Why is my inverter only 5kW but my panels are 6.6kW?

PublicCouncils

12 May

Inverter sizes (kW) can be efficiently matched with rooftop solar panel array sizes (kW) that are up to 33% bigger. There are a couple of reasons for this.

1. Getting the best value from your inverter

The inverter converts the DC power from the solar panels into AC power that can be used in the house or sent to the grid. It controls the output of the system. The inverter is an expensive component of your solar PV system and to get the best value from it and your system in general you want it to be operating close to its maximum efficiency as much as possible.

A 5kW inverter that is converting 5kW of DC solar panel power to (nearly) 5kW of AC power for use in the home or the electricity grid is operating at peak performance. But in reality, 5kW of solar panels will not sustain 5kW of DC power production for long, even in sunny, perfect conditions. At best, under perfect conditions it will peak at or near 5kW around midday, and will be below that for the rest of the day.

Plenty of factors will reduce the kW output of the solar panels including:-

time of day, seasonal and regional variations in solar radiation
the angle and orientation of the panels
dirt or soiling on the panels
shading, smoke
losses due to the wiring or degradation of the panels
high temperatures.

So 5kW of solar panels will only occasionally be delivering 5 kW to the inverter. But 6.6kW of solar panels will reach or exceed 5kW of DC solar power output more regularly and for more hours in a day.

6.6kW of panels will allow the 5kW inverter to operate at optimal capacity for longer each day, creating more (free!) AC power for use in your home.

Summary:
An inverter that is the same size (in kW) or larger than your solar array is being under-utilised.
An inverter that is paired with a solar array of up to 33% higher powery will be operating at maximum power for longer each day.

2. Regulatory requirements

But why a 6.6kW array of solar panels with a 5kW inverter?

Clean Energy Council regulations dictate that solar panel arrays cannot be more than 33% larger than the inverter they are paired with, otherwise the STC rebate will not be applicable.

(The amount of the STC rebate is based upon the DC power output from the array of panels. So in this example, the STC is based on the 6.6kW of panels.)