Most electrical quantities (resistance, voltage etc) are not immediately useful to consumers. Electrical power on the other hand, is very useful, and is often mentioned during appliance retail (eg. this is a 200W stereo system). Power (and electrical power) is a measurement of how many joules of energy can be changed from one form of energy to another form. It is measured in joules per second (or, equivalently, watts).
These slides introduce the concepts of power, electrical power, the kilowatt-hour, and provide some calculations.