OverSizing a Solar inverter

Have you received solar quotes with a 5kW inverter and 6.6kW of panels, and wondered why?

The answer is all about the real world as against the laboratory.
The accepted rule is to take off 25% from the rated value of the panel.
The Goverment subsidy is entirely based on that rule.
So 6.6kW of solar panels, take off 25% = 4.95kW. That's why you get a 5kW inverter

The subsidy is paid on the full 6.6kW of panels.
There ARE times of the year when the sunshine is so strong that the solar panels don't lose 25%. They might only lose 20% on those special days. In this case, you won't see the benefit of that extra production because the inverter can only process 5kW (if its a 5kW inverter). The panels will only produce what the inverter needs.

Oversizing when you connect a battery
This is where things get quite interesting, because you can connect even more panels when you connect a battery.
The 5kW Huawei inverters allow 9.62kW of 370W panels, 8kW with Goodwe, Sungrow and Growatt, but strangely only 7.5kW allowed with Fronius.
The subsidy is also paid on those extra 8 or 6 panels respectively and you can get whatever the subsidy is, years later as long as everything else meets the approval criteria, you don't need to install the extra panels straight away.

One of the crazier decisions by the Clean Energy Council, alongside rooftop isolators and so many switchboard, inverter, battery warning stickers that no-one ever reads them...is allowing this extra subsidy when you connect an AC coupled battery.
It make sense with DC coupled, where the extra panel power is fed by the hybrid inverter directly into charging the battery.
AC coupling, or connecting a Tesla Powerwall 2 as an example, happens AFTER the solar has done all it's DC to AC conversion and as the inverter is limited to 5kW that extra panel production does very little good indeed. It might extend the 5kW production window for a bit longer in the day, but that is all.

If that's all Greek to you, and it may be, then there's no simple way of explaining it. My advice is to buy a Huawei, Fronius or the new SolarEdge three phase hybrid. That way you will get the most benefit and a guarantee of subsidy on the extra panels.

This review was written by Andrew MacKeith, Solar4Ever service manager since 2011.
Most recent update April 2021