Question

An electricity utility company charges ₹7 per kWh. 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%

Hint:

Always convert watts to kilowatts and use the formula: Energy = Power × Time × Days. Then multiply by rate to calculate cost.

Answer 1

The Correct Option is B

First, convert the power rating to kilowatts:
\[ 40\ \text{W} = \frac{40}{1000} = 0.04\ \text{kW} \]

Case 1: Desk light used for 10 hours per day
\[ \text{Energy} = 0.04 \times 10 \times 180 = 72\ \text{kWh} \]
\[ \text{Cost} = 72 \times 7 = \u20B9504 \]

Case 2: Desk light used for 12 hours per day
\[ \text{Energy} = 0.04 \times 12 \times 180 = 86.4\ \text{kWh} \]
\[ \text{Cost} = 86.4 \times 7 = \u20B9604.8 \]

Percentage Increase:
\[ \frac{604.8 - 504}{504} \times 100 = \frac{100.8}{504} \times 100 \approx 20\% \]

Therefore, the percentage increase in cost is 20% and the original cost is ₹504.