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© Oxford University Press 2011

IP1.15.5 Electricity meters

Electricity meters



Energy used is measured in kilowatt-hours (kWh) and is

calculated by the following formula:

energy transferred (kWh) = power (kW) time (hours)



A television uses 400 W and is switched on for 2 hours. Calculate the energy transferred.

Energy transferred = power time

= (400 1000) kW 2 hours

= 0.8 kWh



A 150 W hi-fi is turned on for one and a half hours. Calculate the energy transferred.


= (150 1000) kW 1.5 hours

= 0.225 kWh



On your electricity bill, kilowatt-hours are called units of electricity. We can

calculate the cost of electricity by using the formula:

cost of electricity = energy transferred cost per unit



A 4 kW cooker is turned on for one hour. If the cost of a unit of electricity is 9p, calculate the cost of the total energy transferred.


= 4 kW 1 hour

= 4 kWh

Cost of electricity = energy transferred cost per unit

= 4 kWh 9p/kWh

= 36 pence



A computer uses 250 W and is turned on for 5 hours. If the cost of a unit of electricity is 10.2p, calculate the cost of the total energy transferred.


= (250 1000) kW 5 hours

= 1.25 kWh


= 1.25 kWh 10.2p/kWh

= 12.75 pence



The amount of electricity used in your home is recorded on an electricity

meter.



If the total cost of electricity for the period shown below is £19.72, calculate the cost per unit. In this example the cost per unit is the same for all units used.

Total energy transferred = 72 516 kWh – 72 346 kWh

= 170 kWh


£19.72 = 170 kWh cost per unit

1972/170 p/kWh = cost per unit

Cost per unit = 11.6p/kWh

7 2 643

kWh

7 2 615

kWh



If the cost of electricity for the period shown on the meters below is £20.80 and the cost per unit is 10.4p, calculate the meter reading shown on the first electricity meter.


£20.80 = energy transferred 10.4p/kWh

2080p = energy transferred 10.4p/kWh

2080 ÷ 10.4 kWh = energy transferred

energy transferred = 200 kWh = 200 units

Meter reading = 84 324 – 200 = 84 124

? ? ???

kWh

8 4 423

kWh



Look at the two meter readings below. These meter readings were taken at the beginning and end of a three month period. If the first 250 kWh are charged at 15.7p and the remainder at 10.2p, calculate the total cost for the period.

Total energy transferred = 93 092 kWh – 92 645 kWh

= 447 kWh


Cost of electricity = (250 kWh 15.7p/kWh) + ((447 kWh – 250 kWh) 10.2p/kWh)

= (250 kWh 15.7p/kWh) + (197 kWh 10.2p/kWh)

= 3925p + 2009.4p

= 5934.4p = £59.34

9 2 546

kWh

9 3 290

kWh