For this discussion, we mean energy as defined by physics. It is a concept that meny find difficult to understand.
It is the ability to do work. Work is done when a force is moved a distance. (W = F x D)
Energy may exist in many forms, here are some common examples:
 Heat (thermal energy)
 Light (radiant energy)
 Motion (kinetic energy)
 Height and weight (potential energy)
 Electrical
 Chemical
 Nuclear
In physics, energy is mostly measured in joules and watt hours. As these are quite small, on this website, we'll try to stick to MJ and kWh.
It takes about 4.2 joules of energy to raise 1 gram of water 1 degree celsius.
Power is a rate of using or generating energy. A watt is a joule per second. A kilowatt hour is 1,000 x 60 x 60 = 3,600,000 joules = 3.6 megajoules.
Electrical energy is usually measured in kWh (kilowatt hours)  1 kWh = the amount of energy delivered in one hour at a rate 1000 watts
A practical example,
 A kettle with 1 litre (1,000 g) of water starting with cold water at 20ºC.
 To heat 1,000 g water 80^{o}C would take: 1,000 x 80 x 4.2 / 1,000,000 = 0.336 MJ.
 The retail price for electricity is about 25 cents per kWh,
 So to heat a liter of water to boiling costs: $0.25 x 0.336 /3.6 = $0.02 = 2 cents
Units of Energy
Unit

Abbr

Joules


Megajoule

MJ

10^{6}

Kilojoule

kJ

10^{3}

Joule

J

1

Kilowatt Hour

kWh

3.6 x 10^{6}

Watt Second

Ws

1

calorie (physics calorie)

cal

≈4.184

Calorie (Food Calorie)

Cal

≈4,184

British Thermal Unit

Btu

≈ 1,055
