Power: power, P, in watts, W

The idea

Power P is measured in watts W, where one watt is one joule per second. This unit tells you how quickly energy is transferred or work is done. A 2000 W kettle transfers 2000 J every second, while a 10 W lamp transfers only 10 J each second. In electrical circuits, power can also be calculated using P = V I, where V is potential difference in volts and I is current in amperes, or P = I² R. The watt is the same unit whichever equation is used. When answering questions, always give power in watts unless a question asks for kilowatts or megawatts, and convert carefully: 1 kW = 1000 W and 1 MW = 1 000 000 W.

What earns the marks

• State that power is measured in watts, W, and that 1 W = 1 J/s.
• Use the unit W correctly in calculations and final answers, including kW and MW conversions.
• Link the watt to rate of energy transfer, explaining that a higher power means more energy transferred each second.
• Apply electrical power relationships such as P = V I or P = I² R and give the answer in watts.
• Compare the power ratings of appliances and interpret what the values mean in terms of energy transferred per second.

Common mistakes and how to put them right

• Writing power in joules.

  Put it right by remembering that joules measure energy or work done, while watts measure the rate of energy transfer.

• Treating kW and W as interchangeable.

  Put it right by multiplying kW by 1000 to convert to W.

• Confusing power with energy in a comparison.

  Put it right by explaining that power is the rate, so a higher wattage transfers more energy each second.

Exam tips

• Include the unit W with every power answer unless the question specifies kW or MW.
• Show conversions clearly, for example 2.5 kW = 2500 W, before using the value in an equation.
• When comparing appliances, refer to energy transferred per second rather than total energy used.