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The energy added to the pack is the battery size times the change in state of charge. Because charging is not perfectly efficient, the grid must supply a little more — energy drawn = energy added / efficiency — and you pay for that grid energy at your electricity price. The time is the energy added divided by the effective charging power (charger power times efficiency). Fast DC charging usually slows down above about 80% SoC, so real times past 80% are longer than this linear estimate.
| Quantity | Formula |
|---|---|
| Energy added | E = Battery × (SoCend − SoCstart)/100 |
| Energy from grid | Egrid = E / η |
| Cost | Cost = Egrid × Price per kWh |
| Time | t = E / (Power × η) |
Home AC charging is typically 3.3–7.4 kW (single phase) or up to 11–22 kW (three phase); public DC fast chargers range from 50 kW to 350 kW. Enter the price in your own currency — the cost result is in the same currency.
Find the energy added (battery kWh × the change in state of charge), divide by the charging efficiency to get the grid energy, then multiply by your price per kWh: Cost = Battery×ΔSoC/η × Price.
Divide the energy you need to add by the effective charging power (charger power × efficiency): t = Energy / (Power×η). Adding 36 kWh at 7.4 kW and 90% takes about 5.4 hours.
Charging loses a few percent as heat in the charger, cables and battery. If efficiency is 90%, you draw about 1.11 kWh from the grid for every 1 kWh that reaches the battery, and you pay for the grid figure.
Home AC charging is typically 85–92% efficient; DC fast charging is often 90–95%. Use around 90% as a reasonable default if you do not have a measured value.
To protect the cells, the charger reduces power as the battery fills, especially on DC fast chargers. So the last 20% can take as long as the first 60%, and this linear estimate understates the time past 80%.
It depends on your battery size, how much you add, and your electricity price. As a rule, cost = kWh added / efficiency × price. Charging on an off-peak tariff can roughly halve the cost.
AC (home/destination) charging uses the car's onboard charger and is slower (3–22 kW). DC fast charging bypasses it to deliver 50–350 kW directly to the battery, charging much faster but usually at a higher price.
Divide the charging cost by the range it provides. If a charge costs a certain amount and gives, say, 225 km, the cost per km is that amount divided by 225.
Only up to the car's onboard charger limit for AC, or the battery's accepted rate for DC. A 22 kW wall box will not charge faster than a car whose onboard charger is limited to 7.4 kW.
Take the full battery size, divide by efficiency, and multiply by the price. A 60 kWh battery at 90% efficiency draws about 66.7 kWh from the grid.
Home charging on a standard or off-peak tariff is almost always cheaper per kWh than public DC fast charging, which carries a premium for the speed and infrastructure.
Yes. In cold weather the car may heat the battery before and during charging, which uses extra energy and can slow the charge, raising both the time and the effective cost.
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