kW versus kWh: calculate energy from appliance power and time
Understand why a kilowatt measures instantaneous power and a kilowatt-hour measures consumed energy on a bill.
How to calculate it
A kilowatt is 1,000 watts of power at an instant. A kilowatt-hour is the energy from using one kilowatt for one hour. For constant power, kWh = kW × hours. If wattage changes, sum each operating period or use an energy meter. This distinction matters when comparing a charger rating in kW, a battery capacity in kWh and an electricity tariff per kWh. Multiplying a kWh capacity by hours again gives the wrong unit.
Worked example
A 2 kW heater running for 2.5 hours uses 5 kWh under the constant-power assumption. At 0.20 currency units per kWh, the energy portion is 1.00. A 10 W lamp for 100 hours also uses 1 kWh: 10 ÷ 1,000 × 100 = 1. If the heater thermostat switches it off for half the time, actual use may be near 2.5 kWh, plus any other operating states. A 2 kW rating is not a monthly consumption amount.
What can change the result
Electricity bills can include fixed charges, taxes, tiered rates or time-of-use pricing. Nameplate power is often a maximum, not average draw. Energy loss between wall and battery matters for charging. Read the tariff period and measure variable appliances to avoid false precision. Do not compare a peak kW figure with a daily kWh value as if they are the same.
Use the calculators and keep reading
Use the electrical power calculator to confirm volts, current and watts where appropriate, then the electricity cost calculator for kWh and tariff. The linked watts-to-kWh guide explains the units and how to check a bill.
Try the electrical power, electricity calculator and read this related article for the context and assumptions.