Definition: A British Thermal Unit (BTU) is the amount of heat energy needed to raise the temperature of one pound of water by one degree F. This is the standard measurement used to state the amount of energy that a fuel has as well as the amount of output of any heat generating device.
You might be able to imagine it this way. Take one gallon (8 pounds) of water and put it on your stove. If the water it 60 degrees F/15 degrees C and you want to bring it to a boil (212 degrees F/100 degrees C) then you will need about 1,200 BTUs to do this.
This implies a closed system, of which your stove, gas grill, or any other piece of cooking equipment isn't. BTUs are wasted, of course, but this is the general rule.
All combustible materials have a BTU rating. For instance, propane has about 15,000 BTUs per pound. Charcoal has about 9,000 BTUs per pound and wood (dry) has about 7,000 BTUs per pound. This gives you an idea of how much fuel you'd need to, say, cook something.
When it comes to gas grills they all have a maximum BTU rating per hour. If you see a 35,000 BTU gas grill that means that that grill puts out 35,000 BTUs from all its main burners combined in one hour, or uses a little more than 2 pounds of propane an hour. While the BTU rating on a gas grill doesn't necessarily tell you how much heat it will produce, it does give you a general idea of its heat output, and it tells you how much fuel you'll be burning. Other factors related to how hot a grill will get, or how well it will do high temperature cooking have to do with construction, how heat is held inside the grill, and the mass of the cooking grates.
Also Known As: British thermal unit, BTU, Btu, B.Th.U.