How is joule watt watt-sec kilowatt-hr related?

Answer 1

The Joule is the unit of energy (how much work is done?), and the Watt is the unit of power (how quickly is the work done?) Power is the RATE at which energy is used. If a load draws one watt of power from the source, it consumes one Joule of energy every second, so a Joule is the same as a Watt-second. The Watt-second is an old unit for measuring energy, and has been replaced by the Joule, which is now the internationally accepted unit of energy. A 1000 Watt heater uses 1000 Joules of energy every second, and that's (1000w x 60minutes x 60seconds) Joules of energy per hour, or 3,600,000 Joules (3,600 kilo-Joules or 3.6 Mega-Joules).

A 100 Watt light bulb that's turned on for 10 hours will use the same amount of energy, (100w x 10h x 60m x 60s) but it uses it at 1/10th of the rate. It has a lower power, so it uses less energy per second. In both of the examples above, though, the same energy has been used despite the different powers, because the product of power x time is the same.

Fairly obviously, the Joule and the obsolete Watt-second are not big units of energy. If you got your energy bill in Joules there would be LOTS of zeros on it, so suppliers use Mega-Joules as their unit for cooking gas billing. Electrical energy is measured in kilowatt-hours, where one kilowatt hour is 3.6 Mega-Joules, or 3,600,000 Watt seconds of energy.