Protect. If the performance of your panels has deteriorated, Palmetto can advise you on steps you can take to generate the electricity your home needs.
Increasing the average efficiency of solar panels remains a key driver of developments in the solar panel industry. Improvements to efficiency is good news for everyone involved in the New Utility Revolution, as it means we can power our homes and businesses with more of the sun’s rays and fewer fossil fuels.
Since solar panel efficiency depends upon materials, wiring, reflectivity, age, and temperature, researchers pursue every possible lead to enhance those factors so that more electricity can be generated more effectively.
At Palmetto, we use some of the most efficient solar panels in the market to maximize your solar production. We know that saving money on your utility bills and helping to save the planet is important to you, which means we’ll help you create the best possible solar panel system for your home.
Interested in adding efficient solar panels to your roof? Learn how much you could be saving with our Estimate Savings Tool today!
Understanding solar panel efficiency isn’t rocket science. except when it is
Be honest—if your kid came home beaming with pride for getting a 20% on a spelling test, you’d be questioning their understanding of math (as well as language arts). Because 20% just doesn’t sound impressive. But what if your grade-schooler was being quizzed on a list of 500 doctoral-level vocabulary words? You may then be tempted to call Mensa.
The same can be said of solar panel efficiency ratings. Once you understand what’s behind the grading system, where peak performers register percentages in the low 20s, you may have a better appreciation for the advances made in solar panel efficiency over time.
How is solar panel efficiency measured?
Simply put, solar panel efficiency is the ratio of energy from sunlight that a panel can transform into electricity. If you want to be a little more technical, here’s the formula:
Even without plugging in any numbers, you can see that the more watts a photovoltaic (PV) cell can produce (using the same amount of sunlight), the more efficient a solar panel will be. This can be important when evaluating commercial solar panels, but we’ll cover that later. First, let’s start with a little history.
A short history of solar cell technology
Humans have been tapping the power of the sun for centuries. Magnifying glasses were used to concentrate the sun’s rays to ignite fires in the 7th century B.C., and architects throughout history have built structures with an eye toward the sky to capture as much radiant heat as possible. But it wasn’t until the 1800s when the PV revolution really kicked into high gear. Here are some highlights:
- 1839: Edmund Becquerel discovered the photovoltaic effect in France when he noticed the voltage between two metal electrodes in an electrically-conductive liquid increased when exposed to sunlight.
- 1873: Willoughby Smith discovered that the element Selenium has photoconductive properties.
- 1876:William Grylls Adams and Richard Evans Day created a selenium solar cell that produced electricity when exposed to light, proving that solid matter is capable of transforming light into electricity (less than 1% efficient).
- 1932: The photovoltaic effect of cadmium sulfide was discovered by Audobert and Stora.
- 1954: Bell Labs developed the first silicon photovoltaic cell capable of producing usable electricity, with an initial solar panel efficiency rate of 4%.
- 1958: Hoffman Electronics created PV cells with 9% efficiency.
- 1959: Hoffman Electronics followed up with 10% efficient cells.
- 1960: Solar panel efficiency hit 14% by Hoffman Electronics.
- 1985: A 20% efficiency level was reached for silicon solar cells.
- 1994: The National Renewable Energy Laboratory (NREL) created a solar cell that exceeded 30% conversion efficiency.
- 1999: Spectorlab, Inc. and NREL developed a solar cell that was 32.3% efficient. (Three layers of photovoltaic material were combined into a single solar cell to achieve this level.)
- 2012: Solar Frontier achieved 17.8% efficiency on thin-film PV.
- 2015:SunPower recorded 22.8% efficiency for high-rate production commercial photovoltaic modules.
- 2016: NREL and the Swiss Center for Electronics and Microtechnology (CSEM) recorded 29.8% efficiency on a lab cell.
- 2016: SunPower broke their previous world record for the highest-efficiency (24.1%) commercial solar panel using silicon cells.
- 2019: EnergySage ranks SunPower #1 among the top 5 companies who produce the most efficient solar panels.
The major take-away from this quick skip through time is that the history of solar panel efficiency shows impressive advances at first, but is now running up against the barriers of physics. Higher efficiencies require technologies that are harder to produce at scale at an agreeable cost. In other words, don’t expect any major jumps in efficiency for commercially-available solar panels any time soon.
In fact, depending on which physicist you talk to, there is also a very real theoretical limit to how much efficiency is possible for commercially-viable modules. Most authorities put that upper cap at about 30%. For some perspective, the average commercial solar panels manufactured today (with a few exceptions) hit a range of 17% to 20%.
Bottom line: While the future of solar technology is bright, advances will likely be modest for high-efficiency commercial solar panels.
