Calculate the cost of an EV charge

The energy entering the battery is the capacity multiplied by the percentage gap. But some electricity is lost as heat in the charger and the cables, so the grid supplies more: you divide by the efficiency. That grid energy is what gets billed, and it is what the tool multiplies by the price per kilowatt-hour. If you give the vehicle's consumption, the cost per 100 km applies the same efficiency.

battery energy = capacity × (target − start) ÷ 100

grid energy = battery energy ÷ efficiency

cost = grid energy × price per kWh

Charging efficiency depends on power, temperature and hardware. Slow charging at home often sits around 85 to 90%, rapid charging a little better with a warm battery, and markedly worse in hard frost when the car has to heat the pack. The tool models neither the charging curve, nor the time taken, nor the fixed session fees some networks apply.

Battery capacity

The usable capacity, not the gross figure sometimes advertised.

Starting charge

The level displayed when you plug in.

Target charge

The level you are aiming for. It must exceed the starting level.

Price per kWh

The rate that applies: home overnight, or the public charge point.

Charging efficiency

The share of grid energy that reaches the battery, between 1 and 100%.

Vehicle consumption

Optional: the vehicle's consumption, to get a cost per 100 km.

Assumptions

• Efficiency stays constant throughout the charge.
• The price per kWh does not change during the session.
• The capacity entered is the usable capacity.

Limits

• No charging curve and no charging time.
• No fixed session fees or network subscription.
• Cold weather degrades real efficiency far more than a single figure suggests.

The number to keep is the cost per 100 km: that is what compares with fuel. At 0.22 per kWh and 18 kWh per 100 km you land around 4.40, against roughly 12 for a petrol car at 6.5 L/100 km. On a rapid charger at 0.55 per kWh the advantage shrinks: the same distance costs close to 11. The price per kilowatt-hour matters more than the vehicle.

• Billing the energy that entered the battery.

  The meter measures energy drawn from the grid, losses included.

• Entering the final level instead of the gap.

  The tool asks for start and target, and works out the gap itself.

• Keeping 90% efficiency in mid-winter.

  In hard frost, preconditioning the battery drags real efficiency down.

• Appliance electricity costPower, running time and the price of a kilowatt-hour are enough. The tool returns consumption and spend per day, per month and per year.
• Fuel costThree numbers are enough: distance, economy per 100 km and price per litre. The tool returns litres, total cost and cost per kilometre.
• Fuel economyThe four usual notations for one number. The US gallon and the imperial gallon give two different mpg figures for the same car.
• Energy converterEvery energy routes through the joule. A kilowatt-hour is exactly 3.6 million joules, and a food kilocalorie 4,184 joules.

Written by Rédaction HexaCalc (editorial team). Content last updated: August 25, 2026. No third-party medical or financial review is claimed.

• NIST Guide to the SI, Appendix B: Conversion Factors — National Institute of Standards and Technology