For electrical devices, power can also be calculated using voltage and current. This is important because electricity bills are based on the energy used — and energy used depends on both power and time.
P = I × V Power (W) = Current (A) × Voltage (V)
Rearranging gives us more useful forms:
- P = I²R — useful when you know current and resistance
- P = V²/R — useful when you know voltage and resistance
In domestic settings, energy companies charge by the kilowatt-hour (kWh):
Energy (kWh) = Power (kW) × Time (h)
For example, a 3 kW kettle running for 0.05 hours (3 minutes) uses: 3 × 0.05 = 0.15 kWh of electrical energy.
This how it works covers Why Power Matters in Electrical Circuits within Power for GCSE Physics. Revise Power in Energy for GCSE Physics with 14 exam-style questions and 25 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 4 of 12 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Power
Which of the following is the correct definition of power?
A microwave oven transfers 54,000 J of energy. It has a power rating of 900 W. Calculate how long the microwave was switched on. Use the equation: power = energy transferred / time Give your answer in seconds (s).