Beginner · 9 min
Power and energy use
Calculate the rate of energy transfer and connect electric power to voltage and current.
Power is a rate
Power is the rate at which energy is transferred: P = E ÷ t. It is measured in watts (W), where one watt means one joule per second. Two devices can transfer the same amount of energy but have different average powers if they take different times. To find energy transferred at constant power, multiply power by time: E = P × t. Use seconds when power is in watts and energy is in joules.
Worked example
A device transfers 300 J in 10 s. Average power = 300 ÷ 10 = 30 W
Electric power
For a direct-current device operating at a steady voltage and current, electric power is P = V × I. This is the rate at which electrical energy is transferred to the device. The energy may become light, motion, heating, or a mixture of these. A power rating does not by itself tell you the total energy used; you also need the operating time.
Worked example
A device has a steady 5 V across it and draws 2 A. Power = 5 × 2 = 10 W Energy used in 6 s = 10 × 6 = 60 J
Compare energy use
- Calculate the energy transferred by a 10 W device running for 60 s and a 20 W device running for 30 s.
- Explain how different power ratings can produce the same total energy use.