Free miles from a panel on the fence. It's a lovely idea. Sadly the sums are not on its side, and it's worth seeing why before you buy a kit for the wrong reason.
How much power does a plug-in kit make in a day?
The government's figures say an 800 W kit makes between 378 and 690 kWh a year, depending on where it sits. Spread over a year, that's about 1 to 2 kWh a day. Summer days give more. Winter days give a lot less. A kettle uses about 0.1 kWh to boil once, so this is small beer.
How many miles is that?
We've assumed a car that does 3 to 4 miles on 1 kWh, which is typical. So 1 to 2 kWh a day is roughly three to eight miles. That's the shops and back. We've also assumed a 60 kWh battery, a common size. To fill it from empty, you'd need every bit of the kit's output for one to two months.
Why can't the kit just charge the car slowly?
Because the kit doesn't feed the car. It feeds your home's wiring. A standard home car charger pulls about 7 kW. The kit gives 0.8 kW at the very most, on a perfect sunny day. So when the car is charging, the panel covers a small slice and the grid does the rest. You'll save a few pence an hour. You won't be driving on sunshine.
What can charge a car on sunshine?
A full roof of panels. A typical rooftop system makes about 3,150 to 4,050 kWh a year. That's roughly five to ten times what a plug-in kit makes. Pair it with a charger that follows the sun and a good share of your miles can come from your own roof. If that's the goal, start with how many panels you need.
So should EV owners bother with plug-in solar?
Yes, but for the house, not the car. The kit still cuts about £70 to £110 a year off your bill, car or no car. Just don't buy it thinking the fuel is now free. Our guide to whether plug-in solar is worth it has the full sums.
The quick version
- A kit makes about 1 to 2 kWh a day on average.
- That's roughly three to eight miles of driving.
- Rooftop solar is what really charges a car.