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.

Best Solar Output So Far - Today Breaks 20kWh Barrier

With a reasonable breeze today and slightly lower temperatures we have broken the 20kWh for the first time and got a new record for the electricity generated. By the time the sun set today we had generated 20.6kWh which is a full 1kWh more than the previous record set at the beginning of April. As per my blog post yesterday about the effect of temperature on solar panel electricity generation the cooler day as well as the breeze look to have contributed.

I think at 20kWh we are now getting very close to the maximum electricity that can be generated in the UK from 3kWp solar panels. I'm hoping we can hit the 21kWh target which would mean each 1kWp of panels had generated 7kWh which seems to be the maximum possible for us.

Maximum solar panel electricity output in UK

How does Temperature Affect Electricity Generated by Solar PV Panels?

How much variation does temperature cause for generating electricity from Solar PV Panels?

With one of the hottest days in the UK so far this year it is interesting to compare the output for a hot sunny day with a cooler sunny day earlier in the year.

The best generating day in April was 19kWh for a clear sunny day with temperature of 12C. Yesterday generated 17kWh for a clear sunny day with a temperature of 27C despite the longer generating time with the lengthening days.

Solar panel output with different temperatures.

As you can see the output was for longer on the blue day (24 May) but the peak was lower than on the green day (2 April) as the temperature was far lower. The equation is the temperature coefficient which for our panels is 0.5% meaning output drops by 0.5% per degree over 25C. So for panels at 35C which is 10 degrees higher the output would be 5% lower.

As another comparison the maximum peak generation was around 2.3kW yesterday but for cooler days this has been as high as 2.5kW and for cloudy days with sunny spells it has peaked as high as 3kW showing the effect that temperature has on solar power generation.

One company was selling a hybrid solar panel that included water heating as well as electricity generation. As a method for cooling the solar panels to improve their efficiency it seems like an effective method based on this data as the water will extract the heat that can then be used for hot water in the house.

Solar panel generation in high temperatures