Kilowatt-Hour (kWh)

The kilowatt-hour (kWh) is the unit of energy used by electricity companies to calculate bills. One kilowatt-hour is the energy transferred by a 1 kW appliance running for 1 hour.

Calculating energy in kWh

energy (kWh) = power (kW) x time (hours)

Note: power must be in kilowatts (not watts) and time must be in hours (not seconds).

Example

A 2 kW electric heater runs for 3 hours.

energy = 2 x 3 = 6 kWh

Converting between kWh and joules

1 kWh = 1000 W x 3600 s = 3,600,000 J = 3.6 MJ

The joule is the SI unit of energy, but it is too small for household energy calculations. A single joule would cost a tiny fraction of a sen. The kilowatt-hour is a more practical unit for billing.

Calculating cost

cost = energy (kWh) x price per kWh

In Malaysia, the domestic electricity tariff varies by usage block. For example, if the first 200 kWh costs RM 0.218 per kWh:

cost for 150 kWh = 150 x 0.218 = RM 32.70

Common exam mistakes

1. Forgetting to convert watts to kilowatts (divide by 1000).
2. Forgetting to convert minutes to hours (divide by 60).
3. Confusing kW (power) with kWh (energy). Power is the rate of energy transfer. Energy is the total amount transferred.