Be Careful When Calculating Solar Panel Efficiency

Crunching the numbers on your solar panels’ performance is fun!

One from the mailbag today that is bound to please fellow solar nerds:

From Andrew in Cairns who has just had his system installed:

Finn,

I Finally got my 5KW system installed.   I have a question for you if I may:

If power output is calculated as:

Area of solar panels x solar panel efficiency x daily solar radiation x efficiency (losses in the inverter, wiring, and solar panel temperature coefficient)

Then, on a bright sunny day up here in Cairns, FNQ, I would expect to generate:

30m² x 15% x 6kWh/m²/day x 90% = 24.3kWh

Right?

Can I use the figures quoted daily by the Bureau of Meteorology.  Do you know?  Yesterday, they quoted 6.7kWh/m²at the weather station just up the road, but I only generated 20.0kWh which suggests that my efficiency is more like 65% than 90% (30m² x 15% x 6.7kWh/m² x 66.3% = 20.0kWh).

Any thoughts?

Regards,

Andrew

Here’s my reply:

You are on the right track – but I think you’ve made 2 subtle but common mistakes

Mistake #1 Using Ambient temperature instead of panel temperature in the Temperature Coefficient calculation.

It is all explained here:

https://www.solarquotes.com.au/blog/how-to-read-a-solar-panel-specification-part-1-power-temperature-specs/

To summarise the nerdtastic post above: If it is 40 degrees outside then your panels are probably sitting at 65°C so you have to multiply your temp coeff (which is usually about -0.5%/°C ) by the temperature of the panels above 25°C:

= 0.5 x (65°C-25°C )  = 20% losses, assuming it was a 40°C day.

Mistake #2 Underestimating the other system losses

You have estimated the system losses including temperature losses as 0nly 10%. They are likely to be closer to 20%, as described here:

https://www.solarquotes.com.au/blog/5-reasons-your-solar-system-power-is-lower-than-you-expected/

If you have a temperature loss of  20% and “other system losses” at 5%, then the calculated output (using the BOM data which, yes,  is a good source) is:

30m² x 15% x 6.7kWh/m² x 80% x 90% = 21.7kWh

Which is fairly close to your recorded power of 20kWh. The other thing that may affect your power output is the angle and direction of your panels. If they are not at the perfect angle (17° from horizontal) and perfectly north facing you will need to factor that in using these tables or similar:

CEC-guidelines

Having said all that:  I wouldn’t make a judgement on your solar panels’ performance without at least 1 month of data, as this will be a lot more statistically valid, then you can use the method described here to determine if your panels are performing to their specification or not. Here’s how to use a free online tool do just that:

https://www.solarquotes.com.au/blog/how-to-work-out-if-your-solar-system-is-performing-properly/