1.9 - How Appliances Transfer Energy
The kilowatt-hour as a unit of energy
- The kilowatt-hour (kWh) is a unit of energy used by electricity companies to measure domestic energy supply.
- A kilowatt-hour is the energy transferred by a 1 kW device running for one hour.
- 1 kWh = 3,600,000 J (3.6 MJ).
Formula for energy transferred in kilowatt-hours
Energy transferred (kWh) = power (kW) × time (h)
- To convert power from watts to kilowatts, divide by 1,000.
- To convert time from minutes to hours, divide by 60.
Worked example - Energy in kilowatt-hours
A 1,200 W microwave oven is used for 15 minutes. Calculate the energy transferred in kilowatt-hours.
Step 1 - Convert the power to kilowatts and the time to hours
Power = 1,200 ÷ 1,000 = 1.2 kW
Time = 15 ÷ 60 = 0.25 h
Step 2 - Substitute into the kilowatt-hour equation
Energy transferred = power × time = 1.2 × 0.25 = 0.3 kWh
The cost of domestic electricity
- Electricity bills are based on the number of kilowatt-hours a household uses.
- Electricity companies charge a set price per kilowatt-hour.
Formula for cost
Cost = energy transferred (kWh) × price per kWh
The price is usually given in pence per kilowatt-hour, so the cost comes out in pence.
Worked example - Cost of using an appliance
An electricity company charges 17.4p per kWh. A 1,400 W electric lawnmower is used for 24 minutes. Calculate the cost of using it, to the nearest penny.
Step 1 - Convert the power to kilowatts and the time to hours
Power = 1,400 ÷ 1,000 = 1.4 kW
Time = 24 ÷ 60 = 0.4 h
Step 2 - Find the energy transferred in kilowatt-hours
Energy transferred = power × time = 1.4 × 0.4 = 0.56 kWh
Step 3 - Find the cost
Cost = energy transferred × price per kWh = 0.56 × 17.4 = 9.744p = 10p (to the nearest penny)
TermDefinition
Kilowatt-hour (kWh)The energy transferred by a 1 kW device running for one hour.