EV charging cost and time calculator

Two things about charging an electric car surprise people who work it out on paper. The first is that the energy on the bill is more than the energy in the battery: some of it warms the on-board charger while it turns alternating current into direct, some runs the pumps and electronics that stay awake during the charge, and none of it appears on the dashboard. ADAC measured this across five cars and found 12.7 % to 24.2 % gone at a household socket and 5.1 % to 7.0 % at a wallbox. The second is that a bigger wallbox often changes nothing, because on AC the ceiling is the charger inside the car. This page takes the battery, the two states of charge, the two power ratings and your own price, and gives you the energy, the cost and the time.

How it is calculated

stored = kWh × (to − from) ÷ 100 · billed = stored ÷ (1 − loss) · hours = billed ÷ power

Start with what actually has to reach the battery: the usable capacity times the gap between the two states of charge. Divide that by one minus the loss to get the energy the meter sees, which is the energy you pay for — dividing, not multiplying, because the loss is a share of what goes in rather than of what arrives. Time is that same billed energy divided by the power actually flowing, and on AC the power flowing is the lower of the charge point and the car's on-board charger. The US Department of Energy lists that internal charger capacity among the things charging time depends on, alongside the state of charge and the equipment.

Source: US DOE Alternative Fuels Data Center, Charging Infrastructure — charging times vary with state of charge, capacity, battery type, "the vehicle's internal charger capacity, and the type of charging equipment"

Questions people ask

Why is the energy billed more than the energy stored?

Because the conversion is not free. ADAC put the bulk of it on the on-board charger turning alternating current into direct current, with the rest on the auxiliaries running during the charge and smaller losses in the cable and the battery itself. Their measurements, taken between 10 % and 90 % state of charge with the battery at 20–30 °C, ran from 5.1 % at an 11 kW wallbox to 24.2 % at a household socket. The trip computer shows what the battery received, so the difference never appears anywhere except the bill.

Does a bigger wallbox charge my car faster?

Only up to the car. On AC the current passes through the on-board charger, and that unit has a rating in kilowatts fixed by the manufacturer: a 22 kW wallbox feeding a car whose on-board charger stops at 7.4 kW delivers 7.4 kW. The US Department of Energy lists a Level 2 unit as anywhere from 2.9 to 19.2 kW of output, and names the vehicle's internal charger capacity as one of the things charging time depends on. DC rapid charging bypasses the on-board charger entirely, which is why the numbers there are so much larger.

Why does the socket lose so much more than the wallbox?

Partly because the loss has longer to run — 2.3 kW through a household socket means a charge lasting most of a day, and the auxiliaries draw their share the whole time — and partly because a converter running far below its rating is less efficient. ADAC saw one car whose on-board charger pulled 8 A rather than 10 A at the socket and lost 24.2 % as a result. The same car at a wallbox lost 6.9 %.

Why is my real charge slower than the time shown here?

This page assumes constant power, and charging is not constant. Power tapers as the battery fills, which is why manufacturers quote rapid charging from 10 % to 80 % rather than to 100 %; the last fifth can take as long as the first three. Cold matters too — ADAC found rapid charging a cold, unconditioned battery spent extra energy heating it, and the Department of Energy notes plainly that charging power varies by vehicle and state of charge.

What should I put in the loss field?

Whatever matches how you charge. From the ADAC measurements: about 5 to 7 % on an 11 or 22 kW wallbox, about 8 to 13 % when a wallbox is throttled down to solar surplus, 13 to 24 % on a household socket, and 5 to 15 % on a DC rapid charger once the energy spent warming the battery is counted. The default here is 6.9 %, which is what two of the five test cars showed at an 11 kW wallbox. If you have a sub-meter on the circuit, measure your own and use that instead.

Sources

The documents this page reads its numbers out of, linked so you can check them yourself.

1. US DOE Alternative Fuels Data Center, Charging Infrastructure — charging times vary with state of charge, capacity, battery type, "the vehicle's internal charger capacity, and the type of charging equipment"
2. ADAC charging loss study, 6 August 2026 — losses over a 10–90 % charge: 12.7 % to 24.2 % at a household socket, 5.1 % to 7.0 % at an 11 or 22 kW wallbox

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