What this calculator does
Charging an EV costs the energy added divided by the charger efficiency, multiplied by the electricity price. Taking a 75 kWh battery from 20% to 80% adds 45 kWh, draws 50 kWh from the wall at 90% efficiency, and costs $13.50 at 30 cents a kWh.
The cost does not depend on the charger at all. A 3.7 kW socket and a 22 kW wallbox deliver exactly the same energy for the same money; only the time differs, from 13 hours 30 minutes down to 2 hours 16. Charger power buys convenience, not cheaper electricity.
The formula
The energy added is the battery capacity multiplied by the difference between target and current charge levels. The energy drawn from the wall divides that by the charging efficiency, since some is lost as heat, and the cost applies the electricity price to the drawn figure. Charging time is the energy added divided by the charger power multiplied by efficiency.
| Term | Meaning |
|---|---|
| Energy added | What goes into the battery, in kWh. |
| Energy drawn | What comes out of the wall, which is higher because of charging losses. |
| Charging efficiency | Around 0.85 to 0.92 for AC home charging. Lower in cold weather. |
| 20 to 80% | The commonly recommended daily charging window, which avoids the slow taper above 80% and the stress of very low states of charge. |
The inputs explained
| Field | What to enter |
|---|---|
| Battery capacity (kWh) | Usable battery capacity in kWh. |
| Current charge (%) | Current charge percentage. |
| Target charge (%) | Target charge percentage. 80% is the usual daily recommendation. |
| Charger power (kW) | Charger power in kW. A standard socket is about 2.3, a 32 A single-phase wallbox 7.4, and three-phase up to 22. |
| Charging efficiency | Charging efficiency as a decimal. |
| Electricity price ($/kWh) | Electricity price per kWh. Overnight or off-peak tariffs are often a fraction of the standard rate. |
When to use it
Working out running costs
Comparing the cost per charge against the distance it delivers gives a cost per kilometre that can be set against petrol.
Deciding whether to install a wallbox
The cost is identical either way, so the question is purely whether the time saved justifies the installation, which the time column answers.
Planning an overnight charge
Checking whether the charge fits inside an off-peak window is the practical use, since the tariff difference is often larger than any other factor.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How long does each charger take?
The same charge at a range of charger power levels.
| Charger power | Charging time | Charging cost | Energy added | Energy drawn from the wall |
|---|---|---|---|---|
| 3.7 kW | 13 h 30 min | $13.50 | 45.0 kWh | 50.0 kWh |
| 7.4 kW | 6 h 45 min | $13.50 | 45.0 kWh | 50.0 kWh |
| 11 kW | 4 h 32 min | $13.50 | 45.0 kWh | 50.0 kWh |
| 22 kW | 2 h 16 min | $13.50 | 45.0 kWh | 50.0 kWh |
Questions
Does a faster charger cost more?
No, not at home. The same energy goes into the battery either way and you pay for the same kWh. Faster charging is very slightly less efficient because of higher losses, but the difference is small. Public rapid charging is a different matter and is usually priced at a premium.
Why does the wall draw more than the battery receives?
Charging losses, mostly heat in the onboard charger and the battery itself. Home AC charging typically runs 85 to 92% efficient, so 10 to 15% of what you pay for never reaches the battery. Cold weather makes it worse, since energy also goes into conditioning the pack.
Why charge only to 80%?
Because charging slows markedly above 80% as the current tapers to protect the cells, and because sustained time at a high state of charge accelerates battery ageing. Charging to 100% before a long trip is fine; doing it daily is not recommended by most manufacturers.
How does this compare with petrol?
Divide the charging cost by the range it delivers to get a cost per kilometre, then compare against fuel cost per kilometre for a petrol car. Home charging on an off-peak tariff is typically a third to a fifth of petrol, while public rapid charging can approach parity.
For petrol running costs, see the cost per mile calculator. For household electricity generally, see the electricity bill calculator.