Why is The Peak Output of Solar Panels So Low? Why do my solar panels generate less than their rating?

When we got our Solar Panels I expected that they would generate electricity at a constant rate and that would be the same whenever the sun was shining. The reality is rather different. The output of your solar panels is affected by the length of the day, the temperature, the height of the sun in the sky and the cloud cover.

If you are finding your solar panels have a low Peak Output, much lower than the nominal kWp output that they are rated at then the most likely cause is temperature. PV Solar panels are less efficient the hotter they get and their rated output is measured at 25C. It is very noticeable that on a cloudy day the output can peak at very high levels, close to the maximum rating of the panels.

For some mixed sunny/cloudy days in April we got 3kW peak output from our 3kWp solar panels when the sun came out from behind a cloud. For consistently sunny days during May the daily peak has been far lower at around 2.3kW.



Solar panel low peak output on hot days

You can see the difference in the graph above from April with the peak on the cloudy then sunny day reaching 3kW but the completely sunny day warmed the solar panels so the peak was lower at only 2.5kW. In May the peak electricity output was even lower due to the heat at 2.3kW.