Energy transfers in everyday appliances: power, P, in watts, W

The idea

Power, P, measured in watts, W, is the rate at which energy is transferred or transformed. One watt equals one joule per second, so a 100 W lamp transfers 100 J every second. In everyday appliances, power ratings on labels tell you how quickly electrical energy is transferred from the mains into useful stores, such as light, thermal energy or kinetic energy. You can calculate power using P = E ÷ t, where E is energy transferred in joules and t is time in seconds, or P = V × I for electrical devices.

For example: If a heater transfers 36 000 J in 60 s, its power is 36 000 ÷ 60 = 600 W. Higher power means faster energy transfer, not necessarily more total energy, because total energy also depends on how long the appliance runs.

What earns the marks

• State that power is the rate of energy transfer and is measured in watts, W, where 1 W = 1 J/s.
• Use the equation P = E ÷ t to calculate power in watts when energy in joules and time in seconds are given.
• Convert time to seconds and energy to joules where necessary before calculating power.
• Compare appliances by their power ratings, explaining that a higher power transfers the same energy in less time.
• Link power ratings on appliance labels to the rate of useful and wasted energy transfers in everyday contexts.

Common mistakes and how to put them right

• Treating power and energy as the same quantity.

  Put it right by defining power as energy transferred per second, measured in watts.

• Dividing time by energy instead of energy by time.

  Put it right by using P = E ÷ t with energy as the numerator.

• Leaving time in minutes.

  Put it right by converting minutes to seconds before dividing.

Exam tips

• Write P = E ÷ t and show the substitution with units to make your method clear.
• Check that your answer is in watts and that the time used is in seconds.
• When comparing appliances, refer to both power and time to explain differences in energy transferred.