Oversizing a Solar inverter - Updated August 2024
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.
Usually, single phase hybrid inverters allow a larger amount of oversizing than 3 phase inverters.
For example, the Sungrow 5kW single phase hybrid allows 12kW of panels or 240% oversizing,
whereas their 10kW three phase hybrid inverter allows 16kW of panels, or 160% oversizing.
Remember, this is only when you connect a battery to the inverter.
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.
Reality is usually different to theory.
If you have 10kW of panels on the roof the inverter will never be able to pull down 10kW of power.
There are always losses whether from cable resistance, the angle of the sun, intensity of the solar irradiation etc.
At best, 10kW of panels will deliver around 8.5kW of power.
Still, that's not bad. 5kW to house loads + export and 3.5kW to the battery.
Not all inverters are the same.
Most hybrid inverters can do this 'dual supply' function very well.
Not so with Fronius GEN24Plus inverters which can only do a combined AC + DC that equals the AC rating...
e.g. say we had a Sungrow 10kW inverter and 15kW of panels and a battery.
In good solar conditions we might see 13-14kW of solar power pulled down from the panels.
Some would go to charge the battery, the rest for AC conversion for loads.
Now take the same situation but with a 10kW Fronius GEN24Plus, 15kW of panels and battery.
It will never pull down more than 10kW of AC + DC.
No-one at Fronius can explain why they can't do 'dual charge' above the inverter AC rating.
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.
Oversizing by inverter brand model with a DC coupled battery.
Note: Whatever the maximum set by the manufacturer is, it still has to comply with AS4777.1 standard.
That currently limits the total voltage of a string of panels, open circuit at minimum temperature, to 600Vdc.
Therefore the choice of panel, and it's electrical specifications, are very relevant to oversizing.
Fronius...all models, maximum 150%
iStore, 250% for single and three phase 5kW and 6kW hybrid inverters. TBA for larger models due Q4 2024.
Sungrow single phase 5kW, 240%, 8kW and 10kW 200%. Three phase 160%, except for new 15-30kW models which are 200%.
Goodwe, single phase 170-200% depending on model, 150% for three phase.
SolarEdge single phase 300%, Three phase not available just yet.
Sigenergy single phase, 200%, three phase 160%
Enphase micro inverters are a bit harder to define oversizing but typically the usual 133%
Other brands, SAJ, Growatt, Solis, SolaX etc are typically, but not always, 200% single phase, 150% three phase.
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'.
If you want a quote for solar in Perth (South as far as Mandurah, North to Yanchep/Chittering) call us on...
or email
Written by Andrew MacKeith, Solar4Ever Service Manager since 2011.
