Kristy Hoare from Mysolarquotes looked at almost 300 quotes and found that bigger systems were likely to be paid back more quickly.

As she said: "A huge part of the system cost is actually in the labour. You get someone out to your house and you have to put scaffolding on… just that cost alone makes the smaller systems not as good for payback."

In our view, having a bit more roof juice is wise and it always pays to plan ahead. When you need to charge up your batteries, fuel up the EV and heat your water and rooms, you'll need more electricity (as our Electric Homes and Vehicles report showed, an average fully electric home uses about twice as much electricity as an average fossil fuelled home).

Solar is the cheapest way to get it so it's not surprising that we're seeing significant growth in the number and size of solar installs in New Zealand.

9kW systems are the norm now and that's about what the average has been in Australia in recent years. Over there, system size is still increasing in part because of the battery subsidy and growing EV adoption.

We know people love a payback period, but we do like to point out that it's better to look at ROI or yearly net savings in comparison to doing nothing. And solar should deliver around $1000 a year in savings on top of the cost of paying off the system.

SEANZ's Gareth Williams said: "A 7 kW system may cost $14,000. The cost of another kW - roughly two panels more - would add only about $1000. The effective cost of energy that the extra panels generate is only around 5c/kWh - which is therefore worthwhile based just on export value. Furthermore, in winter or on overcast days the extra generation will have higher value in offsetting homeload."

More solar can also help keep water in the hydro lakes and reduce the need for fossil fuel electricity generation.

More than half of the solar quotes included a battery and around 30% went on to install them. It's less about the economics with a battery, but, as Hoare says, "the value isn't only in the payback with batteries, it's the energy resilience side. Being able to power your home during a power outage is incredibly valuable."

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