Solar Panel Efficiency Calculator
Convert panel wattage and physical size into an efficiency figure, and see how temperature changes that number in the field.
Results
What efficiency actually measures
Efficiency is the share of the sunlight landing on the panel that leaves it as electricity. Standard test conditions assume 1,000 W of irradiance per square metre, so a 400 W panel covering 1.95 m² converts about 20.5 % of what hits it.
The figure matters most when roof space is the constraint. If there is room for ten panels, a higher-efficiency module is the only way to raise the system total.
Why the field figure is lower
Cells lose roughly 0.3 to 0.4 % of their output per degree above 25 °C. On a still summer roof the cells sit some 20 °C above air temperature, which alone takes 7 % off the nameplate figure before soiling, wiring losses or inverter conversion are counted.
Common mistakes
Comparing panels on efficiency alone. A 21 % panel and a 19 % panel of the same wattage produce the same energy; the first simply takes less roof. Efficiency matters when space is limited, and price per watt matters when it is not.
Questions people ask
Do panels lose efficiency in the cold? The opposite. Output rises as cells cool, which is why a bright February day can produce more per hour than a hazy July afternoon.
How much does dirt cost? Typically 2 to 5 % a year in a temperate climate, far more near dust, agriculture or heavy pollen.