All solar panels are not made equally, so how do you decide on which solar panel to get? One way is to look at the efficiency of the solar panel. ANTON WILLEMSE explains how this can be calculated and shares a few thoughts on solar panel efficiency in general.
In order to calculate the approximate efficiency of a solar panel as a percentage, there is a simple equation that can be used: panel power (measured in kW) divided by the length and width of the panel (measured in metres), and then multiplied by 100 percent.
From the get-go, it is important to understand that the efficiency of a solar panel is a matter of surface area, not power. A 10 percent efficient 100W panel and a 20 percent efficient 100W panel will produce exactly the same amount of power. However, you should expect the 20 percent efficient solar panel to be half the total size of the 10 percent version.
To calculate the efficiency of the panel, we need to consider that the power the sun provides for the panel and compare that to the electricity that is produced. If we had a panel that was able to convert all of the light that shines on the panel to electricity, we would have 100 percent efficiency. This, however, is not possible.
In theory, the maximum efficiency of a solar cell made of ideal material is 33.7 percent. This is known as the Shockley-Quiesser limit and is a consequence of the laws of physics and how solar cells absorb power. Multi-layered solar cells can exceed this limit in laboratory conditions but are significantly harder to manufacture.
They are also considerably more expensive, so are typically only used for satellites and other hi-tech systems where space is extremely limited.
For silicon solar cells (the kind used in almost every panel you can buy) the theoretical limit is about 32 percent as silicon, though close, is not a perfect solar cell material. Mass-market solar cells will always experience some power loss though and can only achieve a maximum of up to 25 percent in ideal conditions. The efficiency of a panel as a whole will always be lower than its component cells due to the frame, reflective metal contacts, and gaps between the cells.
To compare the power from the sun to the panel’s electricity output, we first need to know what the sun’s power actually is. At midday near the equator, just over 1kW (1 000W) of sunlight reaches every square metre (m2) of the ground. Away from the equator and depending on the season, weather conditions and time of day this will be less. However, the 1kW/m2 value is used when testing panels to give the power rating at which they are sold. This is part of the Standard Test Conditions (STC) used by the solar power industry and all panels are rated in the same way.
Imagine a panel with an area of 1m2. If it produced 1kW of electricity at noon on the equator we could say that it was 100 percent efficient as it would receive 1kW of sunlight and turn that into the same amount of electricity. If a panel of the same size instead produced 200W of power, then its efficiency is 200W/1000W x 100 = 20 percent. If we express this in kW the equation becomes even simpler, as 0.2kW/1kW = 0.2. The sun’s power can be ignored in the calculation and the ratio is multiplied by 100 to give an answer in percentage.
To perform this calculation for any solar panel that isn’t 1m2, we need to know the surface area of the panel. If a panel is half as big and produces the same power it is twice as efficient, and vice versa. The surface area can easily be calculated from the panel’s dimensions by multiplying the width by the length. Remember to convert the dimensions into metres first as areas do not convert in the same way as lengths (i.e. 1m2 is not 1 000mm2, it is 1 000 000m2).
Finally, to calculate the maximum efficiency of the solar panel we need to divide the ratio of panel power to sun power by the area of the panel in square meters, then multiply it by 100 to get a percentage. Make sure the measurement units of all the values are correct or you will end up with very strange results.
Take, for example, a 300W rigid-frame panel with monocrystalline silicon cells. Its power is 300W or 0.3kW, it is 1.64m long and 0.99m wide. Looking at the math from earlier, the efficiency would be 18.1%.
This is the approximate efficiency of the panel as a whole, so as mentioned we would expect it to be lower than the efficiency of the cells because of the frame and gaps between the solar cells (increasing the area) and normal losses as the electricity travels through the panel and wires. For a higher-efficiency panel of the same area, the power would be higher than 300W. For a higher efficiency panel of the same power, the area would be smaller.
