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© Oxford University Press 2011 IP1.15.5 Electricity meters Electricity meters

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© Oxford University Press 2011 IP1.15.5 Electricity meters Energy used is measured in kilowatt-hours (kWh) and is calculated by the following formula: energy transferred (kWh) = power (kW) time (hours)

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© Oxford University Press 2011 IP1.15.5 Electricity meters 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

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© Oxford University Press 2011 IP1.15.5 Electricity meters A 150 W hi-fi is turned on for one and a half hours. Calculate the energy transferred. Energy transferred= power time = (150 1000) kW 1.5 hours = 0.225 kWh

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© Oxford University Press 2011 IP1.15.5 Electricity meters 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

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© Oxford University Press 2011 IP1.15.5 Electricity meters 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. Energy transferred= power time = 4 kW 1 hour = 4 kWh Cost of electricity= energy transferred cost per unit = 4 kWh 9p/kWh = 36 pence

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© Oxford University Press 2011 IP1.15.5 Electricity meters 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. Energy transferred= power time = (250 1000) kW 5 hours = 1.25 kWh Cost of electricity= energy transferred cost per unit = 1.25 kWh 10.2p/kWh = 12.75 pence

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© Oxford University Press 2011 IP1.15.5 Electricity meters The amount of electricity used in your home is recorded on an electricity meter.

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© Oxford University Press 2011 IP1.15.5 Electricity meters 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 Cost of electricity = energy transferred cost per unit £19.72 = 170 kWh cost per unit 1972/170 p/kWh = cost per unit Cost per unit= 11.6p/kWh 72643 kWh 72615

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© Oxford University Press 2011 IP1.15.5 Electricity meters 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. Cost of electricity= energy transferred cost per unit £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 84423

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© Oxford University Press 2011 IP1.15.5 Electricity meters 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= energy transferred cost per unit 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 92546 kWh 93290

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