Power and Efficiency

What is power measured in?

• Joules (J)

• Watts (W)

• Kiliwatt-hours (kWh)

• Volts (V)

How can you find the power of an appliance?

• see how hot it gets when switched on

• see what voltage it needs

• look for the efficiency label

• look for the power rating on the label

What type of diagram is used to show energy efficiency?

• Bar chart

• Sankey diagram

• Flow chart

• Power diagram

How do we calculate percentage efficiency?

• (useful energy out ÷ total energy in) × 100

• (useful energy in ÷ total energy out) x 100

• (total energy out ÷ useful energy in) x 100

Calculate the efficiency of a bulb with an input energy of 50J, which outputs 40J of thermal energy and 10J of light energy.

• 10%

• 20%

• 25%

• 50%

Calculate the payback time in years of double glazing, which costs £2000 and saves you £40 a year.

• 2 years

• 5 years

• 40 years

• 50 years

You have used 784 kWh of electricity since your last meter reading. Each kWh costs 14p. Calculate the cost of your bill.

• £56.00

• £109.76

• £560.00

• £10976.00

Which kinds of appliances transfer the most energy?

• heaters and cookers

• lights

• TVs and radios

• computers

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What does ‘payback time’ mean?

• The time it takes to pay off the loan on a new appliance.

• How long a new appliance will last without needing repairs.

• The time it takes to save enough money on energy bills to pay for a change.

• The time it takes a new appliance to reduce your energy bills.

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Freezer A costs £200 and freezer B costs £150. Someone buying freezer A would save £20 per year on energy bills. What is the payback time if someone buys freezer A instead of B?

• 0.4 years

• 0.6 years

• 1.6 years

• 2.5 years

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