© Oxford University Press 2011 IP Electricity meters Electricity meters

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

© Oxford University Press 2011 IP 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

© Oxford University Press 2011 IP 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 = kWh

© Oxford University Press 2011 IP 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

© Oxford University Press 2011 IP 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

© Oxford University Press 2011 IP 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 = pence

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

© Oxford University Press 2011 IP 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= kWh – 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 kWh 72615

© Oxford University Press 2011 IP 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 = – 200 = ????? kWh 84423

© Oxford University Press 2011 IP 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= kWh – 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 p = p = £ kWh 93290