Energy and Power
Power is also called wattage, because its unit is watt (W) and kilowatt (1 kW = 1000 W).
Energy and power are closely related, but they are not identical.
Energy is an absolute quantity, like the chemical energy in a cord of wood or a litre of gasoline, or the electrochemical energy in a battery, or the electrical energy on your Hydro-Quebec bill.
Power is the energy per time. You can burn a cord of wood very quickly, in a large fire (high power), or you can burn it slowly, in a small fire (low power, lasts longer). In the end, the result is the same: all the chemical energy will be transformed into heat.
The same is true for a car. The harder you push the gas pedal, the more power your engine develops, the more millilitres of gasoline are transformed into motion and heat per second. Power is the rate at which energy is transformed.
In everyday life, power is used as a property for machines and appliances.
Toaster example
A 1400 W toaster consumes 1400 Wh per hour, or 1.4 kWh per hour. Of course, a toaster does not run for an hour without stopping. If we assume that two pieces of toast take 3 minutes to toast, we can calculate the energy required:
1400 W * 3 min = 1400 W * 0.05 h = 70 Wh
Battery example
We have a battery with an energy capacity of 1200 Wh, or 1.5 kWh. (For example a 12V-battery with 100 Ah) We would like to know, how many hours we can run a television that consumes 50 W:
1200 Wh / 50 W = 24 h.
In theory, we can watch TV for 24 hours. In real life, there will be losses, for example from the inverter. This means we won't quite get 24 hours of run time. For this kind of calculation, you should always keep a margin of error.