General · Glossary
kW vs kWh
kW is power (rate of energy use); kWh is energy (amount used over time). A 7kW charger pulling for 1 hour delivers 7kWh.
The kW vs kWh distinction trips up almost everyone. kW (kilowatt) is power — the rate at which energy is used or produced at any given instant. kWh (kilowatt-hour) is energy — power multiplied by time. A 1kW heater running for 1 hour uses 1kWh; running for 24 hours it uses 24kWh.
In renewable energy land:
- Heat pumps are sized in kW (e.g. 8kW Vaillant aroTHERM)
- Solar arrays are sized in kWp (peak power capacity)
- Batteries are sized in kWh (storage capacity) and have separate kW power ratings (charge/discharge speed)
- EV chargers are rated in kW (charge rate)
- Bills are charged in kWh (energy consumed)
A common error: assuming a 5kW battery means 5 hours of storage. It does not — 5kW is the discharge speed, the storage capacity might be 10kWh or 13.5kWh depending on the model. Always check both numbers. See our battery storage page.