In order to determine whether your solar system is working properly, it’s necessary to measure its performance.
Based on the Clean Energy Council Guidelines, systems in each capital around Australia should perform within at least 90% of these daily kWh outputs per kW installed, when averaged out over the course of a year:

Adelaide 4.2 kWh
Brisbane 4.2 kWh
Canberra 4.3 kWh
Darwin 4.4 kWh
Hobart 3.5 kWh
Melbourne 3.6 kWh
Perth 4.4 kWh
Sydney 3.9 kWh

That means if you’ve got a 4kW system in Adelaide, you multiply 4.2 x 4 to work out your performance standard. Once you’ve had your system for at least a year, your inverter will be able to report your daily average. If you have any issues finding this information, your installer will be able to direct you.

That might seem straight forward enough, but for most of us who are skeptical about the performance of our solar panels, we want to know whether we’ve got a bad system fairly soon after installation, rather than having to wait for a whole year.

Those figures are averages across the year, which takes into account different seasons and different temperatures. But for when you want to check performance for a specific month of the year, for example, you’ll have to be more scrutinising. What if you installed your panels in winter, and know that they’re not performing at their peak? What if your system is west-facing, rather than the optimal north? All these factors come into play, and need to be considered when you’re accurately assessing the performance of your system.

The good news is that there’s a handy tool for this, called PVWatts, developed by then National Renewable Energy Laboratory in America. Go to rredc.nrel.gov/solar/calculators/PVWATTS/version1/ and type in your city location. Click the next arrow, and input the required figures based on your system.

DC Rating: size of your system in kW
Array tilt: if you don’t know, most Australian homes are between 18–22.
Array azimuth: north-facing is 0, east is 90, south is 180 and west is 270.

Then click ‘calculate’ and there you have a guide for what you should be seeing for every month of the year. If it’s been even less than a month, just divide your result by 30 and you’ll get a crude daily average.