Be Careful When Calculating Solar Panel Efficiency

Solar Panels

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:


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


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?



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:

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:

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:


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:


About Finn Peacock

I'm a Chartered Electrical Engineer, Solar and Energy Efficiency nut, dad, and founder of My last "real job" was working for the CSIRO in their renewable energy division.


  1. Finn
    more confused than ever with the extra quotes but narrowed down to 2 suppliers and panels
    5kw system Solarworld Sunmodule SW 20 poly $12k with a SMA inverter upgrade
    5kw system with MArk -5.04 KW BP SUN OASIS system (21 240w Panels)with a Samil Power Inverter : $8,999 – what am I getting for extra $3k that I wont wiht the bp system ?

    • You are paying for a more expensive (German made) inverter and panels.

      • ….and wouldn’t go near it!!
        The days of paying extra for brand-names/reputations are long gone.

        If you WANT to pay over the odds pay extra for the better warranty after careful comparison , including the reputation and likely longevity of who-ever is liable for warranty-matters.
        Oh….and make a point of making the ‘warranty-provider’ PERSONALLY responsible for any issues…whether the company folds up or not….even 25 years down the track.

        I wrote up my own warranty ‘terms and conditions’ (copy available if you want it), and so long as it’s reasonable a reputable installer acting in good faith won’t balk at the idea.

  2. Hi Finn,
    I have received quotes for 5kw system and some include the Aurora PVI 5000 and others include the SMA SB5000Tl-21.
    I am considering adding 2 extra panels to make 5.5 kw of panels to counter some of the loss in real world operation.
    Which of these two inverters would be best to handle having extra panels ?

    • Your CEC accredited designer will advise you – there are stricter rules in place since Feb 2013 – on oversizing the panels to the inverter. A good designer will be all over these rules and will look up each inverter tech spec and calculate the max panel array size you can use with each inverter.

      If he can’t, then he’s not qualified to design your system!

      • Thanks for the advice.
        Is the anywhere (site) I can search for myself to confirm the current rules ?
        I am in Brisbane and connect to Energex network.

Speak Your Mind

Please keep the SolarQuotes blog constructive and useful with these 4 rules:

1) Real names are preferred - you should be happy to put your name to your comments.
2) Put down your weapons.
3) Assume positive intention.
4) If you are in the solar industry - try to get to the truth, not the sale.


%d bloggers like this: