Charging a car from your roof.

01How much a car uses

An electric car uses roughly fifteen to twenty kilowatt-hours for every hundred kilometres. For someone driving an ordinary distance each week, that adds up to a real share of a household's electricity: often as much again as a small home uses.

It takes the place of petrol, so it isn't simply an extra cost. But it changes what a solar system should be.

02Where is the car at midday?

This is the question that decides everything. A car that's home during the day can drink straight from the roof, and nothing is cheaper. Working from home, a second car, school runs with hours parked in between: all of these suit solar charging very well.

A car that leaves at seven and returns at six misses the sun altogether on weekdays. It can charge at weekends, from a home battery in the evening, or overnight on an off-peak rate. Each works. They're different designs.

The best time to charge a car is while it's parked in the sun doing nothing.

03Chargers

The lead that comes with most cars plugs into an ordinary power point and adds range slowly, which is fine for a car that's home a lot and doesn't travel far.

A dedicated wall charger is several times faster. On single-phase power that's around seven kilowatts. Three-phase homes can go higher, though many cars accept less than the charger can offer. For most households the single-phase size refills a day's driving in a couple of hours.

04Chargers that follow the sun

The useful feature to look for is solar-aware charging. The charger watches what the roof has spare and sends the car that much, speeding up as the sun comes out and slowing as a cloud passes or the oven goes on.

It means the car fills on power you'd otherwise have exported for a small credit, without anyone having to think about it.

05Sizing solar for a car

A car is the clearest case for a larger system than today's bill suggests. If one is coming in the next few years, tell us now. Allowing for it at the start is much cheaper than adding panels later, and it affects the choice of inverter as well.

06Your switchboard

A charger is a large, continuous load, and not every switchboard has room for one. Before we recommend anything we look at what's there: the main switch, the space on the board, and whether the home is single-phase or three-phase. A photo of the switchboard is usually enough to start.