Lesson: Chapter 10 — Power and energy in electrical appliances

10.5. Measuring energy in kilowatt-hours 2 of 2

Counting the units used

Knowing each appliance's power and how long it runs, the energy used is easy to work out.

Units used

kWh = (watts ÷ 1000) × hours

Example — four 100 W lamps lit 3 hours a day and five 60 W lamps lit 4 hours a day. Units in a month?

100 W lamps: 100 × 4 × 3 = 1200 Wh a day

60 W lamps: 60 × 5 × 4 = 1200 Wh a day

A month: (1200 + 1200) × 30 = 72 000 Wh

= 72 kWh

So 72 units of electricity — kilowatt-hours — are used in a month.