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Energy and units
Energy can be transferred and converted between forms. Power is the rate of energy transfer, measured in watts. Energy is measured in joules, or commonly watt-hours for electrical use.
Do not treat power and energy as interchangeable. A device running longer can use more energy even if its power is lower. Real devices can have varying power rather than a constant rating.
Worked example
A constant 10 W lamp used for 3 h consumes 30 Wh, equal to 0.03 kWh.
Try it yourself
Compare 10 W for 3 h with 20 W for 1 h.
Check your understanding
How much energy is 10 W used for 3 h?
Multiply power by time to obtain energy.
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