Calculate energy from power and time
Distinguish watts from watt-hours and convert to kilowatt-hours.
Supplied materials
- Paper-only fictional constant-power device: it uses 60 W while operating, for 4 hours. Do not touch wiring or measure a real device.
- Rules: energy in Wh = power in W × time in hours; 1,000 Wh = 1 kWh. Actual devices may vary power over time.
Learn the idea
Power describes the rate of energy transfer. Energy describes the amount transferred over a period. Multiplying a constant power by operating time gives energy; the units become watt-hours.
A lower power does not guarantee a lower total energy use if operating times differ. Keep both power and time visible, and state the constant-power assumption. No cost can be calculated until a relevant price per kWh is supplied.
- Multiply 60 W by 4 h.
- Divide Wh by 1,000 to express the result in kWh.
- Label the quantity as energy, not power.
- Calculate the new device-and-time case in the question.
See the method in action
60 W × 4 h = 240 Wh = 0.24 kWh. The device’s modelled power is still 60 W; 0.24 kWh is the energy for that four-hour period.
Check your reasoning
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See the answer and explanation
0.06 kWh
12 × 5 = 60 Wh, then 60 ÷ 1,000 = 0.06 kWh. Multiplying power by time gives energy; the conversion is required to express it in kWh.
Take it further
Compare a 10 W device for 8 h with a 20 W device for 3 h. Check: 80 Wh versus 60 Wh, so the higher-power device uses less total energy in these periods.
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