IDK if this is going to get any responses, but if you have any experience with using propane with a portable generator maybe you can explain what's going on.
So I got a big generator, I previously only used gas, but as this was new and dual fuel, it seemed like propane might be a big win. Propane as far as I know doesn't get old like ethanol gas does, won't gum up small engine carbs like gas does.
However, it had some downsides - the tanks are not able to just have an extra gas can to refill while the generator is running if needed, and for some reason I can't tell the manual gives 0 estimated runtime with propane, but lots for the gas fuel. Ok, well some searching found a Y connector with a kind of switch/indicator that is supposed to auto failover to the second connected tank if the first one empties so you can then change out the tank while the generator is running.
Now my problem. Power goes out last night, it's 20F and I fire up the new generator for the first time. First hour, no issue, however it then starts almost stalling out and then restarting over and over again. I go look at it seems like the "switch" indicated it tried to change tanks but... maybe didn't? It went straight up and down, not pointed to the other side (though IDK if I actually understand the switch, there were like 0 instructions with it from Amazon). OK, I'll just figure out which tank is empty (wondering how it went empty in like an hour on a 20lb tank) and move the switch to the other one and then change out the tank with my spare. Did all this, no change. Cannot get the generator to run right, and cause constant brownouts to my house and the generator makes a sound like it's backfiring every so often. I give up on backup power for the night.
Today, I go look at it again, and it starts up and runs fine today at 43F. However I haven't put a load on it, but it wasn't running right without a load last night, so I don't think it was overloading the generator (and I know it wouldn't given earlier uses when it was warmer). OK, well lets at least use the valves on the top of the 20lb tanks to test the switch over thing. I tried turning off one of the tanks (right), the one the switch / arrow is pointing towards. Nada, generator keeps running, switch doesn't do anything. I turn off left and generator stalls out. Weird. I then reset, restart, and try turning off the left tank - no change keeps running. I then re-open left and close right, no change, generator keeps running. Just to not lose my mind, I also close left and as expected generator stalls out.
Ok, so - do I have a worthless amazon transfer thingy, is the propane just not working at 20F or below? This seems really weird as I have a 500 gallon tank for my entire life for heat and stove and it got down to like -15F a few times with no issue.(I asked my provider, they say they can't hook up the portable generator, or even provide me a propane hose / valve/ anything I could hook it up to.) The 20lb tanks are brand new... So some googling seems to say maybe the tanks need to be warmer? I could get some tank heaters I guess and plug them into the generator also assuming it can run long enough for the heater to do anything and bootstrap stuff when it's cold. However, I'm also concerned about not having any estimate how long it should run from what should be 40lbs of propane. I don't know if the first tank leaked over the months since I set it up (the other 2 didn't), but 1 hour seems really fast to run out, and I would expect at least 8 hours when the gas tank is supposed to be good for 18.
So - do I just give up on propane for this generator? It seems silly to keep propane for the warm months and then switch to gas for the winter...
So I'm a propane guy (I'm in the biz) but not a generator guy, so I'll give my knowledge on best guess.
So your 500gallon house tank you never had issue with never ran a higher demand thing like a generator. They suck more gas. Not an issue with a 500g tank though (which when full holds 400 gallons). The liquid doesn't have to quickly boil off to make more gas because there's so much more room and the size resists the rapid temp swings from the phase change going from liquid to gas.
A 20 pound cylinder only holds about 4.6 gallons of propane (and that's if they actually filled it all the way. Many will only fill them to 15 pounds). So your issue is as the number one (at this time, anyhow) poster says. The tank was having to convert so much propane from liquid to gas so quickly that it dropped the tank down to a temp too close to below the boiling point of propane, so there wasn't the 10 or more psi of pressure moving to keep your generator running. Once the liquid gets down to about -30f you wont have enough gas pressure. So you'd have to keep your tank warmer, which isn't really an easy thing to do without being dangerous.
What you need to do is get a bigger tank(at least if this is made to be a permanent generator and not a movable/temporary generator).Call around to some propane companies. You'll probably find someone who will lease you a 330 gallon tank for like $75 a year and hook the system up, but they probably won't do it if it is a temporary style generator. If that's the case, get yourself a 100 gallon tank if you think you can load/unload it from your vehicle when you need to go fill it back up. The 100 gallon ones are just tall versions of 20 and 30 pound tanks. Same fittings and stuff.
Now to answer your question about fuel and how long it should last. Like I said before, a 20 pound tank holds around 4.6 gallons of propane. Propane has almost 3/4 the energy as gasoline. So a 20 pound tank (if filled all the way and not just filled to 15 pounds) should last about as long as however long a bit over 3 gallons of gas lasts.