The effect of heat on solar power output

The first really hot day of summer we always get several phone calls from customers who are eagerly watching their PV monitors, expecting to see the highest output yet - and are disappointed to find that actually their array is producing a fair bit less than normal.

If that's you, don't worry. It's not a problem with your inverter, or a dodgy panel. It's just that unfortunately panels become quite a lot less efficient as the cells get warmer. On a really hot day, particularly if there is no breeze to cool them down, panels can easily reach temperatures of 60 or 70 degrees - too hot to touch.

The power output of panels is measured under Standard Test Conditions, which are a cell temperature of 25 degrees C, an irradiance of 1,000 W/m2, and an 'Air Mass' of 1.5. The 'Air Mass' is a measure of how much atmosphere the sunlight passes through before it hits the panel. If the sun was directly overhead (and the panel was at sea level), the Air Mass would be 1. By the time the sun is low on the horizon in the evening, the Air Mass can be as high as 4 or 5. But a value of 1.5 isn't unreasonable for the height of summer in the UK - it's what you would get with the sun being about 45 degrees above the horizon. The irradiance value of 1,000 W/m2 is also a reasonable approximation.

The cell temperature however is normally far above the 25 degrees of 'standard test conditions' on a summer's day. With each degree increase in temperature, a panel will lose around 0.4% of its power. So if the cell temperature reaches 65 degrees, the actual power of a 250W panel becomes: 

250 x (1-0.004)^(65-25)

= 213W

This is about a 15% drop in power compared to what you might expect if you go by the standard test condition values.

Take a look at the power output on a cold, clear spring morning however, and you should see a considerably more pleasing figure!