Power is a rate and energy is an amount. Use watts or kilowatts for the rate an appliance, charger, or device draws; use watt-hours or kilowatt-hours for the amount used over time. A 2 kW device running for 3 hours transfers 6 kWh of energy because 2 kW × 3 h = 6 kWh.
How can I convert power to energy?
A kettle rated 2,000 W is rated 2 kW. If it heats for 6 minutes, convert 6 minutes to 0.1 hour, then multiply 2 kW by 0.1 h to get 0.2 kWh. A 2 kW space heater running for 3 hours uses 6 kWh, thirty times as much energy because its duration is thirty times longer. Equal power does not mean equal energy unless time is equal.
What is the difference between watts and kilowatt-hours?
Ask whether the question says “how fast,” “how much at once,” or “how much over a period.” Use W or kW for the first two and Wh or kWh for the last. Convert watts to kilowatts by dividing by 1,000, convert minutes to hours when calculating kWh, then use the appliance-energy calculator for a duty-cycle scenario. The power converter changes rate units, while the energy converter changes energy units.
Why do I need time to convert kW to kWh?
Power is energy per unit time. The watt is one joule per second. Energy is measured in joules or watt-hours; 1 kWh equals 3.6 megajoules. The units themselves provide a check: kW multiplied by h gives kWh, but kW cannot be “converted” to kWh unless a duration is supplied.
How do I calculate kWh from an appliance rating?
Do not say that a 10 kWh battery has “10 kW of power” without a discharge-rate condition. Do not multiply kW by a per-kWh statement amount without hours. Do not divide kWh by a voltage to get a general power answer. Do not compare a charger’s maximum kW with a battery’s kWh as though they were competing capacities.
When does power not predict energy use?
This distinction does not predict a device’s actual energy use, battery runtime, heat output, or bill by itself. Those depend on time, efficiency, load, control behaviour, rate structure, and sometimes environmental conditions. A nameplate maximum is not necessarily an observed average.
Source for the external fact
NIST defines the watt as one joule per second in its watt definition and lists 1 kilowatt-hour as 3.6 megajoules in its energy conversion tables. These facts make the rate-versus-amount distinction checkable.
Related calculators
Estimate a running appliance in the energy-cost tool, convert a device rating in the power converter, and use an energy converter when the result must be expressed in megajoules.