1. Energy Calculations Revision Energy = Power x Time (Joule) (Watt) (Second) Energy Power x Time Example 1: How much energy does a 40 Watt bulb use in 2 minutes? Answer: Energy = ? Power = 40 Watt Time = 120 seconds (which is 2 minutes) Energy = Power x Time Energy = 40 x 120 = 4800 Joules

2. Energy Calculations Revision Energy = Power x Time (Joule) (Watt) (Second) Energy Power x Time Example 2: What is the power rating of an electric device that uses 5000 Joules in 50 seconds? Answer: Power = ? Energy = 5000 Joules Time = 50 seconds Power = Energy / Time Power = 5000 / 50 = 100 Watt

3. Energy Calculations Revision Energy = Power x Time (Joule) (Watt) (Second) Energy Power x Time Example 3: How much energy does a 1 kW hairdryer use in half an hour? Answer: Energy = ? Power = 1000 Watt (which is 1 kW) Time = 1800 seconds (which is 30 x 60) Energy = Power x Time Energy = 1000 x 1800 = 1,800,000 Joules

4. Energy Calculations Revision Energy = Power x Time (Joule) (Watt) (Second) Energy Power x Time Example 4: How long does it need a 1500 Watt vacuum cleaner to produce 3000 Joules of energy? Answer: Time = ? Power = 1500 Watt Energy = 3000 Joules Time = Energy / Power Time = 3000 / 1500 = 2 seconds

5. Sometimes you will be asked to work with kW and hours not in Watts and seconds. The equation will be as follows: Energy = Power x Time (Joule) (Watt) (Second) (kW.h) (kW) (hour) Energy Power x Time Example 5: How much energy does a 2kW heater use in 24 hours? Answer: Energy = ? Power = 2 kW Time = 24 hours Energy = 2 x 24 = 48 kW.h

6. Sometimes you will be asked to work with kW and hours not in Watts and seconds. The equation will be as follows: Energy = Power x Time (Joule) (Watt) (Second) (kW.h) (kW) (hour) Energy Power x Time Example 6: What is the power rating of a cooker which uses 20 kW.h in 4 hours? Answer: Power = ? Energy = 20 kW.h Time = 4 hours Power = 20 / 4 = 5 kW

7. The electricity companies charge us for the amount of energy we consume (in kW.h) not the current or voltage supplied. They use the term “unit” to describe 1 kW.h Energy = Power x Time (kW.h) (kW) (hour) Energy Power x Time Example 7: How many units of electricity are used when you run a 2kW cooker for 4 hours? Answer: Power = 2 kW Time = 4 hours Energy used (no. of units used) = 2 x 4 = 8 kW.h

8. Calculating the cost of electricity Cost of electricity = no. of units used x cost of 1 unit (£) (kW.h) (£/kW.h) Example 8: How much would it cost to run a 3kW heater for 10 hours. (assume the cost of 1 unit of electricity is 6p). Answer: Power = 3 kW Time = 10 hours Cost of 1 unit = £0.06 per unit Energy (no. of units used) = 3 x 10 = 30 kW.h Cost of electricity = 30 x 0.06 = £1.80

9. Which of these are units of energy: Joule (J) Watt (W) Mega Watt (MW) kilo Watt hour (kW.h) kilo Joule (kJ) kilo Watt (kW) Second (s) Hour (h) Mega Joule (MJ)

10. Which of these are units of power: Joule (J) Watt (W) Mega Watt (MW) kilo Watt hour (kW.h) kilo Joule (kJ) kilo Watt (kW) Second (s) Hour (h) Mega Joule (MJ)

11. Which of these are units of time: Joule (J) Watt (W) Mega Watt (MW) kilo Watt hour (kW.h) kilo Joule (kJ) kilo Watt (kW) Second (s) Hour (h) Mega Joule (MJ)

12. Q1. An electricity meter reading is 11600 kW.h. One month later the reading is 11800 kW.h. a. How many units (kW.h) were used? b. If the cost of electricity is 8p per unit of kW.h. How much should the bill be that month. Q2. A 2 kW electric fire is run for 4 hour. How many units are used during that time. If the electricity costs 10p per unit of kW.h. How much would the cost be? Work out the following problems

13. The following table summarises the initial and running costs of a 100 W filament lamp and a 20 W compact fluorescent lamp (CFL). Type of lamp Lifetime of lamp (h) Power (kW) Price of lamp (£) Running cost per hour (£) Total running and purchase cost for 12000 hours (£) CFL 1200010.00 Filament 10000.50 * Cost of unit of electricity is £0.10 ** The 20 W CFL gives the same brightness as the 100 W filament lamp

14. The following table summarises the initial and running costs of a 100 W filament lamp and a 20 W compact fluorescent lamp (CFL). Type of lamp Lifetime of lamp (h) Power (kW) Price of lamp (£) Running cost per hour (£) Total running and purchase cost for 12000 hours (£) CFL 120000.0210.00 (0.02x1x0.10)= 0.002 (0.002x12000)+10 = £34.00 Filament 10000.100.50 (0.1x1x0.10)= 0.01 (0.01x12000)+6 = £126.00 * Cost of unit of electricity is £0.10 ** The 20 W CFL gives the same brightness as the 100 W filament lamp