Electric power is the rate at which electricity does work--measured at a point in time, that is, with no time dimension. The unit of measure for electric power is a watt. The maximum amount of electric power that a piece of electrical equipment can accommodate is the capacity or capability of that equipment.

Example: A 500 watt hair dryer cannot handle more than 500 watts of power at any one time.

Electric energy is the amount of work that can be done by electricity. The unit of measure for electric energy is a watt-hour. Electric energy is measured over a period of time and has a time dimension as well an energy dimension. The amount of electric energy produced or used during a specified period of time by a piece of electrical equipment is referred to as generation or consumption.

Example: A 500 watt hair dryer that ran for ½ and hour at its medium setting (say, 250 watts) would consume 250 watts x ½ hour = 125 watt-hours of electrical energy.

 Example: Power Generation

A gas turbine has a maximum power rating of 50 MW (fifty megawatts, or 50 million watts).  If you ran it at 50 MW for an hour, you would have generated 50 MWh (fifty megawatt-hours, or 50 million watt-hours) of electric energy.

If it was a particulary difficult hour, and the turbine kept turning on and off, you may have been running it at 50 MW at any one time, but only for a few minutes at a time.  It may have only generated 20 MWh (twenty megawatt-hours) of energy over that hour of operation, even though it only ran at 50MW when it did run.

Some people like to think of the power rating (watt) as “watt-hour per hour”, but personally, I find that more confusing.  If that doesn’t confuse you, than think of this: power is like the speed of a car – kilometers per hour - and energy is like the distance traveled – kilometers.