The useful starting point
kW tells you how fast energy is flowing. kWh tells you how much energy is transferred or stored. You pay an electricity rate per kWh; the charger’s kW rating helps estimate how long it takes.
Three numbers you will see when charging
| Example | Meaning | Use it for |
|---|---|---|
| 7.4 kW | Power: energy transferred per hour | Charging speed |
| 60 kWh | Energy: usable battery capacity | Amount needed |
| 30p/kWh | Price per unit of supplied energy | Electricity cost |
A kilowatt-hour is the energy supplied by one kilowatt of power sustained for one hour. It is not “kilowatts per hour”. At a constant 7.4 kW, a charger supplies 7.4 kWh in one hour or 14.8 kWh in two hours. The battery stores less if part of that energy is used elsewhere during charging.
Battery capacity is normally stated in kWh; charger power is stated in kW. Check whether a vehicle specification gives usable capacity or a larger gross/total capacity. The calculator needs usable capacity for a charge-percentage calculation.
Worked example: what will a four-hour window add?
Suppose your car can accept a steady 7.4 kW AC supply throughout a four-hour window. Assume a 60 kWh usable battery, a 20% starting charge and 10% charging losses.
- Energy supplied: 7.4 kW × 4 hours = 29.6 kWh.
- Energy stored: 29.6 × 0.90 = 26.64 kWh.
- Battery increase: 26.64 ÷ 60 × 100 = 44.4 percentage points.
- Ending charge: 20% + 44.4 = about 64%.
- Energy cost at 30p/kWh: 29.6 × £0.30 = £8.88.
This is a steady-power illustration, not a guarantee for every car or installation. A lower vehicle limit, load sharing or battery conditioning can change the outcome. In the calculator, choose By charging time to estimate a top-up within a fixed window. If the battery reaches full before the window ends, the calculation stops adding energy.
Worked example: how long from 20% to 80%?
For the same 60 kWh usable battery, the increase is 60 percentage points—not 80% of a full battery.
Energy divided by power gives time in hours. To convert a decimal hour to minutes, multiply the decimal part by 60: 5.405 hours is about 5 hours and 24 minutes, not 5 hours and 40 minutes.
At a flat rate, the cost depends on energy supplied, not simply the charger’s advertised speed. A four-hour cheap-rate window would not cover this entire example. Calculate the energy supplied in each tariff period and price each part at its applicable rate; one flat-rate estimate does not automatically account for a price change halfway through.
Why a 150 kW charger does not mean 150 kW all session
The Energy Saving Trust explains that a charging point’s rating and the vehicle’s charging capability both affect speed, and charging slows as the battery fills. A high number on a charger is a limit, not a promise of constant power.
- Vehicle limit: a car limited to 11 kW AC cannot accept 22 kW AC just because the charger offers it.
- Supply and connector: AC phase/current support and the installation can impose a lower limit. A headline 11 kW rating does not mean the same power is available on every supply.
- Battery conditions: temperature and state of charge affect the power the car requests.
- DC taper: rapid charging power varies through the session. Dividing all the required energy by advertised peak power can give an unrealistically short time.
EV Calculated caps power by the entered car and charger limits and uses simplified average-power stages for DC. It is a planning model, not a live prediction of a measured charging curve. Read how the time estimate works before planning a tight stop.
Put the right numbers into your estimate
- Choose the exact vehicle. Use the real vehicle picker and confirm the usable battery, AC limit and DC limit. Similar-looking trims can have different charging capabilities.
- Choose a target or a time window. Use your current battery percentage as the starting point. In target mode, enter the desired ending percentage.
- Check the charger and price. Enter kW for power and the price per kWh for electricity. Example rates are not live tariff quotes.
- Check the loss assumption. The charging-loss guide explains how to choose and validate it.
For range, efficiency connects battery energy to distance. At an illustrative 4 miles per battery kWh, adding 36 kWh corresponds to 144 miles. Weather, speed and driving style affect the real result; catalogue efficiency is a starting assumption you can edit.
Quick answers
Is a bigger battery always more expensive to charge?
Charging a larger usable battery through the same percentage range needs more energy. But adding the same 20 kWh costs the same at the same electricity rate and loss allowance, regardless of total battery size.
Does 50 kW for 30 minutes mean 25 kWh?
Yes, if 50 kW is sustained for the full half-hour at the measurement point. That is supplied energy; battery energy is lower when losses apply. A charger labelled 50 kW does not establish that the session actually averaged 50 kW.
Can I divide battery size by charger power?
That gives an idealised empty-to-full time at constant power with no losses. For a useful estimate, use the energy needed for your actual percentage change, allow for losses and account for vehicle limits and DC taper.
Sources & scope
Sources checked on 3 November 2026. Worked examples use the stated illustrative inputs; they are not measurements made by EV Calculated.
- Energy Saving Trust: charging electric vehicles
UK charging guidance on charger ratings, vehicle limits and slowing charge rates as the battery fills.