Mike Casey, chief executive at Rewiring Aotearoa, said some people were not installing big enough systems because their demand for power was likely to increase over time.

"They're going with the cheaper option and ultimately then outgrowing that solar system very, very quickly … they'll get a solar installer who will ask for their power bills over the last few months and they'll design a solar system that's optimised for return for that particular movement in time but then you add more electricity demand.

"Another way of explaining it is the most savings that you're going to get comes from avoiding buying electricity from the grid at 40 cents… the way to maximise your return on investment is potentially to look at a smaller system where you're using almost all of the generation immediately in your hot water, in your fridge and freezer and EV or whatever it might be.

"But actually that's not practical because as your electricity consumption grows, your solar install needs to grow as well. And the most expensive thing isn't the panels, the most expensive thing is getting the bloke on your roof to drill the roof. So really you should be overbuilding your solar system."

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