But records are made to be broken, right?
Creating high-efficiency commercial solar panels that break records is a worthy goal. However, when you see claims of a new efficiency milestone being reached, be warned the results are often confined to the laboratory with no practical real-world applications on the horizon. The maximum efficiency number noted earlier is for something called “single junction cells.” These are your standard, mass-produced (and relatively-inexpensive) solar cells. When you see efficiency numbers climbing into the high 20s or beyond, be sure to read carefully. It usually means there’s some fancy tech going on that won’t likely hit the commercial market any time soon (if ever).
There are some other processes that can produce greater solar efficiency rates that do offer real commercial potential. For example, layers of different materials can be sandwiched into a single cell to capture different light spectrums, resulting in greater efficiency (more light is captured). Another approach uses lenses to concentrate sunlight onto small, highly-efficient cells (more sunlight goes into the cell, so more energy is produced). These applications can vastly improve solar panel efficiency, but likely at a considerably higher cost. The added expense may be worth it if you’re trying to power an orbiting satellite—where size, weight and amount of power generated are key. For commercial solar power systems, however, they simply aren’t practical.
Comparing efficiency ratings
Now that you have a better understanding of what you’re looking at when you see a solar panel efficiency rating, here are a few other things to think about:
- Not all solar cells are made the same. Seems obvious, but it’s worth noting. Take the industry-dominant silicon solar cell. Is it mono- or polycrystalline silicon? Monocrystalline silicon is purer and produces a more efficient solar panel; however, it is also more expensive to produce. For an interesting overview of the pros and cons of different types of solar panels, check out this article from Energy Informative.
- Not all solar equipment providers readily offer up the efficiency of the panels they sell. It could be the company doesn’t want to provide the information if it doesn’t help sales. Or, they could actually believe that efficiency is not important. Having this information is required by law, so don’t hesitate to ask.
So how important are efficiency ratings?
Are efficiency ratings important if the differences are within a few percentage points? That depends who you ask.
- Some argue that as solar cell costs go down and efficiency improves, it becomes less important because mounting hardware, inverters and other components of a solar power system have a bigger impact on the overall install.
- Others counter that solar panel efficiency is important because more efficient solar panels can produce more power in the same amount of space. This is great for constrained areas or if you just want to get maximum output. It could also mean fewer solar modules to generate the energy you need, which can lead to lower installation costs and less preventative maintenance down the line.
When you realize that commercial solar is a long-term commitment, it would be short-sighted to FOCUS solely on upfront costs. Considering solar panel efficiency ratings, as well as other aspects of your investment that could lead to the most savings over the life of your system, is not only important—it’s the Smart thing to do
What Is Solar Panel Efficiency? Definition and Importance
David Kuchta, Ph.D. has 10 years of experience in gardening and has read widely in environmental history and the energy transition. An environmental activist since the 1970s, he is also a historian, author, gardener, and educator.
In a single hour, the sun provides enough energy to fuel human civilization for an entire year. Solar panels can only capture at most a quarter of the sun’s energy hitting them and convert it into electricity—a great improvement since the first photovoltaic cell was created in 1839—but research is ongoing to increase the efficiency of solar electricity and accelerate the transition to clean, renewable power.
There are many factors that go into creating an efficient solar panel, so knowing what to look for can help you save money on an installation and help you maintain their efficiency over time. Keep in mind, however, that the actual hardware in a solar system is only about one-third (35%) of the total cost of a rooftop solar system. The rest is “soft costs” such as labor, permitting, and design. So while solar panel efficiency is important, it’s only one element in a larger package.
Why Efficiency Matters
If you have unlimited space and are ground-mounting solar panels in a field or empty lot, efficiency matters less than if you are installing them on a roof, where getting the most out of limited space is important. Higher efficiency reduces the overall cost of a solar system and decreases the time it takes solar owners to recoup their installation costs. The environmental impact of producing solar panels is also reduced, as panels with higher efficiency can more quickly repay the energy used to produce the panels in the first place, and fewer, more efficient, panels need to be produced to generate the same amount of electricity.
Which Factors Determine Solar Panel Efficiency?
Solar cells convert photons (packets of energy) from the sun into currents of electrons, measured in volts, thus the term photovoltaic (PV). PV cells commonly used in solar panels are made from silicon crystals, though other elements (such as selenium and germanium) also have photovoltaic properties. Finding the most efficient element or combination of elements in the right crystalline structure determines how efficient solar panels can be, but other factors are involved as well.
Untreated, 30% or more of the photons that strike a PV cell will be reflected back as light. Minimizing reflection involves coating and texturing PV cells to absorb rather than reflect light, which is why solar panels are dark in color.
