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Question:

An electric heater is rated 1500 W. How much energy does it use in 10 hours?

Solution:

Time for which the heater is used t = 10 hours = 10 × 3600 s = 36000 s
Energy consumed by the heater E = P × t
where P is the power of the heater
∴ E = (1500) W × 36000 s
⇒ E = 5.4 × 10⁷ J
E = 15 kWh (∵ 1 kWh = 3.6 × 10⁶ J)