kW describes power: the rate at which energy is transferred. kWh describes an amount of energy. To estimate charging time, divide the energy needed by the average power delivered over the session, keeping the units consistent.
The U.S. Department of Energy’s workplace charging guide uses separate columns for energy, charger power and estimated charging time. That distinction is useful even when you are planning a private journey rather than workplace infrastructure.
The units help show how the calculation works. Dividing an energy amount in kWh by an average power in kW leaves a duration in hours. Writing those units beside the numbers makes the estimate easier to follow and to adapt for another session.
Follow one calculation
Suppose a hypothetical session needs to deliver 20 kWh at an average of 5 kW:
20 kWh ÷ 5 kW = 4 hours
The result describes the stated assumptions. It does not mean a charger advertised at 5 kW will always add exactly that amount to the battery in four hours. The vehicle, equipment and conditions need to support the assumed average, and charging losses need to be accounted for at the relevant measurement boundary.
Use average power for a session estimate
| Figure | What it describes |
|---|---|
| Charger rating in kW | A capability, not a promise for the whole session |
| Session energy in kWh | The amount recorded at the stated meter or boundary |
| Average charging power | Session energy divided by elapsed charging time |
| Battery capacity in kWh | Stored energy capacity, not charger speed |
With only a peak figure available, label the calculation as idealised and keep the practical session estimate open.
Ask where the energy was counted
An electricity meter, charger receipt and car display may refer to different parts of the process. Record which one supplies the figure you are using. For planning, prefer the car’s applicable guidance and a session estimate that reflects your actual starting charge and equipment.
Connect battery percentage with an energy estimate
For a fictional battery with 60 kWh of usable capacity, moving from 20% to 70% represents a 50-percentage-point change. Under that simplified assumption, the stored-energy change is 60 x 0.50 = 30 kWh. If the relevant average battery-side charging power were 10 kW, the corresponding idealised duration would be three hours.
Keep the usable-capacity assumption, charge interval and measurement boundary with the answer. A charger receipt may count energy at a different point from the stored-energy figure. The vehicle’s current guidance is needed for a practical estimate.
Calculate an average from a completed session
Suppose an invented receipt records 24 kWh over 40 minutes. Convert time to hours: 40 / 60, or two-thirds of an hour. The average at that receipt’s boundary is then 24 / (40 / 60) = 36 kW.
| Question | Calculation or information needed |
|---|---|
| How much energy? | kWh over the stated interval |
| What average power? | kWh divided by hours |
| How long at that average? | Required kWh divided by kW |
| What would it cost? | Applicable billed energy and tariff conditions |
These calculations can share the same session record while answering different questions. Keep each result labelled so it can be reused appropriately in journey or cost planning.
For an additional invented example, 18 kWh at an average 6 kW gives three hours. Place the average-power assumption beside the answer. That makes it straightforward to recalculate the estimate when a more suitable session figure becomes available.
Sources consulted
- Federal Workplace Charging Program Guide — U.S. Department of Energy; consulted 3 November 2026.