How many btu does a whole house generator use?

The BTU usage depends on the size generator. What you need to know to figure this out is the fuel type (propane or natural gas) and the size of the generator (usually measured in kilowatts or KW) and the size of the engine. Once you know these things, you can calculate the BTU. The easiest way is to ask the sales person or manufacturer. They can tell you the BTU.