COMMERCIAL UNIT OF ENERGY
The unit joule is too small and hence is inconvenient to express large quantities of energy. We use a bigger unit of energy called kilowatt hour (kW h).
Let us say we have a machine that uses 1000 J of energy every second. If this machine is used continuously for one hour, it will consume 1 kW h of energy. Thus, 1 kW h is the energy used in one hour at the rate of 1000 Js–1 (or 1 kW).
1 kW h = 1 kW ×1 h
= 1000 W × 3600 s
= 3600000 J 1 kW h
= 3.6 × 106 J.
The energy used in households, industries and commercial establishments are usually expressed in kilowatt hour. For example, electrical energy used during a month is expressed in terms of ‘units’. Here, 1 ‘unit’ means 1 kilowatt hour.
A certain household has consumed 250 units of energy during a month. How much energy is this in joules?