The SI unit for power is the Watt (W).
A 100 W light bulb is left on for 8 hours overnight, how much energy did the bulb consume?
First we need to convert 8 hours into seconds (SI unit of time):
Next calculate the energy transferred:
That answer is of limited usefulness. Most people don't think in terms of large standard form numbers in their daily life. Energy companies charge people based on how many units of electricity/gas they used. A unit of electricity is 1 kWh (note this is a unit of energy not power).
1 kWh = power rating in kW × time used in hours.
For the lightbulb used overnight in the question above the energy consumption in kWh is:
The bulb consumed just under one unit of electricity, which may cost in the region of 20p.
Really simple with a good simple example here: http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel/generation_transmission_electricity/electrical_quantitiesrev3.shtml