Under-Sizing a Solar Array Inverter
Times have changed since the last time I installed a solar array 16 years ago. Back in those days, it was common to right-size or over-size an inverter based on the size of the solar array. In our particular case, I installed 4kW worth of solar panels and (2) 2.5kW inverters for a total of 5kW of inverter.
I really didn’t have to pay attention to our power transformer at that time. This transformer is a 10kVA transformer and my solar array and inverter were well within the tolerances of our transformer. A 10kVA transformer means that I could install up to 10kW of inverter.
Fast forward to this year, my goal for our new solar array was to size it so that we could get ourselves back to net zero — meaning that the amount of power that we use to power our house and electric vehicles would be offset by our renewable energy production. I figured that we needed to size an array to around 6kW in order to get slightly above net-zero.
The problem however was the fact that because our transformer was rated at 10kVA, I could not produce more than 5kW per hour in order to stay within the tolerance of the transformer. (5kW of existing inverter + 5kW of new inverter = 10kW or 10kVA) MiEnergy stated that they could increase the size of the transformer but this would cost between $800 and $1,500 to do so. So was there a way to increase production enough to get us to net-zero without having to upgrade our transformer?
Scott at Viking Electric had the answer. By keeping the inverter at 5kW but increasing the solar array to 6.4kW would produce enough extra power to get us there and still keep us in tolerance of our existing transformer.
In fact, nowadays it is normal to oversize the array by 20%-30% compared to the output of the inverter. This keeps the cost of the inverter low and increased production by using more solar panels. Solar panels are much more reasonable priced than they were 15+ years ago and inverters are now made to handle over-production.
Here’s our latest video that goes into greater detail on this method of solar array design:
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— Alan Stankevitz