Ask a new EV driver how big their battery is and how fast their charger is, and the answers often come back in the wrong unit. The confusion is understandable: kilowatt (kW) and kilowatt-hour (kWh) look almost identical. Yet they describe different things, and every charging decision, from hardware selection to pricing, depends on keeping them apart.
- kW is power: how fast energy flows at a given moment. It describes charger speed.
- kWh is energy: how much has flowed in total. It describes battery size and what is billed.
- Energy (kWh) = power (kW) × time (hours).
- OCPP chargers report both: instantaneous power and a cumulative energy register.
A simple analogy
Think of filling a bath. The tap's flow rate, litres per minute, is like kW: it tells you how fast water enters. The amount of water in the bath, in litres, is like kWh: it tells you how much has entered. A powerful tap fills the bath faster, but the bath holds the same volume either way.
kW: charging power
Charger ratings are given in kW: a 7.4 kW wallbox, an 11 kW AC point, a 150 kW DC charger. This is the maximum power the charger can deliver. The power that actually flows depends on the car, the battery's state of charge and temperature, and any limits set by load management. A car with an 11 kW on-board charger will draw at most 11 kW from a 22 kW AC point.
kWh: energy
Battery capacity is given in kWh: a car might have a 60 kWh battery. Energy consumption is often expressed as kWh per 100 km, for example 16 kWh/100 km. And public charging is usually priced per kWh, because that is the energy the driver actually received.
The formula that connects them
Energy (kWh) = Power (kW) × Time (h)
| Example | Calculation | Result |
|---|---|---|
| 11 kW for 2 hours | 11 × 2 | 22 kWh |
| 50 kW for 30 minutes | 50 × 0.5 | 25 kWh |
| 150 kW for 20 minutes | 150 × 0.33 | about 50 kWh |
| Adding 30 kWh at 7.4 kW | 30 ÷ 7.4 | about 4 hours |
These are idealised figures. In real sessions, power changes over time, especially on DC, where it falls as the battery fills. Our article on the EV charging curve explains why, and our guide on how long it takes to charge an electric car turns this into practical estimates.
Why the difference matters for operators
Pricing
Billing per kWh charges for the energy delivered, which is fair and comparable. In the EU, ad hoc charging at chargers of 50 kW and above must be priced per kWh. Time-based fees, if used, are typically for occupation after charging. See our guide to pricing EV charging.
Grid connection
Your grid connection is sized in kW (or kVA), because it limits how much power can flow at once. Ten 22 kW chargers do not need 220 kW of connection if load management shares the available power. The energy you buy, however, is billed in kWh, and some tariffs add charges based on peak kW.
Hardware choice
Higher kW reduces charging time but costs more and needs more grid capacity. The right power level depends on how long cars stay and how much energy they need, not simply on "faster is better".
kW and kWh in OCPP data
OCPP chargers report measurements through MeterValues messages. Two values are especially common:
| Measurand | Unit | Meaning |
|---|---|---|
| Energy.Active.Import.Register | Wh or kWh | Cumulative energy delivered, used for billing |
| Power.Active.Import | W or kW | Instantaneous power at that moment |
A frequent integration mistake is mixing Wh and kWh, which makes a 20 kWh session appear as 20,000 kWh or 0.02 kWh. A good management system reads the unit attribute and normalises values. Our guide to OCPP MeterValues covers measurands and units in detail.
Common mistakes
- "My battery is 60 kW." Batteries store energy, so the correct unit is kWh.
- "The charger delivered 150 kWh in an hour." A 150 kW charger can deliver at most 150 kWh in one hour at full power, and real sessions are lower.
- Comparing kW with kWh prices: a price per kWh and a charger power in kW are not directly comparable; only energy delivered is billed.
Frequently asked questions
Is kW or kWh more important when choosing a charger?
For the charger itself, kW matters because it sets the maximum speed. For planning, both matter: how much energy (kWh) cars need and how much time they have determine the power (kW) required.
How many kWh does it take to fully charge an EV?
Roughly the usable battery capacity plus charging losses. A 60 kWh battery charged from 10% to 80% needs about 42 kWh in the battery, and a little more from the grid.
Why do I pay per kWh and not per kW?
Because kWh measures the energy you actually received. Power describes speed, not quantity.
What is the difference between kW and kVA?
kVA is apparent power and kW is real power. Grid connections are often specified in kVA; for most charging calculations they are close, but electrical design should use the correct value.
See live power and energy per connector, normalised and ready for billing, from any OCPP charger. Explore Üreticy OCPP software or contact us.