EV charging cost calculator: methodology

The mistake this avoids

Most EV calculators price the car’s electricity at the cheap overnight rate and stop there. But an EV tariff is a whole-home tariff. Its overnight rate is low, around 7.6p a kWh, and its day rate is high: about 36.7p on average, against 26.32p on the price cap. Everything the house uses outside the cheap window moves onto that day rate.

In a typical home only 15% of electricity is used from 23:30 to 05:30. The other 85% gets more expensive. For a low-mileage driver that costs more than the car saves, and the “cheap” tariff raises the bill.

So, as in the battery calculator, we compare two whole annual bills, house and car together, rather than pricing the car alone.

The calculation

battery kWh       = miles ÷ miles per kWh
home meter kWh    = battery kWh × home share ÷ (1 − charging loss)
public kWh        = battery kWh × (1 − home share)

standard bill     = (house kWh + home meter kWh) × unit rate
                    + standing charge × 365

EV tariff bill    = house kWh × night share × overnight rate
                  + house kWh × (1 − night share) × day rate
                  + home meter kWh × overnight rate
                  + EV standing charge × 365

switching saving  = standard bill − EV tariff bill
charging cost     = cheaper bill − house-only standard bill
                    + public kWh × public rate
fuel cost         = miles ÷ mpg × 4.54609 × pump price

The break-even mileage is where the switching saving crosses zero. The saving grows in a straight line with miles, so it is the house’s extra cost on the EV tariff divided by what each mile of home charging saves.

“Charging cost” is what the car adds to your energy spending on whichever tariff is cheaper for you, plus public charging. That is the fair figure to set against fuel.

Where the numbers come from

The EV tariff

We start from Intelligent Octopus Go (12-month fixed), the largest EV tariff, because its rates are published for every region. That lets us state a national average and a range instead of one region’s price. It is a starting point, not a recommendation: put in your own quote.

Overnight rate 7.6p a kWh, the same in every region, 23:30 to 05:30.
Day rate 36.7p a kWh, our simple average of the 14 regional rates, which range from 34.96p to 38.82p.
Standing charge 54.34p a day, averaged the same way, ranging 42.29p to 66.74p.
In force from 2 October 2026; checked 23 October 2026.

Intelligent tariffs bill any charging the supplier schedules outside the window at the overnight rate too, so with smart charging on, all home charging is priced overnight. On a fixed-window tariff, charging beyond what the charger can deliver in the window is priced at the day rate.

The house

The share of the house’s own use that falls in the cheap window comes from the OpenSynth / Faraday (Centre for Net Zero), no-LCT cohort load shape: 509,082 synthetic profiles, filtered to homes with no solar, battery, EV or heat pump. The default consumption is Ofgem’s typical medium home, 2,500 kWh a year.

Public charging and fuel

Public charging: Zapmap Price Index, September 2026. Rapid and ultra-rapid 79p a kWh; slow and fast 54p. Pay-as-you-go list prices. Subscriptions and network deals can be lower.
Fuel: DESNZ weekly road fuel prices, UK average for the week of 19 October 2026. Petrol 174.57p a litre; diesel 199.33p. UK mpg uses the imperial gallon of 4.54609 litres.

The car and the charger

Efficiency: 3.3 miles per kWh, the real-world average across every car EV Database lists (303 Wh a mile). Cold weather can cut this by a quarter. Your trip computer’s figure is better.
Charging losses: ADAC measured 5% to 7% on an 11 kW wallbox and 13% to 24% on a household socket. UK wallboxes are almost all 7 kW single-phase, between the two, so we use 8%. For a three-pin plug we use 15%.
Mileage and mpg are round placeholders, not national averages. Your MOT history shows your real mileage.

What this leaves out, and where it could be wrong

Homes with a heat pump, solar or a home battery. They use more of their electricity overnight or less from the grid by day, so the house’s penalty here is likely too high for them, and the EV tariff will look better than this shows.
Charging in one go. The window check spreads charging evenly across the year. A weekly top-up from empty needs more than one night on a plug.
VAT. Electricity rates include the temporary zero rate of VAT on domestic electricity in Great Britain, which ends 31 March 2027. Pump prices are unaffected, so the gap to petrol narrows slightly from April.
Running costs only. Purchase price, insurance, servicing, road tax and depreciation are not in it.
Regional prices. The price cap figures are national averages, as everywhere on this site. Your own rates are better.

Sources

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