A lamp consumes 500 J of electrical energy in 20 seconds. Find the energy(kWh) consumed by the lamp in units if it operated for 2 hours per day for 20 days.

Given:
Energy = 500J
Time = 20s
$\mathrm{Power}=\frac{ \mathrm{Energy} }{ \mathrm{Time} }$$=\frac{500 \mathrm{J}}{20 \mathrm{s}}$
$\Rightarrow \mathrm{P}=25 \mathrm{W}$
Total time for 20 days at the rate 2h per day = 20 x 2 = 40 h
Units of energy consumed = Pt
$\Rightarrow \mathrm{E}=0.025\times 40$
$\Rightarrow \mathrm{E}=1 \mathrm{kWh}$
$\Rightarrow \mathrm{E}=1 \mathrm{unit}$              [Since, 1kWh = 1 unit] 
 A lamp consumes 500J of electrical energy in 20seconds. The power of the lamp is 25W and the energy consumed by the lamp in units if it operated for 2hours per day for 20days is 1unit.