#### notes

**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 J^{s–1} (or 1 kW).

1 kW h = 1 kW ×1 h

= 1000 W × 3600 s

= 3600000 J 1 kW h

= 3.6 × 10^{6} 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.