CLASS 10 SCIENCE · ELECTRICITY

Electrical energy (kWh) and electricity bill

Energy (kWh) = power (kW) × time (h). 1 kWh = 1 unit = 3.6 × 10⁶ J. Bill = units × rate.

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Worked examples

Example 1 · level 1

A 750 W appliance runs for 2 h. How many units (kWh) of energy does it use?

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E = 0.75 × 2 = 1.5 kWh.

Answer: 1.5 kWh

Example 2 · level 2

A home in Surat uses a 750 W appliance for 2 h a day and 5 LED tube lights of 60 W each for 7 h a day. Find the electricity bill for 30 days at ₹7 per unit.

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Daily energy = 1500 + 2100 = 3600 Wh = 3.6 kWh.
Monthly units = 3.6 × 30 = 108 kWh.
Bill = 108 × ₹7 = ₹756.

Answer: ₹756

Example 3 · level 3

An appliance uses 1.8 × 10⁶ J of energy. How many units (kWh) is this?

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1.8 ÷ 3.6 = 0.5 kWh.

Answer: 0.5 kWh