There are 4 electric bulbs of 50 W (each) are used for 5 hours per day. Calculate the energy consumed in the month of January.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 31 kWh
Electrical energy used = power × time. With power in kilowatts and time in hours, the energy comes out in kilowatt-hours (kWh), the “units” of an electricity bill.
Power of the four bulbs \(=4\times50=200\,\mathrm{W}=0.2\,\mathrm{kW}\)
Energy used in one day (5 hours) \(=0.2\times5=1\,\mathrm{kWh}\)
January has 31 days, so the energy for the month \(=1\times31=\) 31 kWh
Since \(1\,\mathrm{kWh}=3.6\times10^{6}\,\mathrm{J}\), the same energy is \(31\times3.6\times10^{6}\) J. Option (4) prints this number with the unit “J kWh”, which mixes two units, so only Option (1) states the energy correctly.