A 60 W electric lamp is used for 5 hours daily. Calculate the energy consumed in 'units' (kilowatt-hours) in 30 days.

Correct — A. Convert the lamp's power to kilowatts, then multiply by the hours of use.

Power = 60 W ÷ 1,000 = 0.06 kW
Energy per day = 0.06 kW × 5 h = 0.3 kWh
Energy in 30 days = 0.3 × 30 = 9 kWh
1 unit = 1 kWh, so the lamp uses 9 units → option (a)

Why the others are wrong

• (b)
  30 units — 30 units would need 1 kWh a day, which is a 200 W lamp run for 5 hours. A 60 W lamp uses only 0.3 kWh a day.
• (c)
  0.3 units — 0.3 units is the energy for one day. The question asks for 30 days, so it still has to be multiplied by 30.
• (d)
  90 units — 90 units is ten times the answer. It comes from dividing 9,000 Wh by 100 instead of 1,000 when converting watt-hours to kilowatt-hours.

The unit on an electricity bill is the kilowatt-hour (kWh): the energy used when a 1 kW appliance runs for one hour.

Energy = power × time. With power in kilowatts and time in hours, the product comes out directly in units. Converting watts to kilowatts first (divide by 1,000) keeps the arithmetic short and avoids the decimal slips behind the wrong options.

Key facts

• One unit of electrical energy on a bill is 1 kilowatt-hour.
• 1 kWh = 1,000 W × 3,600 s = 3.6 × 10⁶ joules.
• Energy in kWh = power in kW × time in hours.

Common traps

• Stopping at the daily figure of 0.3 kWh and forgetting to multiply by 30 days.
• Dividing watt-hours by 100 instead of 1,000 when converting to kilowatt-hours.

No directly related past PYQ was found.