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Over Sizing a solar inverter.

Oversizing is putting more panel power on an inverter than its rated maximum.

For example 6.6kW of panels on a 5kVA inverter.

The reason why this works is because solar panels never make their rated output. Not even close. When the panels are flash tested in the factory it's at a frosty 5°C and with a really bright 1000W per square meter of irradiance. In the real word, its much warmer and less bright and the light is less directly aimed at the cells inside the panel.

Real life conditions result in 25-30% less output than the maximum...even when the sun is perfectly overhead.

So that's why it works. Oversizing pushes the output closer to the inverter upper limit for longer througout the day so you get more power without having to buy a larger and more expensive inverter.

The Clean Energy Regulator allows oversizing by 33.33%. Go beyond that and you lose the $170 or so rebate per panel. Recent changes have allowed oversizing by more than 33.33% as long as a battery is connected to the inverter, but the manufacturer of the inverter doesn't always allow that.

Undersizing an inverter is also possible, but again is limited to no more than 33.55 % of the inverter capcity. So for example a 5kVA inverter can have no less than 3.75kW of panels.




To many, it defies logic, but yes, it totally makes sense to add more panels to your inverter than it's rated maximum. Take any inverter maximum rated AC output and multiply by 1.3333, and that's the maximum amount of solar panel power you can install (and qualify for the solar rebate)
5kW inverter x 1.3333 = 6.66 kW of solar panels
3kW inverter x 1.3333 = 4kW of solar panels

In reality, even this level of over sizing is less than you could do, in theory, especially if your panels are installed on a West or East facing roof, but if you want the solar rebate (and you do), then 1.3333 times is the limit to over sizing.


Mostly losses from the angle of the sun and the light intensity but also cable run distances, dirt on the panels, conversion from DC to AC within the inverter and more, mean that most of the time a 5kW system is producing about 3kW per hour and never, ever hits 5kW. That's a theoretical lab result only, not real World. When we send you a quote we include a month to month break down of what your system will produce. Over sizing by up to 33.333% works.

Under Sizing
If you were paying attention to your TV a couple of years ago, you would have seen solar floggers promoting systems with 1.5kW of panels on a 5kW inverter (so you can expand later). Their business model was to get you in, and then charge you the absolute earth for the upgrade. The Clean Energy Council has put a stop to that. Now you can't over size an inverter by more than 33.33%, and you can't under size it by the same amount either.

Is a bigger solar installation, better?

A typical scenario is this.
Based on your power bills, your consumption might suggest that a 3kW system was the most suitable size for you,. It might cost an extra $1,000 to get a 5kW system, but why buy bigger than you need if all the extra 2kW of solar is going to do is be exported back to Synergy for a payment to you of just 7 cents a unit?

If we assume the extra 2kW of panels is installed on a North roof in Perth. With Tier 1 quality solar panels and a good inverter it's going to make 9.6 kWh (units) of power a day. Multiply that by 7 cents paid to you per kWh by Synergy and that works out to 9.6 x 365 x $0.07 = $245 a year. You paid $1,000 more for those 2kW, so it's going to take four years to pay off that extra $1,000 investment. After that, you are getting free money and, AND this is the real point, by 2020 or sooner, it's pretty much guaranteed that solar battery storage will have dropped in price far enough to make it affordable and sensible. All that extra power stops being exported for a 7 cents pittance and gets stored in batteries instead for use at night. Yes, bigger is better for you, future batteries or not, as long as you stay in your home for at least four years.