Q.3 An electricity utility company charges ₹ 7 per kWh (kilo watt–hour). If a 40–watt
desk light is left on for 10 hours each night for 180 days, what would be the cost of
energy consumption? If the desk light is on for 2 more hours each night for the 180
days, what would be the percentage–increase in the cost of energy consumption?
(A) ₹ 604.8; 10%
(B) ₹ 504; 20%
(C) ₹ 604.8; 12%
(D) ₹ 720; 15%

40-Watt Desk Light Energy Cost Calculation: ₹7/kWh for 10 Hours Nightly Over 180 Days

The cost of running a 40-watt desk light for 10 hours each night over 180 days at ₹7 per kWh is ₹504, with a 20% increase if used for 2 more hours daily.

Step-by-Step Calculation

Convert power to kilowatts: $40\text{W}=0.04\text{kW}$.

Energy for 10 hours: $0.04\times 10\times 180=72\text{kWh}$. Cost: $72\times 7=₹504$.

For 12 hours: $0.04\times 12\times 180=86.4\text{kWh}$. Cost: $86.4\times 7=₹604.8$.

Percentage increase: 604.8−504504×100=20%504604.8−504×100=20%.

Option Analysis

• (A) ₹604.8; 10%: Matches second cost but understates increase (actual 20%). Incorrect.

• (B) ₹504; 20%: Correct original cost and precise increase. Matches calculations.

• (C) ₹604.8; 12%: Second cost right, but increase wrong (20%, not 12%). Incorrect.

• (D) ₹720; 15%: Overstates cost (assumes 50W or error); increase wrong. Incorrect.

GATE Exam Context

This question from GATE CE 2025 Set 2 tests unit conversion and percentage change, common in aptitude sections for engineering exams like CSIR NET.