Propane pressure from tanks

Ask your propane supplier, they're the experts. When I was doing my gas fitter training, we spent about 10 minutes on propane tank/cylinder sizing because, as my instructor said, it's only the supplier who really needs to know.

Also concider having them install the tank under ground.
Sure looks better then having a baby hipo out in the yard.
With a larger tank it will need to be located away from the house not next to it.

2-250 gallons tanks. Or 1-500 gallon tank. Generators use a large amount of gas, so the tank needs to have a lot of surface area for the propane to boil off to vapor. 100 gallons tanks don't have enough surface area, so if you were on the generator on a cold night, you won't have enough gas pressure to run the furnaces also, and maybe not enough to keep running the generator.

2-100 gallon tanks, only gives you 160 gallons of propane after they are filled(they only fill them to 80% to have room for expansion).

For reference information only, I have one 250 gal. above ground tank that does a good job supplying a 80,000 btu furnace at the same time as a 36,000 btu water heater when the outdoor ambient temperatures is -21°F.

Generators use a large amount of gas, so the tank needs to have a lot of surface area for the propane to boil off to vapor. 100 gallons tanks don't have enough surface area, so if you were on the generator on a cold night, you won't have enough gas pressure to run the furnaces also, and maybe not enough to keep running the generator.

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Ayuh,... Agreed,... The tank valves tend to freeze up, under high usage like that too,...

Fairview said:

For reference information only, I have one 250 gal. above ground tank that does a good job supplying a 80,000 btu furnace at the same time as a 36,000 btu water heater when the outdoor ambient temperatures is -21°F.

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A single 250 gallon tank should have enough surface area to handle a 5000 watt generator.

With a 5000 watt generator averaging 4000 watts continuous output, it will use roughly 1 gallon an hour. Your 80,000 BTU furnace if its running continuous would be using .87 GPH. So your 250 gallon tank right after it was filled would handle the furnace and a 5000 watt generator for 106 hours by content. It would only suffer when the tank got low. Say 50 gallons left.

The reason I recommend 2-250 gallon tanks. is because you the OP may not be able to get a propane delivery just before a power outage. And 80 gallons of propane may only last him 40 hours. After that, he might as well not have a generator.