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What ever you figure out you need add 50%. And go with diesel or propane, storing large amounts of gas sucks
Just curious, but if a generator can last, say, 5 hours on a tank of gas, how long could it last on a cylinder of propane (the kind I have for my grill)?
Rough estimates are fine here...
Propane has abt 90,000 BTU per gallon.
Gasoline abt 140,000 BTU per gallon.
Roughly.
A 20 propane take filled w/ 18 pounds of propane has about 4 gallons of 90,000 BTU per gallon fuel.
So that's roughly abt 2 2/3 gallons of gasoline equivalent in a 20# propane tank w/out doing the calculations.
You're going to need a bigger gen set than 15kw. I'm thinking at least 25kw if not closer to 30kw. You don't want to be running that thing at 100% capacity for very long.
EXPY37 is right about these figures, so why don't we look at them a little closer.
Engines waste 2/3 of the fuel they burn. Basically, only 1/3 of the fuel burned is used in creating mechanical motion, 1/3 goes out the exhaust, and 1/3 is lost to the cooling system.
What if you were to forget trying to power up that massive 220vac/100amp monster heater, and instead bought a smaller gen set and tried to recoup the waste going out your exhaust and cooling system to heat your home, or help heat your home?
If you burn one gallon of fuel an hour and created 114,000 BTU's, and you could use the waste just in the cooling system, you would have a 38,000 BTU's heater in your home. Since one watt is equal to 3.4 BTU's you are saving around 11,000 watts of power that a gen set would need to create.
If you were to mount a radiator with quick connects inside your home, and put an electric floor fan behind it, now it is just like having a huge heater core in your home. This is what they do in cars to heat them in the winter time. The quick disconnects are so you can use the gen set in the summer time to run the AC and cool your house. Just mount the radiator outside.
Now if you use one of those chimney heat exchangers in the exhaust, you could recoup even more wasted heat.
The money you save from not buying a bigger gen set would make these two projects easy to afford. You're still going to need a good size gen set from the sound of it, so that means a liquid cooled engine. You will be saving on fuel as well because you're not burning as much to make huge amounts of electricity, but I still think you may be close to a gallon an hour by the sound of things.
I'm not sure what your AC needs are, but here there's no getting around it, you need electricity. Your AC is only lowering the temp of your home say 20 degrees, but your heater maybe trying to raise the temp as much as 80 to 100 degrees from the outside temp so heating demands are much larger.
AC units this big are not rated in BTU's, they're rated in TONS. The ton rating is actually pretty simple. When AC was first invented, it was never designed for cooling a home, it was made to make ice. Since ice is sold by the pound, people who sold ice wanted to know how much water it could freeze in a 24 hour period, and the TON rating was born. If a unit was 5 ton, then it made 5 tons of ice in 24 hours, or froze about 1250 gallons of water every 24 hours. One ton of water in about 250 gallons.
Just for ease of numbers, there's 12,000 BTU's per ton of AC, or about 3,530 watts, so you can do the math to size your gen set.