$(a)$ What is the commercial unit of energy? Give its relationship with the $SI$ unit of energy.
$(b)$ An electric bulb of $60 \,W$ is used for $10$ hours every day. How many units of electrical energy are consumed by this bulb in one day?

(N/A) The commercial unit of energy is kilowatt-hour $(kWh)$.
$1 \,kWh = 1 \,kW \times 1 \,h = 1000 \,W \times 3600 \,s = 3.6 \times 10^6 \,J$.
$(b)$ Given: Power $(P) = 60 \,W = 0.06 \,kW$,Time $(t) = 10 \,h$.
Energy consumed $(E) = P \times t = 0.06 \,kW \times 10 \,h = 0.6 \,kWh$.
Since $1 \,kWh = 1 \text{ unit}$,the bulb consumes $0.6 \text{ units}$ of electrical energy in one day.