The solar radiation that reaches Earth includes most of the electromagnetic spectrum, from X-rays to radio waves, with about half of that radiation coming in the Band from ultraviolet to infrared. As the wavelengths get shorter, the energy of the photons increases, which is why the color blue has more energy than red. Designing PV cells involves taking into account these different wavelengths to maximize the efficiency of generating electricity from photons with different wavelengths and different levels of energy.
Recombination is the opposite of generation. When photons from the sun are absorbed by a PV cell, the photons excite the electrons in the crystals and make them jump to a conductive material, generating a current of “free electrons” (electricity). But if an electron’s energy is weak, it recombines with the “hole” left behind by another electron and never leaves the silicon crystal. Instead, it releases heat or light rather than generating a current.
Recombination can be caused by defects or impurities in the crystal structure of the PV cell. Yet impurities in the crystal are necessary to move the electrons in a particular direction; otherwise, no current is created. The challenge is to reduce the level of recombination while maintaining an electric current.
Augusta, Maine receives roughly 4.8 sun-hours per day, slightly less than the 5.0 sun-hours per day received in Augusta, Georgia. Yet PV cells work better at lower temperatures, so panels on a rooftop in Augusta, Maine may be more efficient at producing electricity than those on a rooftop in Augusta, Georgia, even if their daily insolation is lower.
What Is Insolation?
Insolation is a measurement of the average solar radiation of an area over a period of time.
Solar panels are at their maximum efficiency in temperatures between 15°C (59°F) and 35°C (95°F), according to EnergySage, but the panels themselves can rise to 65°C (150°F). Panels will be labeled with a temperature coefficient, which is the rate at which they lose efficiency for every degree above 25°C (77°F ). A panel with a temperature coefficient of.0.50% will lose a half percent efficiency for every degree above 25°C.
How Are Solar Panels Tested for Efficiency?
Essentially, testing the efficiency of a solar panel means finding the ratio between the amount of electricity that the solar panel is able to produce and the amount of solar irradiance the panel is exposed to. Here is how that test is conducted:
Solar panels are tested at 25°C and exposed to 1,000 watts (or 1 kWh) per square meter of solar irradiance—what is known as standard test conditions (STC), then their electricity output is measured.
A panel’s power output rating (Pmax), measured in watts, is the maximum amount of power a solar panel is designed to produce under STC. A standard residential panel might have an output rating of 275-400 watts.
As an example: A 2-square meter panel under STC would be exposed to 2,000 watts. If it has a power output rating (Pmax) of 350 watts, it will have an efficiency rating of 17.50%.
To calculate a panel’s efficiency, then, divide the Pmax by the panel’s solar irradiance, then multiply by 100%. So, 350 / 2000 =.1750, and.1750 x 100 = 17.50%.
Tips for Maximizing Efficiency
The most efficient panels may not be the best use of your money. Consider the entire system cost for the panels (separate from “soft costs”). Given the panels’ efficiency, how many watts will they generate over the next 25 years (assuming standard test conditions)? How many watts do you need? Perhaps you are overbuilding, while a less efficient system will supply all your needs at lower cost.
Once you’ve installed a solar system, keep your panels clean. Regular rainfall will do the job, but if you live in a dry climate, use plain water (no soap, which can leave a film) twice a year to remove dust and dirt. Trim back branches if they’re over-hanging your roof, and remove any debris between the panels and your roof, since greater air circulation keeps your panels cooler. If needed, get a solar easement to remove shade from neighboring obstructions.
Software that comes with a solar system will monitor its output in kilowatt-hours (kWh). If you find output declining over time, all other conditions being equal, have your system tested. An ampmeter and multimeter are needed for these tests: Consult a professional, as you can damage your panels by doing the tests incorrectly.
Solar’s Future Is Bright
In June 2021, the maximum efficiency of a solar PV panel on the market was 22.6%, while a number of other manufacturers had cells over 20%. That’s why research is underway to create more efficient combinations of materials that can be commercially viable. Perovskites or organic PV cells may reach commercialization soon, while more inventive methods such as artificial photosynthesis show promise, even if they are still at an early stage of development. Research in the lab has produced PV cells with efficiencies nearing 50%, but bringing that research to market is key to the future of solar technology.
Solar panels designed for home use are typically 15% to 18% efficient. Anything between 18% and 25% would be considered premium.
Although reports over the years have claimed that solar panels have reached 100% efficiency in testing, no such solar panel is commercially available. Some of the most efficient solar panels currently on the market are just above 20% efficient.
The efficiency of solar panels seems low because not all the light that hits the panel can be processed as energy due to imperfect glass, lenses, and reflectors; the temperature of the solar panel; and the fact that much of it is not direct sunlight but diffused.
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