[Dr. Jerryrigger]

Okay, it would be good to have a ball park idea of how much propane this thing uses. So I'm just making up most of these numbers, if anyone has a better idea of what would be accurate please plug those in.
Okay so at full load it can put out 3400watts, so lets say the genorator and pulley has an ef of 65%, so it would need about 5230watts from the engine.
Giving the engine an ef of 15% it would take about 35,000 watts of propane, which is equivalent to about 120,000BTU per hour. A gallon of propane has about 91500 BTUs so that's 1.33gal/hr and there are 36 cubic feet of propane in one gal of liquid propane so:

It will take something like 48 cubic feet of propane per hour to run this generator at full load.

I think I've used nearly that from a grill tank before (a really big burner that was run at 7psi off of a welding regulator). But that is well above the capacity of a normal grill regulator (i think).