What is this generator being used for-- a commercial building? It must
be huge to comsume that much fuel. If you are correct about the
consumption and want it to be able to run 40 hours, you will need 112
gallons.. The most common size tank in that range, with some to spare,
is 250 gal. All propane tanks are only filled to 80% capacity, which
would be 200. The main thing is that most propane companies will not
deliver less than 100 gallons, and the few that do, charge so much extra
per gallon that you will pay as much for 50 or 60 as you would for 100.