What Does the EV Charging Cost Calculator Do?
This calculator estimates the electricity cost of a defined charging session. It starts with usable battery capacity and the increase in state of charge, then allows for energy lost between the meter and the battery. That separates battery energy gained from electricity purchased. An optional vehicle-efficiency entry converts the stored energy into an indicative number of miles, so you can see the energy cost per mile as well as the session price.
How to Use This Calculator
- Enter usable battery capacity in kWh where you know it. Gross advertised capacity can include a buffer that the displayed state of charge does not make available, so check the vehicle's stated basis.
- Enter the starting and target charge percentages. Target must exceed start. A rise from 20% to 80% represents 60% of the entered battery capacity, not 80% of it.
- Enter the electricity price in pence per kWh and an overall charging efficiency. Use a realistic measured assumption if available. If your input basis already includes the losses, avoid applying them a second time.
- Optionally enter miles per battery kWh for an indicative added range, or zero to omit it. Calculate and compare battery energy with purchased energy before reading the bill. Add session or parking fees separately.
The Maths & Assumptions Behind the Tool
Energy added to the battery = capacity × (target − start) ÷ 100. Purchased energy = battery energy ÷ (charging efficiency ÷ 100). Session energy cost = purchased kWh × pence per kWh ÷ 100. At 90% efficiency, 36 kWh reaching the battery needs 40 kWh at the chosen purchase boundary. The optional range uses battery energy multiplied by miles per battery kWh, rather than the larger purchased-energy quantity.
Worked example
A 60 kWh usable battery charged from 20% to 80% gains 36 kWh. At 90% efficiency, grid energy is 36 ÷ 0.9 = 40 kWh. At 25p/kWh, the session costs £10.00, including 4 kWh of modelled losses.
At 3.5 miles per battery kWh, this represents approximately 126 miles of energy and 7.94p/mile. The range is conditional on efficiency; it is not a prediction that the car will travel exactly 126 miles.
Why This Matters for UK Drivers
Use usable battery capacity where the vehicle specification distinguishes it from gross capacity. Enter the starting and target percentages for the session, rather than assuming every charge goes from empty to full. The tariff should be the rate applying to the electricity bought during that session. If a session crosses price windows, calculate the priced portions separately or use a weighted rate that reflects the energy in each window.
Charging efficiency is an explicit assumption. The meter can supply energy for conversion losses, battery conditioning and other vehicle loads, so battery percentage alone cannot reproduce a precise bill. A charger or smart-meter reading is better evidence for an actual completed session. Public parking, connection or idle fees are not included here. Add those separately before comparing the total cost of two charging locations, especially when the energy-price difference is small.
Interpreting the result in practice
The charging boundary matters. A home electricity meter, a public charger's billed energy and energy stored in the battery do not necessarily measure the same point in the process. Choose the efficiency assumption that bridges your price measurement and usable stored energy. The default is illustrative and does not establish the losses of your particular charger or vehicle.
If a charging session crosses two tariff windows, a single price can only represent a weighted average. Split the purchased kWh by price period or use a charging-mix calculation. A fixed connection fee should be added after the energy calculation, while a time-based fee needs an estimate of time. Leaving these costs out is acceptable for an energy-only comparison provided the result remains clearly described that way.
Frequently asked questions
Why include charging efficiency?
Some electricity is lost as heat and through charging electronics, so grid energy can exceed battery energy added.
Can I enter an off-peak tariff?
Yes. Use the p/kWh rate you expect to pay during the charging session.
How much does it cost to charge an electric car?
Multiply the battery energy added by the electricity price, then allow for charging losses. The exact cost depends on the start and target charge, battery size, tariff and charging efficiency.
What changes the cost of an EV charge
What the number tells you
Battery percentage does not translate perfectly into electricity drawn from the wall. Charging systems lose some energy as heat and through power electronics, so the calculator allows for charging efficiency rather than assuming every purchased kWh reaches the battery.
Use realistic inputs
Use the tariff that applies during the charging period. If you have an off-peak EV tariff, the relevant rate may be much lower than your daytime household rate. For public charging, enter the actual network price and remember that memberships can alter the effective rate.
What is outside the calculation
The result estimates energy cost for the requested state-of-charge change. It does not model battery taper, parking fees, connection fees or idle charges. For a monthly ownership estimate, pair the charging cost with your real driving efficiency and charging mix.