I was wondering how to start our blog, but the obvious solution is answers to questions we get every day about PV system performance.

There common one is: The hotter the day, the more the output right? And very common: I have looked at my inverter and it is putting out 850 Watts – is the system broken? Well, no…

The thing with a solar PV system is that whilst you do get a kilowatt of PV on your roof, the instantaneous output will vary, and it does so because of various factors:

• It could just be cloudy – this is pretty obvious;
• The atmosphere could be very clear, or not so clear, not as obvious;
• The amount of incoming solar radiation varies with season and weather, reasonably obvious;
• A most important thing though is that the output of PV is strongly influenced by temperature. Not generally known!

PV and temperature

The one thing to note here is that all PV modules are rated at their nominal output (say its a 175W module) at a certain temperature, 20 degrees C, or Nominal Operating Cell Temperature (NOCT), and at a certain level of solar radiation.

The problem for us is that the temperature and level of incoming radiation changes all the time, so when we design a system we have to allow for this variation in radiation, and we deal with the variation in temperature by using average temperatures for each month.

If the cell temperature it gets cooler than NOCT, the cells will actually perform better, and as an illustration, the other day I looked at my 1002 Watt nominal system and found that it was producing 1047 Watts. I got a bit excited, but my excitement was short lived, because later in the day the cells inside the modules heated up and they started to reduce in output.

I went out later and saw that the system was producing at about 850W. So when it hits summer, people sometimes get a bit of a shock when the system is not instantaneously producing a huge deal more wattage than in spring.

The upside is of course that there are more hours of daylight in the summer,and the incoming solar radiation is still more intense, so even after the negative effect of the heating of the array, you still end up with more over the course of the day.

The best time to see this effect is in the spring, when radiation is high, the temperature is cooler, and grid connect systems everywhere seem to tick over quite nicely. I have noticed that a light wind keeps the temperature lower, and the performance higher.

Overall, the output of a system is equal to Watts x Time, and the output is measured in kilowatt hours (kWh, or units on your Synergy bill).

Its just that the Watts will vary constantly, so don’t look at the Watts, look at your daily output – that is the true measure of the performance of your system.

RG.

If you have any questions about this article, or would like another subject explored in the next article, please send me an email to info@renewablelogic.com.au