What does it cost per mile to run an EV on home solar?

The comparison that matters is against the grid at its standard rate. Charging at home on an ordinary tariff costs about 8p per mile, based on an average tested EV efficiency of 3.18 miles per kWh, against 2p per mile on an off-peak or EV rate⁴. Driving 100 miles in an EV costs £7 in electricity, against £18 in a diesel or petrol car¹.

What follows is how those numbers are built, why your own figure will differ, and how much of the car's running cost a household can genuinely take off the grid.

How the figures are worked out: miles per kWh and the price of your electricity

Two numbers set the answer: how far the car travels on a kilowatt-hour, and what that kilowatt-hour cost to produce or buy.

On efficiency, the Government advisory rate for electric vehicles is 3.59 miles per kWh⁶. Independent guidance is more cautious about real-world conditions, putting an EV at around 3.2 miles per kWh in winter and approaching 4 in summer⁵. The 3.18 miles per kWh used in the tariff comparison sits close to the winter figure⁴.

On price, the arithmetic is the same one used for any electrical load: divide the wattage by 1,000 to get kilowatts, multiply by hours used to get kWh, then multiply by the unit rate⁷. For self-generated solar the unit rate is effectively zero, which is why the pence per mile collapses. Independent guidance describes self-consumed solar as locking in a portion of your energy costs at 0p per kWh for the next 25 years⁸.

The generation side has its own rule of thumb. In the UK, one kilowatt of panels generates between 800 and 1,000 units (kilowatt-hours) a year⁹. Typical installed costs are around £2,000 per kWp¹⁰, and a 6kW system averages £6,000 to £8,000 without a battery, a size the guidance associates with large families or households with EVs that charge at home⁸.

Put those together and a 4 kWp array is producing roughly 3,200 to 4,000 kWh a year. At 3.2 to 4 miles per kWh, that is a large share of a typical car's annual mileage, but only the share that is actually diverted into the battery rather than used in the house or exported.

Why your driving pattern and system size change the answer

The headline pence per mile assumes the car is charging when solar is generating, or when a tariff is cheap. Neither is guaranteed by the hardware alone.

Season is the first variable. In summer a reasonably sized system can cover a significant portion of a typical household's EV charging needs; in winter output drops considerably and grid top-ups become more necessary⁸. That is two effects at once: less generation, and a car that is less efficient in the cold, at around 3.2 miles per kWh rather than approaching 4⁵.

Driving pattern is the second. A car that sits on the drive all day can absorb midday generation; a car that leaves at 7am and returns at 7pm cannot, unless a battery or a timed tariff moves the energy to when the car is present. The cost per mile shown by charging calculators is for pure electric driving only, so it excludes any petrol or diesel miles in a plug-in hybrid⁷.

System size is the third. A 6kW system is the size independent guidance associates with households that charge an EV at home, at £6,000 to £8,000 without a battery⁸. Smaller arrays still contribute, but a greater share of charging falls to the grid.

The honest position is that solar sets a floor on cost, not a fixed figure. The floor is zero pence for the units the car actually takes from the roof; everything above it is bought at whatever rate applies when the car is plugged in.

Solar alone versus solar plus a smart tariff: which gets you the lower pence per mile

Solar on its own gives the cheapest possible electrons and the least control over when they arrive. A smart tariff gives control over timing but no ownership of the generation.

The tariff side is well documented. Home charging on an EV tariff costs about 2.3p per mile³. Off-peak home charging is put at 2p per mile against 8p on a standard rate⁴. A household in Bedford charging with a smart meter reports two pence per mile on average².

The solar side is cheaper still where self-consumption is high, because the units are not bought at all. Independent guidance states that charging from your own electricity is significantly cheaper than using grid power and far cheaper than petrol or diesel over the same distance⁸, and that home charging is likely to be cheaper than at a public EV station¹¹.

Combining the two is where the lowest figures come from. Guidance on plug-in solar with a battery and a smart tariff puts the combined savings at potentially £300 to £400 a year¹¹. The reason to favour self-consumption over export is stark: rates for selling electricity to the grid are much lower than tariffs for using electricity from the grid¹². Export rates under the Smart Export Guarantee range from 1p to 25p per kWh¹², and in the scheme's fifth year the lowest tied and untied tariffs were 5p/kWh (Octopus) and 1p/kWh (E Energy) respectively¹³. Every unit exported at 1p and bought back at a charging rate is a unit that cost more than it earned.

What size solar system to plan for, and what it costs

There is no formula that converts a mileage figure into a required array size, because the answer depends on when the car is at home. What the figures do give is the cost of the generation.

A 6kW system is the size independent guidance ties to households with EVs that charge at home⁸. At 800 to 1,000 kWh per kWp a year⁹, that is 4,800 to 6,000 kWh of annual generation, comfortably more than a typical car's consumption, but only if the timing works.

For households that want the car charged from the roof rather than the grid, the practical levers are a home charge point that can be scheduled, and either a battery or a tariff that makes the overnight top-up cheap. Neither changes how much the roof generates; both change how much of it reaches the car.