Question

A table lamp of power $60W$ consumed $9$ (commercial) units of electricity in the month of April. For how many hours per day, on average, was the lamp in use?

• A. $1h$
• B. $3h$
• C. $5h$
• D. $6h$

Answer 1

The correct option is C $5h$

The basic unit of electricity is the Kilowatt-hour ($kWh$). In simple terms, $1kWh$ is the amount of energy used by a $1kW$ ($1000watt$) electric heater for $1hour$.

$9units=9kWh$
For the month of April ($30days$), we get an average of $\frac{9000}{30}=300Wh$ energy per day.
For an appliance of $60W$, this clearly corresponds to

$t=\frac{E}{P}$

$\Rightarrow t=\frac{300}{60}$

$t=5h$