Propane generators are used to generate electricity for various reasons, such as lack of access to power lines, backup for buildings in the event of power failure and being in a remote or temporary location. Generally, the bigger the propane tank is, the longer your generator runs. You can determine how big a tank of propane you need by taking into account what you plan to use the generator for and how long.
Find out how much propane the generator uses an hour. Most propane generators come with documentation that tells you how many gallons of propane it uses an hour to create the amount of electricity it generates. If you do not have the documentation, contact the generator manufacturer and find out fuel consumption (FC) per hour in gallons.
Determine the amount of electricity you plan on using and how many hours (D) you plan on running the generator. The longer you use a generator consistently, the more propane it uses and therefore the bigger tank you should get.
Calculate how much fuel you will need for your project. (FC) x (D) = Propane tank size. For example, if you wanted to power a home with electrical appliances for 48 hours and you knew your generator used 2 gal of propane every hour it was in use, you would calculate (2 gal hr) x (48 hr) = 96 gal. So you would purchase a tank at least 100 gallons in size.