Five bulbs each of 100-watt capacity in a tunnel are illuminated continuously for twenty hours. The consumption of electricity will be:

Five bulbs of 100 W each give a total load of 500 W, which is 0.5 kW. Energy is power multiplied by time, so 0.5 kW x 20 h = 10 kWh. One commercial unit of electricity is one kilowatt-hour, so the consumption is 10 units, option C.

The core concept is the energy-power-time relation and the unit conversion from watt-hours to kilowatt-hours. Mains papers often connect this to energy efficiency: tunnel and street lighting are major consumers, which is why the shift to LEDs under schemes like UJALA matters for distribution losses and subsidies.

• A (one unit) would need only 50 W of total load running for 20 hours, or 100 W for 10 hours. It is a common slip when someone forgets one of the bulbs.
• B (two units) corresponds to about 100 W running for 20 hours, which ignores four of the five bulbs.
• D (twenty units) needs a 1 kW load. It appears if you convert 100 W per bulb into 200 W, or skip the division by 1000 when converting watts to kilowatts.

Trap: Candidates forget to convert watts to kilowatts before multiplying by hours, and then pick a number that is 1000 times too large or small.