What's a Watt?
Techie nerd tidbits relevant to Alternate Energy...
 
When discussing energy issues, it helps to have a rough idea of a few basic definitions and principles.
Definitions of common units of energy
A "Watt" is a unit most commonly used as an expression of electrical power (although it is equally valid as an expression of mechanical power). It is named after James Watt (1736-1819), a Scottish inventor who perfected the steam engine. Common expressions to remember:
        power(in Watts) = voltage (Volts) x current (Amps)
              (example: a 1200 watt hair dryer will draw 10 Amps at 120 Volts)
        milliwatt = 1/1000 of a Watt
        kilowatt (or KW) = 1000 Watts
        megawatt (or MW) = 1,000,000 Watts
 
A Watt is a unit describing the instantanious power that an appliance is drawing at any one moment. The total amount of energy used is found by multiplying the power by the length of time the appliance is on. This is most commonly expressed in "Kilowatt Hours" (or kw-hrs) and is what the power company generally uses to calculate your bill.
Having kilowatts of power available at our fingertips is taken for granted in the modern age. We often don't recognize what it would take to provide this power the old fashon way (which in olden days meant slaves or animal power).  When electricity was being introduced to American homes there were ads and articles calculating the number of human slaves a Roman nobleman needed to get the same power the modern home can have "for just pennies a day" (at least in 1930). A hard working human generates about 100 watts for a few hours at a time.  Given an electrical service of 10 KW it would require 100 humans to generate an equivalent amount of power! A slightly flawed comparison perhaps, but it is still worth keeping in mind.
 
In order to give you a feel for these quantites, here are typical values for everyday household items...
 
  Power  Hrs/Day Energy Notes
Solar Path Light 20 Milliwatts  6   .12 Watt Hrs   
Xmas light Bulb 5 watts   12   60 Watt Hrs   
Cable TV box (off) 10 watts   24   240 Watt Hrs   
Cable TV box (on) 100 watts   4    400 Watt Hrs   
100 W Bulb 100 watts  4   400 Watt Hrs   
PC (active) 200 watts   4   800 Watt Hrs   
Toaster 1000 watts  .25   250 Watt Hrs   
Hair dryer 1500 watts  .25   375 Watt Hrs   
Refrigerator 350 watts  8   2.8 KW Hrs  at 1/3 duty cycle 
Oil Furnace 1000 watts  8   8 KW Hrs  varies by season 
Electric stove+oven 7,000 watts  .5   3.5 KW Hrs  all elements on
Electric clothes dryer 7,500 watts  1   7.5 KW Hrs   
Electric space heating 20,000 watts  8   160 KW Hrs  varies by season 
A few observations worth noting:
Respect your kilowatts!
Other units....
Much of the rest of the world uses International Standard (SI) units based on Metric units of measurement. The common unit of energy is the joule, which equals 1 watt for 1 second. Since there are 3600 seconds in an hour, 1 watt Hr equals 3600 joules and 1 KW hr equals 3.6 million joules. One BTU equals 1055 joules and one kilowatt hour equals 3412 BTU.
 
In the US we tend to use horsepower to measure mechanical power. One horsepower is defined as 550 foot-pounds per second of mechanical work. This is equal to 746 watts. In Europe it common to rate engines using kilowatts. A European car might have an engine rated at 200 kw, which would be advertised as 268 horsepower here in the US.
 
To summarize:
 
A British Thermal Unit (BTU) is defined as the amount of energy needed to raise the temperature of one pound of water by one degree Farenheit. In the US, the BTU serves as the most common unit of measurement in a couple of important areas:
 
Heating and cooling - The oil burner in my furnace is rated at 80,000 BTU per hour. The window air conditioner in the den is rated at 8200 BTU per hour.
 
Describing the energy contained in various fuels - a few examples:
    a gallon of gasoline = 124,000 BTU
    a gallon of diesel fuel or heating oil = 139,000 BTU
    a gallon of propane = 91,000 BTU
    a ton of coal = 20,000,000 BTU
    a full cord of wood = 12,500,000 to 25,000,000 BTU (more on wood values)
 
It is a British Thermal Unit because its part of the old English system of measurements along with feet, pounds, rods, furlongs, etc. (There are some pretty odd measurements, click here) . 
Why is it a Thermal Unit British?
Energy is never created or destroyed (but it sometimes gets misplaced)
One of the first things we learned in high school physics was that energy is never created or destroyed, it simple gets converted from one form to another. In our modern energy infrastructure there are a lot of conversions going on all the time: heat to mechanical motion, motion to electricity, electricity to light or back to heat, etc. When discussing various energy schemes, it is often important to understand that some of these conversions happen more easily and at greater efficiency than others. Here is my simplified efficiency score card:
Sometimes just studying the efficiencies can indicate which path is likely superior. Lets take electric heat as an example. If you use natural gas to generate electricity, perhaps one third of the BTUs in the natural gas are converted to electricity. If you are a significant distance from the power plant, transmission losses may further reduce the power recieved at your home. You may have to burn 4 units of energy at the power plant to get 1 unit of heat at your home. You are far better off if you can pipe the natural gas to a furnace in your home and capture almost all of the heat directly. Efficiency matters. (Of course, you might use the electricity to run a heat pump, which can be a great improvement over direct electric heat in some circumstances.)