# Energy Cost

In this worksheet, students will learn about domestic use of fuel and how the cost is calculated for bills.

Key stage:  KS 3

Curriculum topic:   Physics: Energy

Curriculum subtopic:   Calculation of Fuel Uses and Costs

Difficulty level:

### QUESTION 1 of 10

When an electrical appliance is on and working, it uses electricity from the mains supply. Electrical energy is transferred from the mains to the appliance. The rate at which energy is transferred is measured in kilowatt hours (kWh). It can be calculated by multiplying the appliance power with the time it is being used.

energy transferred  =  power  ×  time

Note that since the unit for energy transferred to an appliance is kWh, when using the equation, you must make sure power is in kW and the time is converted into hours.

Electricity meters (see diagram below) are used at home to measure the energy transferred to all electrical appliances used at that particular house or building. The total cost is calculated using this formula:

total cost  =  number of units  ×  cost per unit

In the formula one unit is 1 kWh.

It is essential that energy is used efficiently by everyone nowadays, as excessive use causes more pollution to the environment and costs more. All appliances waste some energy when on, so only a proportion of the energy is utilised for the actual intended use. For example, a light bulb uses electricity when lit, but some of the output energy is heat, which is not actually needed from a light bulb. Heat is wasted energy going into the into the environment. The more input energy (electricity) utilised by an appliance for the intended use, then the more efficient an appliance is.

Sankey diagrams show the input and output energy of appliances; you will see the Sankey diagram for a light bulb below. The electrical energy transferred to the light bulb is 100 J.  75 J are actually converted to light energy, which is the intended use of a light bulb.  25 J are wasted as heat.

Let's put that information to use in this activity.

What type of energy is used by most appliances at home?

heat

kinetic

electrical

What is the unit of the energy transferred to electrical appliances by mains electricity?

J

W

kWh

Calculate the energy transferred to a 2 kW appliance over 3 hours.

1 kWh

5 kWh

6 kWh

Calculate the energy transferred to a 500 W appliance over 3 hours.

1500 kWh

1.5 kWh

4.5 kWh

Calculate the energy transferred to a 1 kW appliance over 30 minutes.

0.5 kWh

30 kWh

31 kWh

What does an electricity meter measure?

watt

joules

kilowatt-hours

Calculate the cost of electricity in pence for a particular household, if one unit costs 10 pence and 10 units have been used.

1 penny

20 pence

100 pence

Calculate the cost of electricity in pounds for a particular household, if one unit costs 10 pence and 10 units have been used.

1 pound

20 pounds

100 pounds

Calculate the cost of electricity in pounds for a particular household, if one unit costs 20 pence and 10 units have been used.

1 pound

2 pounds

200 pounds

What is essential to do in order to save fuels and pay less for electricity?

Keep the same appliances

Use more efficient appliances.

Use the most expensive appliances

• Question 1

What type of energy is used by most appliances at home?

electrical
EDDIE SAYS
Electrical energy is used by most appliances at home.
• Question 2

What is the unit of the energy transferred to electrical appliances by mains electricity?

kWh
EDDIE SAYS
The unit of electricity transferred to appliances is kilowatt-hour (kWh).
• Question 3

Calculate the energy transferred to a 2 kW appliance over 3 hours.

6 kWh
EDDIE SAYS
energy transferred = 2 kW x 3 h = 6 kWh
• Question 4

Calculate the energy transferred to a 500 W appliance over 3 hours.

1.5 kWh
EDDIE SAYS
Energy transferred = 0.5 kW x 3 h = 1.5 kWh. If the power is given in W, you must divide by 1000 to get the kW, so that your calculation is right.
• Question 5

Calculate the energy transferred to a 1 kW appliance over 30 minutes.

0.5 kWh
EDDIE SAYS
Energy transferred = 1 kW x 0.5 h = 0.5 kWh. If the time is given in minutes, you must divide by 60 to get the time in hours, so that your calculation is right.
• Question 6

What does an electricity meter measure?

kilowatt-hours
EDDIE SAYS
Electricity meters measure the units of electricity, which are kilowatt-hours.
• Question 7

Calculate the cost of electricity in pence for a particular household, if one unit costs 10 pence and 10 units have been used.

100 pence
EDDIE SAYS
cost = number of units x cost per unit = 10 x 10 = 100 pence
• Question 8

Calculate the cost of electricity in pounds for a particular household, if one unit costs 10 pence and 10 units have been used.

1 pound
EDDIE SAYS
Cost = number of units x cost per unit = 10 x 10 = 100 pence = £1.00. Alternatively, one can convert pence into pounds before the calculation. Let's try that at the next question.
• Question 9

Calculate the cost of electricity in pounds for a particular household, if one unit costs 20 pence and 10 units have been used.

2 pounds
EDDIE SAYS
Cost = number of units x cost per unit = 10 x 0.2 = £2.00. There are 100 pence to a pound, so 20 pence would be 20/100 = £0.20 (0.2).
• Question 10

What is essential to do in order to save fuels and pay less for electricity?

Use more efficient appliances.
EDDIE SAYS
The use of more efficient appliances ensures that we do not waste fuels, we reduce pollution and we save money in electricity. Using the most expensive appliances won't necessarily be the most energy-efficient (though they might be - check the label) and whilst it's not good to continually replace working appliances, it doesn't make economic nor environmental sense to keep inefficient ones going for ages.
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