Oversizing a Solar inverter - Updated January 2025
Oversizing an inverter means connecting more panel power than the inverter's rating.e.g. 6.66kW of panels on a 5kW inverter is 133.33% oversizing.
or 13.32kW of panels on a 10kW inverter, also 133.33% oversizing.
If you stay within 133.33% oversizing then you get paid STCs, the still generous Government subsidy on panels.
The rationale for this allowed oversizing is that solar conditions are not always ideal.
Early morning, late afternoon, or entire seasons like Winter, if you only had 5kW of panels on a 5kW inverter
your inverter wouldn't be able to pull down enough power from the panels to hit its 5kW limit.
When you connect DC connect a battery to your solar inverter then you can usually add even more panels.
The rules for claiming STCs change when you connect a battery to the inverter.
With a battery, you can connect as many panels as the manufacturer permits.
iStore for example allow 250% oversizing, so for instance, 25kW of panels on a 10kW iStore inverter with a battery.
Fronius only allow 150%, so just 15kW of panels on a 10kW Fronius inverter with a battery.
Sungrow allow 200% oversize on their single phase inverters and their 15-25kW three phase inverters.
Sungrow's 10kW three phase inverter is being increased to 200% oversizing when the new model comes out soon.
Sigenergy initially allowed 200% on single phase and 160% on three phase, but now it's 200% for all.
Plus, Growatt, Goodwe, Tesla, SolarEdge...they all allow significant oversizing with their hybrid battery inverters.
So why would you add more panels?
Mostly it's about charging the battery.
If for example, there's 10kW of panels on the roof connected to a 5kW inverter and battery, then the inverter,
in theory at least, could pull down 5kW of panel power to convert to AC for loads and export and then pull another
5kW of panel power, leaving it as DC, unconverted, to store in the battery.
Is there such a thing as too much oversizing?
Solar panels, at least at the moment, are so cheap, there's very little reason not to oversize as much as you are allowed.
However the reality is that once the battery is full, the huge oversizing does very little.
I can't see any difference at all between sites with 200% oversizing and those with 240% or more,
apart from the battery is usually charged a bit earlier the more panel power there is.
Can I oversize with an AC Coupled battery?
Yes, you can. The rules recently changed to allow this, but in my opinion, it's almost pointless.
Almost.
An AC Coupled battery is connected AFTER the solar inverter has converted panel power to AC for the home/export.
If the solar inverter is a 5kW inverter then that's all it can do. Convert panel power to 5kW AC.
It can't pull down extra panel power to charge the AC Coupled battery in the way a hybrid inverter with a DC coupled battery can.
So what would be the point of adding extra panels with an AC Coupled battery, and the answer is more production for longer.
Solar production, on a good day, is a bell curve with it starting low, rising to peak around midday and then falling in the afternoon.
If you have more panels on the roof then that bell curve is going to achieve a bit more production through the day.
The bell curve will be shallower with production closer to maximum for longer. That's good, especially in Winter.
It's not going to be a huge difference, but it will help a little, which is why I said 'almost'.
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Written by Andrew MacKeith, Solar4Ever Service Manager since 2011.
