Solar panel power drop.

[Lignafera]

Just over eleven years ago, I had a 16 panel 4 kw array installed on my home. The Inverter was put in the garage, a digital display shows hourly output, the highest is 3610 watts.
I have noticed for several years a power drop in the summer. Recently, blue sky, 38 degrees, the output was between 2.6 and 2.7 kwh. Does anybody know why this is happening?

 

[Spectric]

Have you cleaned the panels? Also these panels degrade over time and like everything get less efficient, maybe they are now less tolerant of higher temperatures.

 

[Lorenzl]

Maximum efficiency of solar panels is around 15 °C and 35 °C The laws of thermodynamics tell us that with increased heat comes decreased power output

 

[Spectric]

Has the temperature in Norfolk reached 35° 's in summer these days, but perhaps the upper limit for max efficiency has fallen due to degradation of the panels.

 

[Lignafera]

The panels are very clean after the heavy rain. Perhaps the panels are worn up like myself..

 

[Spectric]

Don't you get more wind in North Norfolk than hot sunshine, last time I was in Sherringham it was blowing a gale right of the sea.

 

[Lignafera]

I live 10 miles from the coast, and we do get a lot of hot sunshine.

 

[Richard_C]

It might 'only' be 35 degrees air temperature, but the panel itself will be much hotter. Excellent thread started by sideways earlier has good information about panel types and the low rate of degradation of output, I wonder if 10 years ago the panels were less long lasting.

 

[Pedronicus]

The panels are very clean after the heavy rain.

I wouldn't put money on that. I clean ours once or twice a year (depending on Sahara dust) and it is quite surprising how much dirt come off them even though they 'look clean'.

 

[Sideways]

Panels are more efficient at low temperatures. The hotter they are the less output you get for the same amount of solar radiation.
Manufacturers have the ability to tune the composition of the silicon cell to suit different environments. Solar cells for Australia where they may be subject to much higher emperatures than we have in the UK may have a different composition to get max output in that environment.
Solar efficiency it was recently mentioned is very good in Scandinavia in the winter months. Part of this comes from beautiful clear skies, but it will be helped by the low temperatures that allow panels to work at high efficiency.

This may be part of the reason behind what you are seeing. The up side is that your panels will be at the most efficient in winter when there is less sun. At least they will make the best use of what is available.
I imagine in mid summer sunshine, panels could quite easily be at 50 degrees.

Based on the typical performance guarantees for panels with the age of yours, you should expect the output to be 5 to 7 percent less than when they were first installed.