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Energy, power and fuel bills
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Food, fuel and electricity all supply energy. Knowing how much you use helps you understand food labels and bills.
Energy and food
Energy is measured in joules (J). 1 kilojoule (kJ) = 1000 J.
Food labels give the energy in kJ, often per 100 g, so you can compare foods.
Different fuels release different amounts of energy when they burn, so we can compare them too.
Food labels give the energy in kJ, often per 100 g, so you can compare foods.
Different fuels release different amounts of energy when they burn, so we can compare them too.
Power
Power is how quickly energy is transferred, measured in watts (W). 1 kilowatt (kW) = 1000 W.
A 2 kW kettle transfers 2000 J every second.
energy (J) = power (W) × time (s)
A 2 kW kettle transfers 2000 J every second.
energy (J) = power (W) × time (s)
Paying for electricity
Electricity bills use the kilowatt-hour (kWh): the energy a 1 kW appliance uses in 1 hour.
energy (kWh) = power (kW) × time (hours)
cost = energy (kWh) × price per kWh
Appliances with a higher power rating, used for longer, cost more to run.
energy (kWh) = power (kW) × time (hours)
cost = energy (kWh) × price per kWh
Appliances with a higher power rating, used for longer, cost more to run.
A 2 kW heater is on for 3 hours. Electricity costs 25p per kWh. Find the cost.
- Energy = 2 kW × 3 h = 6 kWh
- Cost = 6 × 25p
Answer: 150p (£1.50)
Energy is in joules (1 kJ = 1000 J). Power is in watts (1 kW = 1000 W). Energy (kWh) = power (kW) × time (h). Cost = kWh × price per kWh.
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