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The watt (symbol: W) is the SI derived unit of power, equal to one joule of energy per second.

A human climbing a flight of stairs is doing work at the rate of about 200 watts. An automobile engine produces mechanical energy at a rate of 25,000 watts (approximately 30 horsepower) while cruising. (25kW)

W = V*A 

You'll also see energy refered to as.. 

Amp Hours...
The Amp Hour rating tells you how much amperage is available when discharged over a period. The amp hour rating is cumulative, so for example... in order to know how many constant amps the battery will output for say 20 hours, you have to divide the amp hour rating by 20. Example: If a battery has an amp hour rating of 75, dividing by 20 = 3.75. Such a battery can carry a 3.75 amp load for 20 hours before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.) A battery with an amp hour rating of 55 will carry a 2.75 amp load for 20 hours before dropping to 10.5 volts. -- http://www.1st-optima-batteries.com/amp_hours.asp