In order to describe the amount of electrical power put out by a source of renewable energy people often make reference to the units "kilowatt" or "megawatt," or the related units "kilowatt-hours" and "megawatt-hours." This can get a little jargony unless you're an engineer or other aficionado of electrical terminology. Even in press addressed to the lay person, the terms are hardly ever defined. So here's a quick explanation.
A watt (abbreviated W) is a unit of power. Power is the rate at which energy is generated, consumed, or otherwise transformed. That's the textbook definition, and it's still a bit opaque. Most of us have a rough idea of the difference between a 40W and 75W light bulb, namely that the 75W puts out more heat and light, and it uses more energy to do so.
Watts are closely tied to a couple other units of electrical measurement you've probably heard of. Anyone who's bought batteries has heard of volts, which is a unit of electrical potential, and if you've rummaged through a hardware store to find the right size fuse for your old-style fusebox, you'll be familiar with amperes, aka "amps," which are units of current.
The easiest way to explain the interrelationship of these units is through metaphor. Electricity consists of electrons flowing through a conductor. Imagine your electrical circuit, with a power source at one end and an appliance at the other, is actually water flowing through a dam from a reservoir (the power source) and spinning a turbine (the appliance). The amount of water flowing through the dam is the current (amperes). The height of the dam is the potential (voltage). And if you multiply the amount of current by the height of the drop it's flowing down, you get the amount of power available to spin the turbine (watts). It's not a perfect metaphor, but it's close enough. A watt is the power generated by one ampere of current flowing at one volt.
A watt isn't a huge amount of electrical power: if you move a 12-oz. bag of coffee on supermarket shelf and place it a foot away, and if this task takes you one second to perform, you use about a watt of mechanical power doing so. A moderately strong person with a physically demanding job can average about 75 watts of mechanical power during the course of a workday. Which means if you turn on a 75-watt lightbulb, the amount of power you're using to light your closet could theoretically be building you a new deck instead.
Of course, our appliances tend to use a lot more watts than that. A refrigerator and freezer might use 600 watts each, a microwave between 1,000 watts and 2,000 watts, and an AC unit starting up might consume as much as 5,000 watts. With each household generally possessing quite a few electrical appliances, and millions of households in the United States, writing out all those zeroes would get tiresome. So to describe the numbers, we use the same prefixes found in the metric system: kilo-, mega-, giga-, and so forth. Each unit is 1,000 times the size of the next smaller one:
A thousand watts is a kilowatt.
A million watts (or 1,000 kilowatts) is a megawatt.
A billion watts is a gigawatt.
A trillion watts is a terawatt.
To give some idea of the scale of these progressively larger units:
A typical household rooftop solar installation might have a peak output of 10 kilowatts.
A single large wind turbine, similar to the new ones being installed in the California desert, maxes out at about 5 megawatts.
A moderate-sized utility-scale coal-fired power plant (or big desert solar plant) might run at around 500 megawatts, though some will reach a gigawatt of capacity.
In 2010, the United States had about 1.14 terawatts of electrical generating capacity.
There's one more unit to consider! All the above figures for appliances and generating stations express the unit's power generation, or consumption, at any particular moment in time. But we don't consume electricity for just a moment at a time, nor do we generate it that way. We create and use electrical power over time, so we factor the length of that time into the units we use. Running a one-kilowatt space heater for an hour thus consumes 1 kilowatt-hour of electricity. A 5 megawatt wind turbine turning at full speed for an hour generates 5 megawatt-hours of electricity. If you open your closet door and turn off the 75W lightbulb someone left burning in there, you'll conserve 75 watt-hours for every hour after that until someone turns the thing back on again.