We can calculate the amount of electrical energy transferred by an appliance and how much it costs to run. This is useful for comparing the advantages and disadvantages of using different electrical appliances for a particular purpose.
The amount of electrical energyelectrical energy: Energy transferred by electricity. transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1 kWh.
E = P × t
Note that power is measured in kilowatts here instead of the more usual watts. To convert from W to kW you must divide by 1,000.
For example, 2,000 W = 2,000 ÷ 1,000 = 2 kW.
Also note that time is measured in hours here, instead of the more usual seconds. To convert from seconds to hours you must divide by 3,600.
For example, 7,200 s = 7,200 ÷ 3,600 = 2 h.
A 2 kW electrical fire is switched on for 3 hours. It uses 2 × 3 = 6 kWh of electrical energy.
You also use the equation E = P × t when:
For example, a 20 W lamp lit for 100 seconds transfers 20 × 100 = 2,000 J (2 kJ).
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