Apart from cost and aesthetics, the most important consideration when choosing a solar panel is its efficiency. Efficiency refers to the ability of a solar panel to convert sunlight into electricity. The higher the efficiency, the more sunlight your panel will be able to use.
Most solar panels have efficiency rates of around 15 percent. Low-efficiency panels might have efficiency rates of about 10 percent. But high-efficiency panels have rates approaching 20 percent or more.
Every year seems to bring more efficient solar panels. In 2015, SolarCity achieved a record efficiency of 22 percent. Just one week later, Panasonic broke that record with slightly more efficient panels. Last year, SunPower’s rooftop solar modules broke the 24 percent efficiency barrier. And earlier this year, Japanese solar company Kaneko set another new record with its 26.6-percent-efficient panels.
Kaneko’s high-efficiency panels are premised on a manufacturing technique that allows them to minimize band gaps – spaces where electrons cannot exist and sunlight is, therefore, wasted. The Kaneko engineers didn’t invent the technique, known as thin-film heterojunction optimization, but they improved upon it in order to attain their record-breaking efficiency.
In research settings, solar panel efficiencies have reached even higher. In 2014, researchers from University of New South Wales (UNSW) in Australia developed a panel with an efficiency rating of over 40 percent.
That same year, the Fraunhofer Institute for Solar Energy Systems (FISES), semiconductor and microelectronics manufacturer Soitec, and nanotechnology firm CEA-Leti collaborated to develop solar cells (but not solar panels) that were even more efficient. The French and German research team created cells that were 46 percent efficient.
The researchers from UNSW and from FISES both achieved their high efficiency rates by “splitting” the sunlight with a multijunction solar cell. In multijunction solar cells, four traditional (single-junction) cells are stacked on top of one another. Each single-junction cell captures light of a different bandwidth, enabling the cell to utilize more of the sun’s energy. Traditional single-junction solar cells have theoretical upper efficiency limits of about 36 to 44 percent. Multijunction cells, on the other hand, have theoretical efficiency limits of over 86 percent. Current efficiency rates could be improved even further if the light passed through five single-junction cells rather than just four.
Another cell type that can yield high efficiency is the passivated emitter and rear cell (PERC). PERCs are different from most other solar cells, which are known as aluminum back surface field (Al-BSF) cells. Unlike Al-BSF cells, PERCs are backed by an insulating layer that improves the reflectivity of the aluminum electrode. As a result, PERCs receive more light than Al-BSF cells under identical conditions. Today, Al-BSF cells account for about 80 percent of all solar cells on the market, but PERCs are becoming more common. Analysts expect that PERCs will be utilized in 60 percent of all solar panels by 2027.
The way in which the electric current is transferred from the silicon wafer within the solar cells also affects efficiency. The surface of the solar cell is lined with busbars (thick strips of metal) and fingers (thinner strips of metal) that link cells within a panel and transmit it into your home. But sunlight cannot reach the areas covered by the fingers and busbars. To increase panel efficiency, a growing number of manufacturers are finding ways to produce solar panels without busbars.
At the same time, another contingent of manufacturers is increasing the number of busbars to five, up from the standard three or four. By increasing the number of busbars, the cell receives less sunlight, but it also enjoys improved electrical transmission and, therefore, a higher efficiency rate.
Another high-efficiency design choice is increasing the amount of light that is reflected and trapped within the module. In addition to the PERCs revolution now underway, solar panel manufacturers are finding new ways to capture the reflections that occur inside the panels. For instance, many manufacturers are utilizing better antireflective coating on the panels’ faces, thereby allowing more light to pass into the solar cell.
The materials that are used to create a solar panel also affect efficiency rates. Monocrystalline panels tend to have lower rates of impurities and, therefore, higher efficiencies than polycrystalline solar panels.
Analysts predict that at the current rate of technological development, high-efficiency solar panels in 2027 will be an average of 5 percentage points more efficient than today’s high-efficiency panels. It doesn’t sound like a big improvement, but considering most of today’s high-efficiency panels are only about 21 percent efficient, a jump to 26 percent efficiency represents a 23 percent improvement over today’s panels. Future improvements in efficiency (for both high-efficiency and standard-efficiency panels) are important for making clean solar energy more affordable, since the more efficient solar panels are, the fewer of them you’ll need to power your home.
For the average homeowner, high-efficiency panels are a good investment if you’re installing them in a limited space, since more efficient panels tend to be smaller compared to a lower-efficiency panel of equal wattage. For example, if one 250-watt panel has an efficiency of 20 percent and another 250-watt panel has an efficiency of 15 percent, both will provide 250 watts under ideal conditions, but the panel with the 20 percent efficiency will be smaller. To get the most out of a small area, high-efficiency panels are the way to go.