Question

Calculate the total cost a using a 100 W lamp and a 300 W hair dryer for 10 minutes if a unit of electricity is 14p.

For the lamp:

Number of units used = power rating in kW x time in hours

Power rating = 100 W = \frac{\text{100 kW}}{\text{1000}} = {0.1 kW}

time = 10 minutes = \frac{\text{10 hours}}{\text{60}} = {0.167 hours}

Number of units used = 0.1 kWh x 0.167 h = 0.0167 kWh

total cost = number of units used × cost per unit

= 0.0167 kWh x 14p

= 0.23p

For the hair drier:

Number of units used = power rating in kW x time in hours

Power rating = 300 W = \frac{\text{300 kW}}{\text{1000}} = {0.3 kW}

time = 10 minutes = \frac{\text{10~hours}}{\text{60}} = {0.167~hours}

Number of units used = 0.3 kWh x 0.167 h = 0.0501 kWh

total cost = number of units used × cost per unit

= 0.0501 kWh x 14p

= 0.70p

Total cost = 0.23p + 0.70p = 0.93p

The cost of using the lamp and hair dryer is 0.93p.

Question

A TV needs 250 W. It is switched on for 30 minutes. If each kWh costs 14p, how much does it cost to run the TV?

Number of units used = power rating in kW x time in hours.

Power rating = 250 W = \frac{\text{250 kW}}{\text{1000}} = {0.25 kW}

Time = 30 minutes = \frac{\text{30 hours}}{\text{60}} = {0.5 hours}

Number of units used = 0.25 kWh x 0.5 h = 0.125 kWh

Total cost = number of units used × cost per unit

= 0.125 kWh x 14p

= 1.75p

The cost of running the TV is 1.75p