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As the thermometer rises , solar panels work less well

When solar panels are tested in the factory under what are called Standard Test Conditions (STC), conditions that are determined by the IEC in Switzerland, and the same for every manufacturer, the cells inside the panel must be 25°C.


That's not the ambient (air) temperature, that's the temperature of the cells, which are typically 20-30 degrees C hotter than the air temperature. We even emailed the IEC to ask them about this and they replied that the test is only that the cells are 25 degrees, they do not care, and could not say what the air temperature should be.


Somewhat bizarely, if you look at the solar panel datasheets, they show that under 'Normal' conditions the ambient temperature is 20°C. So you have no mention of ambient temperature in one test, but you do in another. Weird, apples and oranges comparison.


Why is that important?

The way that most people see it is that solar panels perform less well only when it's HOT outside, where 25° C is considered 'normal', and anything over 25°C will start to show losses, but in reality, the starting point is probably more like 5°C, and anything hotter outside than 5°C is going to mean the solar panel is making less power. We rang up Q.Cells technical support and asked them what the ambient temperature was in their factory test facility, and the helpful guy said 'Don't know, but yes, pretty cold, 5° sounds about right to me". So lets go with that.


Temperature Coefficient at PMax

If you look at an old datasheet for a typical 275W polycrystalline panel you'll see that they show a loss of 0.4% per degree C of temperature rise.

So, if it's 30 °C outside, with a starting point of 5 °C then that's 25 x 0.4 = 10% loss of power


Todays monocrystalline panels are showing 0.36% losses, and some, shingled types in particular are down around 0.34%. If we split the difference and make it 0.35%, then again, if it's 30 °C outside, then 25 x 0.35 = 8.75% loss of power.


Therefore it's not unrealistic to expect that over a year 6.6kW of standard polys will make 1.25% less power than some of the latest half cells and shingled monos because they handle temperature increases a little better.


There are also going to be other minor gains with these new types of panels because they handle low light, shading slightly better too.


Let's say that you get 2% more output from the new panels.


For a typical Perth unshaded home with 6.6kW of panels split over North and West, the output is going to be 30kWh a day average. 2% extra gives you an extra 0.6 kWh per day.

However, who uses even 20kWh, let alone 30kWh of their solar power between 8am and 5pm? Almost nobody, so the extra 0.5kWh per day, is going to be exported back to Synergy for a 3.5 cent daily payment worth $13 a year. Not very exciting.


The point of this rather long drawn out piece is to explain that losses, be it from heat, passing clouds, an inconsiderate bird, resistance in cables, or whatever, are just part of the overall solar experience and you really shouldn't lose any sleep over trying to fix them.


Look at it this way if it helps you worry less about all this stuff

Your solar wakes up at dawn and shuts down at dusk, but the lack of powerful solar irradiation means that any time outside of 8am and 4pm isn't doing very much at all.


So in those eight hours, you have a 5kW limited inverter, oversized with 6.6kW of panels then your system should make 40kWh per day (8 x 5 = 40). On a few days a year it might even do that, especially with longer days in Summer, but on average across the year it's going to make 30kWh, and no amount of additional money you throw at it is going to change that to any meaningful degree.