Powering the Big Units
Short Version: Energy is the ability to do something, and is measured in joules or calories or kilowatt-hours or in other ways. Power is how fast you do it, and is measured in watts or horsepower or in other ways. Your 2000-calories-per-day diet is the same as a single 100-watt light bulb burning all day. Let's take a closer look!
Friendlier but Longer Version: Suppose that you are an employee at a Pennsylvania power company. Your customers buy a lot of kilowatt-hours of electricity to run their microwave ovens and music players, but your power plant needs to be turned off for maintenance. Your boss tells you to buy some power from a hydroelectric company in Quebec but they don't have any kilowatt-hours for sale -- all the offer are megajoules. What do you do?