EV COST CHECKER

"Drive on sunshine" is a lovely sales line, and like most sales lines it's about a third true. Solar panels genuinely can put cheap miles in your car. They can also sit there generating electricity at noon while your car is in a car park five miles away. Whether solar charging works for you comes down to two unglamorous questions: how much your roof generates, and whether the car is at home when it does.

What a UK roof actually produces

A well-sited 4 kWp array (about ten panels, south-ish facing) generates roughly 3,400–3,800 kWh a year in most of the UK — more in Cornwall, less in Aberdeen. At a typical 3.5 miles per kWh, that's a theoretical 12,000+ miles of driving. Sounds like your whole annual mileage sorted. It isn't, for two reasons.

Reason one: seasonality. That generation is brutally lopsided. A 4 kWp system might produce 450+ kWh in June but barely 90 kWh in December — when your car is using more energy per mile, not less (see the winter range guide). Solar is a summer game in Britain.

Reason two: the car has to be there. Peak generation is 10am–3pm. If the car commutes, the sunshine and the battery rarely meet. Retired, working from home, or running a second car that lives on the drive? Now the maths starts working.

The opportunity cost most people miss

Here's the bit the solar installers gloss over: charging the car from solar isn't free — it costs you the export payment you'd otherwise have earned. With a decent Smart Export Guarantee rate (around 15p per kWh on the better tariffs as of mid-2026), every kWh you put in the car instead of exporting "costs" 15p of forgone income.

That changes the comparison entirely. Against a standard tariff at 26p, solar charging saves you 11p per kWh — worthwhile. But against an EV tariff's 7p overnight rate, solar charging is worse than exporting by about 8p per kWh. Read that again, because it surprises everyone: if you have a good EV tariff and a good export rate, the financially optimal move is often to export your solar at 15p and charge the car at night for 7p. The sunshine pays for the night-time charging twice over.

Does a home battery fix it?

Partly. A battery catches the midday generation and releases it in the evening, which means the commuter car can finally drink the day's sunshine after work. But batteries cost £2,500–£4,500 installed for a useful size, and the same opportunity-cost logic applies: a battery that shifts solar into 7p overnight-charging hours is saving you very little per cycle. Batteries earn their keep best when paired with smart tariffs (charging cheap, even exporting at peak prices on some tariffs) rather than as a pure solar accessory.

So is solar-plus-EV worth installing?

A 4 kWp system costs roughly £5,000–£7,000 installed (0% VAT helps), and a typical home with an EV will see payback in the 7–11 year range depending on usage patterns, tariffs and how much you're home in daylight. The EV improves the case — it's a big, flexible load that can swallow surplus generation — but it doesn't transform it. The honest framing: buy solar because you want cheap, low-carbon electricity for the house, and treat summer car-charging as a pleasant bonus.

We built a solar & battery payback calculator that does these sums properly — regional generation, your tariff, your export rate, and how many miles you can realistically charge in daylight. It will tell you your payback year rather than ours.