such that:- total energy = power × time.
For example a 40W light bulb kept on all day uses 40W x 24h = 960Wh (Watt hours), or approximately 1kWh.
or a 1000W electrical heater kept on full power for 1 hour uses 1000W x 1h = 1000Wh (Watt hours), or again 1kWh.
To be clear the kWh is used for all forms of energy not just electrical, including gas, petrochemical, and coal etc. You will often find the energy relesased on combustion of these substances quoted in joules or calories, all we have done is convert these numbers to kWh. Incidentally we have found that many of the quoted energy values on the internet for these materials are wrong, particularly on internet sites selling them, so please beware.