The cost of electricity

This lesson covers: 

1. How to calculate the energy an electrical appliance transfers
2. How electricity meters can be used to calculate energy used
3. How to work out the cost of an electricity bill

Calculating energy transferred by electrical appliances

Anything that needs electricity to work is called an electrical appliance.

When switched on, all electrical appliances transfer energy from the mains electricity supply to other stores of energy.

For example, a kettle transfers electrical energy into heat energy.

How to calculate energy transfer

The total energy transferred can be measured in:

• Joules (J), Kilojoules (kJ) or Kilowatt-hours (kWh)

The rate at which an appliance transfers energy is its power and is measured in:

• Watts (W) or Kilowatts (kW) (1 kW = 1000 W)

The total energy transferred depends on both the power and the time the appliance is switched on for.

Calculating energy transfer using Watts and seconds

Use the formula: Energy transferred (J) = power (W) × time (s)

For example: A 2000 watt kettle boils for 150 seconds.

Calculate the energy transferred.

Step 1:

Write the formula: Energy transferred (J) = power (W) × time (s)

Step 2:

Substitute the correct values: Energy transferred = 2000 × 150

Step 3:

Complete the calculation: = 2000 × 150=30 000 J

Calculating energy transfer using Kilowatts and hours

Use the formula: Energy transferred (kWh) = power (kW) × time (h)

For example: Andrea uses a 3 kilowatt heater for 4 hours to heat her house.

Calculate the energy transferred.

Step 1: Formula

Energy transferred (kWh) = power (kW) × time (h)

Step 2: Substitution

Energy transferred = 3 × 4

Step 3: Evaluation

Energy transferred = 3 × 4=12 kWh

Using electricity meters to calculate energy usage

Most houses have an electricity meter that records cumulative energy usage in kilowatt-hours (kWh).

By taking meter readings at different times, you can calculate the energy usage over a specific period.

For example:

• Record meter readings: Start time = 13249. End time = 25827.
• Subtract end time from start time: 25827 - 13249 = 12578.
• This gives get the energy used between those times: = 12578 kWh.

Calculating electricity bills

Electricity companies charge per unit of energy consumed, based on the kilowatt-hours (kWh) recorded by your meter.

The total cost is calculated using: Cost = Energy transferred (kWh) x Price per kWh

Where Price per kWh depends on your tariff but is typically 20-45 pence per kWh.

For example: Electricity costs 36p per kWh.

At the start of the month Jo’s meter read 42729.66 kWh.

At the end it read 43044.91 kWh.

Calculate her electricity cost.

Step 1:

Calculate the energy used in one month: Energy used = 43044.91 - 42729.66 = 315.25 kWh

Step 2:

Use the formula: Cost = Energy used × Price

Step 3:

Substitute the correct values: Cost = 315.25 × 36

Step 3:

Complete the calculation: = 315.25 × 36=£113.49

This same principle of energy x unit price is also used to calculate gas bills based on gas usage in kWh.