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 5kW inverter.

The Clean Energy Regulator allows you to claim STCs, which are the Government discount for solar panels as long as the inverter is not oversized by more than 33.3%, hence a 5kW inverter is fine with 6.6kW of panels.


Better still, if you buy a hybrid inverter and a battery then you can add even more panels, typically 7.5kW to 8kW of panels on a 5kW inverter and claim the extra STCs on those panels as well. If you don't buy the battery yet, then you can't install and claim STCs for the extra panels yet, just leave space on your roof for them to go up, come battery time. The benefit of all this extra extra panel power is because the hybrid inverter can send it straight to the battery and doesn't need to convert it to AC in the house until you need it at night.


The reason for this 33.3% 'oversize' allowance is because panels never perform as well on your roof as they do in the factory. All panels are individually flash tested in the factory under 1000 Watts per square metre of intense artifical sunlight and where the cells inside the panel are 25 °C.


As cells typically operate at about 20°C higher than the ambient (air) temperature, the factory test is pretty frosty. You are never going to get that intensity of sunlight at such a cool temperature on your roof. Not ever. So, a panel that flash tests at 330W might produce 250W on your roof. That's exactly the same for ALL panels no matter how cheap or expensive they are. Same test, same percentage reduction. You can see the STC and NOCT on any panel datasheet. STC is the factory test under 'ideal conditions' and NOCT is the second test under 'Normal' conditions.


So if you had 20 x 330W panels making a total of 6.6kW, but in reality they were actually producing 250W on your roof, then 20 x 250W = 5kW and therefore fine to work with a 5kW inverter. Make sense?


Sometimes the conditions for solar production are better than normal, bright and sunny, but still not particularly hot. In Perth we often see this in October and early November. On these wonderful solar days, we often see examples of 'clipping' where the panels are making slightly more power than the inverter can handle. The inverter's main job is to convert DC power made by the panels into AC power, but it can't make any more than 5kW of AC power, that's its limit in this example, so if it receives 5.5kW of panel power, then the extra 0.5kW is lost. Converted to heat and expelled. The graph below is a really good example of this 'clipping'. The green colour shows a flat top for some of the day not a jagged peak. It's flat because that's the 5kW limit, it can't convert any more, but clearly, for some of that time, there was more panel power.



How much power did this person lose?

It's actually quite easy to work out what they lost. In this case it's a Huawei inverter fitted with a smart meter and 6.6kW of panels. The extra lines and colours in the graph are because of the additional information from the smart meter. Forget about that and just focus on Green.


The production for the day was 36.11kWh. That's the total amount of AC power that was produced after converting it from the DC panel power. Delving further into the Huawei reporting software (below) we find for this day the overall efficiency (conversion of DC to AC) was 96.37%.


The maximum efficiency for this inverter is 98.6%, so 2.23% or 0.82 of a kWh was lost for the day. It's not much, and it only happens a few weeks a year.