If you’ve done any kind of research into solar panels and photovoltaic technology, you’ve heard the term “efficiency” thrown around a few times. But what is efficiency and why does it matter when you are looking to invest in solar panels?
There are two main types of efficiency when it comes to solar panels. They are theoretical efficiency and practical efficiency. Theoretical efficiency is what solar panels have the potential to do under the most perfect of conditions and with the maximum concentration of the sun upon them. (1)
Unfortunately, it is unlikely that this theoretical series of conditions will occur. One of the main reasons is that theoretical efficiency is based on solar panels getting the maximum amount of sunlight, without any disruption from the atmosphere. It also ignores other factors that reduce the efficiency such as temperature, amount of photons that are present in sunlight, angles, and more.
These conditions are what determine practical efficiency. Practical efficiency is the actual percentage the solar panel can produce based on all external factors and natural disruptions in light distribution. (2)
Efficiency determines the actual amount of power you can get from a solar panel. Having more efficient panels reduces the number you have to buy, as well as the space you have to use up.
For people that have plenty of space on their roof or for an array where direct sunlight is available throughout the day, efficiency may not matter as much. Since efficiency is often associated with cost, many people end up paying the same or a little less for less efficient panels to get the same wattage.
However, if you have limited space, then having more efficient solar panels is a must. For residential homes, there is about a difference of 10% between the most common kinds of solar panels.
However, there are solar panels out there that are much too hard to build and expensive for residential use that easily double efficiency. These are used where space is limited, such as on space stations. (3)
As you may know, there are several types of solar panels and each has its pros and cons, including efficiency. For example, monocrystalline silicon panels have a higher efficiency than polycrystalline silicon panels.
Typically, any electronic device, including solar panels, operates at a higher efficiency when it is colder. This has to do with the resistance in circuits increasing with temperature.
Unfortunately, solar panels have to be placed in sunlight to generate power, and energy absorbed by the panels that isn’t converted into electricity is lost as heat, lowering the efficiency of all solar panels.
Direct sunlight is one of the most important aspects of a solar panel’s efficiency. If you live in an area that is cloudy most of the year, or your panels are installed under a tree or some other shade, you can expect to see lower returns and efficiency than what the panel is advertised at. (4)
Solar panel efficiency is also dependent on other parts of your solar system. If your solar panels are installed far away from things like your inverter or a battery bank, you lose some of the power generated over that distance because of resistance.
Because the Earth is round, you have to account for the curvature by angling your solar panels. In the northern hemisphere, people typically angle their panels southward. This positions the solar panels so they can operate at maximum efficiency and have the longest time in the sun possible.
Certain types of panels are built to account for the reflection of sunlight off the panels. Some do this by absorbing a broader range of the light spectrum so less light is reflected off the panel.
Others have a second layer underneath the PV cells in a solar panel that purposely reflects light back at the PV cells to get a second chance at absorption.
Solar panels get dirty over time because of tiny amounts of dust left behind by rain after it dries. This effectively puts a small amount of shade directly over the PV cells of a solar panel, reducing the amount of light they can absorb and thereby reducing the efficiency of the panel.
For residential use, there is an average efficiency of 16%. Depending on the panels you choose, you can expect to see somewhere between 7% and 22% from solar panels under ideal conditions.
In 2020, median efficiencies for solar panels installed on residential systems were around 19.8%, which was an increase from 13.4% in 2002. (5)
As solar panels become easier to mass produce, they also become cheaper to make. This is part of why the average efficiency has gone up. This means more people can afford higher-efficiency systems as solar panels get better over time.
For residential use, monocrystalline panels are the best as far as efficiency. However, they are expensive, which leads people to often choose polycrystalline instead, at a loss of efficiency. Then, for those without space for traditional solar panels, there are also thin-film, which are the lowest in efficiency, but are lightweight and can be molded to unique shapes like an RV roof or heavily slanted house.
When it comes to non-residential uses, the III-V solar panels are the best. While these can be up to 50% efficient, they are impractical for residential use due to the cost.
- Belghachi, A. (2015, October 22). Theoretical Calculation of the Efficiency Limit for Solar Cells. IntechOpen. https://www.intechopen.com/chapters/47490
- Luque, A., & Martí, A. (2005). Theoretical Limits of Photovoltaic Conversion. Handbook of Photovoltaic Science and Engineering, 113–151. https://onlinelibrary.wiley.com/doi/abs/10.1002/0470014008.ch4
- Best Research-Cell Efficiency Chart. (n.d.). Photovoltaic Research | NREL. https://www.nrel.gov/pv/cell-efficiency.html
- Venkateswari, R., & Sreejith, S. (2019). Factors influencing the efficiency of photovoltaic system. Renewable and Sustainable Energy Reviews, 101, 376–394. https://www.sciencedirect.com/science/article/abs/pii/S1364032118307585
- Barbose, G. (2021, September). Tracking the Sun. Pricing and Design Trends for Distributed Photovoltaic System in the United States. https://emp.lbl.gov/sites/default/files/2_tracking_the_sun_2021_report.pdf