1. Energy

Energy is "the capacity for doing work".

The unit of energy (or work) is the joule (J)

• 1 megajoule (MJ) = 1,000,000 J
• 1 gigajoule (GJ) = 1,000 MJ

Example:
1 MJ of gas is enough energy to enable you to do a certain amount of work, i.e. heat water by so many degrees celsius. Just like a litre of petrol can move a car a certain distance.

2. Power

Power is ‘using energy to do work’. Alternatively, power is the rate at which work is done or
the rate at which energy is consumed. The unit of power is the watt (W).

1 watt = 1 joule per second. That is, energy is being consumed at the rate of 1 joule each second, ie car cruising at a set speed might use X litres of petrol per hour.

Example:
A 100 watt light globe uses 100 joules of energy per second.

3. Watt hours and kilowatt hours

A watt hour is the energy transmitted or consumed when 1 watt is delivered in one hour. For example:

• 1000 watt maintained for one hour equals 1 kilowatt hour (kWh) of electrical energy
• ie 1kWh is the same as saying that 1kJ/s was consumed in that hour (since 1 W = 1 J/s).
  

Note:
1 kWh = 1 kJ/s x 60 sec/min x 60 min/hour
=     3600 kJ/h
=     3.6 kJ/h