Question

Commercial unit of power is
A. kilowatt
B. kilowatt hour
C. kVAh
D. All

Answer 1

Hint: This is an informative question, we will be able to answer it if we have the knowledge about different units of electrical power and electrical energy. We will see how power is calculated commercially to answer the question.

Complete answer:
Commercial unit of power is kilowatt hour. One kilowatt hour is defined as the power consumed by an electrical device of $1kW$ power in $1$ hour. It is written as
$kWh$ .So If we need to convert $1kWh$ in calorie, We need to follow the following steps. We will first convert $kWh$ in $Wh$ ,then we will convert $Wh$ in $J$ , then we need to convert $J$ into $cal.$
$1kWh=1000W\times 1h=1000\times 60\times 60J=3.6\times {{10}^{6}}J=\dfrac{3.6\times {{10}^{6}}}{4.2}cal=8.6\times {{10}^{5}}cal$
So let’s say if a $60W$ bulb glows for $10$ hours a day then the energy consumed in a day in $kWh$ is given by
$\dfrac{60}{1000}\times 10=0.6kWh$ ,So it will consume $0.6$ commercial units of power.

Hence, the correct option is (C).

Additional information:
Kilowatt hour is also called Board Of Trade (B.O.T) unit. So $1B.O.T=1kWh$ , To solve numerical problems regarding calculation of B.O.T units, we first need to convert $W$ into $kW$ in case of power and we need to convert the time in $h.$ Then we need to multiply both of them to get the correct value of commercial unit of power.

Note:I
In this question we are using the term ‘power’ for ‘electric energy ‘itself which we use generally as it is supplied to our houses and not for electrical power . Electrical power is different to that. There should not be any confusion about this. Here we need to remember that $kWh$ is a unit of electrical energy and not electrical power. The SI unit of electrical power is watt and it is defined as $1W=1J/s$ . The bigger unit of electrical power is $kW$ . where $1kW=1000W$ . Now in case of $kWh$ ,we are multiplying $kW$ with the unit of time $h$ .So it actually gives the amount of energy consumed and not the power. So we need to remember to multiply the time.