The Roof, the Tariff and the Plug: Does Solar Really Beat Cheap Night Charging?

Solar panels on a UK roof have a lovely, obvious appeal: free electricity, lower bills, a car charged by sunshine. But for an EV driver, the useful question is not whether solar works. It is whether solar beats a cheap overnight tariff by enough to justify several thousand pounds of kit. In November 2026, with off-peak rates still keen and export payments modest, that answer depends heavily on your roof, your mileage and whether your car is actually at home when the sun is out.

Three set-ups, one spreadsheet

Take a fairly ordinary Midlands semi with a 4 kWp array. Call it ten panels, a south-facing roof and annual generation of around 3,400 kWh. The driver does 7,000 miles a year in an EV that returns about 3.5 miles per kWh, so the car wants roughly 2,000 kWh from the plug. The house uses another 2,500 kWh. We will use 26p per kWh for daytime grid electricity, 7p on an overnight EV tariff and 15p for exported solar under a smart export guarantee.

Set-up one: solar plus home charging

A 4 kWp system might cost £6,000 installed. Without a battery, you self-use maybe 40% of generation and export the rest. In this example, the house takes 800 kWh, the EV takes 560 kWh on sunny weekends and days off, and 2,040 kWh goes to the grid. Savings come to about £208 on the home bill, £146 on car charging and £306 from export: roughly £660 a year. Simple payback is just over nine years.

That is not a disaster, but it is not a slam dunk either. If the car is away at work all week and only charges after dark, the EV share drops sharply. The same array might save £500 a year in total, stretching payback towards 12 years. The panels are still doing useful work; they are just not doing much for the car.

Set-up two: solar plus a home battery

Add a 5 kWh battery and the installed cost might rise to £10,500. The battery lets you store daytime sunshine for the evening, so self-consumption climbs to perhaps 75%. In our example, the house uses 2,000 solar kWh, the EV still manages 600 kWh on sunny days, and 800 kWh is exported. Savings reach about £796 a year. Payback is around 13 years.

That looks worse than solar-only on a pure payback basis, and it often is. A battery mainly helps the house, not the car. If nobody is home to plug in during daylight, the battery does not magically fill the EV in the evening unless you have a wallbox that can draw from it. It can reduce your evening import, but it adds cost and embodied carbon. It makes most sense where evening home use is heavy and the roof is productive.

Set-up three: tariff-only charging

No panels, no battery, just a smart meter and an overnight EV tariff. Our 2,000 kWh of car charging costs about £140 at 7p per kWh, or £154 once you allow for charging losses. At the standard 26p rate, the same electricity would cost around £572. So the tariff saves roughly £418 a year compared with charging at the day rate, for zero upfront capital.

This is the comparison that trips people up. Solar-plus-battery saves more in absolute terms, but it costs £10,500 to get there. Tariff-only charging delivers a useful chunk of the EV saving for nothing. If your main goal is cheap motoring, the night tariff is usually the first thing to sort, not the last.

Where the payback stretches

A north-facing roof, heavy shading or an east-west split can cut generation to 2,300 kWh or less. That pushes solar payback out by years. So does a low-mileage driver: at 5,000 miles a year, the EV needs only about 1,430 kWh, so there is less car-related saving to chase.

Daytime parking is the big one. Solar self-consumption for the car depends on the car being there. If you commute at 8am and return at 6pm, most of your generation is exported or soaked up by the house. Export at 15p is not terrible, but it is worth less than avoiding a 26p import.

Export rates can also fall. A few years ago, early solar adopters got generous feed-in tariffs; those schemes are history. Today's export payments are useful but not guaranteed to stay at 15p. And if you move house before the system has paid back, you are relying on the buyer valuing the panels. Many do, but it is not cash in your pocket.

Where the payback shrinks

A clean south roof, a home worker and a higher annual mileage all help. At 12,000 miles, the car needs around 3,430 kWh, and if you can charge on sunny weekends and days off, self-consumption rises. A 30p daytime rate makes every solar kWh used at home more valuable. A battery also starts to look less silly if you have a heat pump, an electric shower and a household that uses plenty of evening power.

Keeping the car for eight or ten years helps the EV case, but remember the panels outlive most car ownership periods. The solar investment is really a property decision with an EV bonus, not the other way round.

Is solar automatically the greenest route?

Not necessarily. Panels and batteries carry manufacturing emissions, and the UK grid is cleaner at some times than others. Overnight charging often coincides with lower-carbon generation, especially wind. Solar reduces demand on the grid during the day, which is genuinely useful, but the greenest choice is still fewer car miles, an efficient EV and charging when the electricity is cleanest. Solar is part of that picture, not a free pass.

The practical verdict

If you want solar for the house, buy it for the house. Do not stretch the budget solely on the promise of free EV miles unless your car is home in daylight, your roof is good and you plan to stay put. For an average 7,000-mile driver who parks away from home during the week, a cheap overnight tariff is likely to win on cash and simplicity. For a high-mileage driver with a south roof, daytime charging and a long-term plan, solar can make a real dent in running costs. A battery is the icing, not the cake. Get several quotes, check the roof and the export terms, and run the numbers with your own mileage before signing anything.