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# Energy transfer

You should be able to calculate the cost of using an electrical appliance when given enough information about it.

## The unit: kilowatt-hours, kWh

The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1kWh.

The equation below shows the relationship between energy transferred, power and time:

energy transfered (kilowatt-hour, kWh) = power (kilowatt, kW) x time (hour, h)

Note that power is measured in kilowatts here, instead of the more usual watts. To convert from W to kW you must divide by 1000. For example, 2000W = 2000 ÷ 1000 = 2kW.

Also note that time is measured in hours here, instead of the more usual seconds. To convert from seconds to hours you must divide by 3600 (this is the number of seconds in 1 hour). For example, 1800s = 0.5 hours (1800 ÷ 3600)

## The cost of electricity

Electricity meters measure the number of units of electricity used in a home or other building. Units (kilowatt-hours) are used instead of joules because a joule is too small a unit of energy.

The more units used, the greater the cost. The cost of the electricity used is calculated using this equation:

total cost = number of units x cost per unit

For example, if 5 units of electricity are used at a cost of 8p per unit, the total cost will be 5 × 8 = 40p.

Check your understanding of working out the cost of electricity by having a go at this activity.

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