I can answer that.
100% gasoline is 114,000 btu per gallon, but with the 10% ethanol blend they are forced to sell us these days (gotta love the farm lobby), that number is reduced to about 108,000 btu per gallon. Propane is about 92,000 btu per gallon. So the raw math works out to 1.17 gallons of propane to create the same energy as 1 gallon of E10 gasoline. So using $4.00 a gallon gasoline, the break even cost would be $3.42 for a gallon of propane to be the same $ cost to operate an engine for the same period of time. However, in the real world of mechanics vs. mathematics it is a little closer than that as the propane is a cleaner more efficient burning fuel so the real number is somewhere in the middle, more like 1.1 gallons of propane being equivalent to 1 gallon on E10 gasoline, so figure the real world break even cost of about $3.64 per gallon. So right now, if you can buy the propane for $3.64 per gallon or less, it is cheaper to operate than a gasoline generator at $4.00 per gallon. Flying J usually runs .10 to .40 cheaper than gas depending on the day, and usually rv stores and the like are usually a little cheaper.
One big benefit of the propane though is engine life. On the average you can double the engine life on propane. Clean dry fuel with no contaminants to get in the oil and sludge up the works and cause wear. One misconception though, the oil will stay nice and clean looking but you still need to change it regularly as it still will break down like the oil in any other motor, it just does not look bad like other engines.
I know none of that answers your question of gallons per hour to operate, but I cannot answer that question for you. Every engine size and generator size will use a different amount of fuel, as will the electrical load you are pulling. If you are just running your lights the generator will idle along and burn very little fuel. Turn on your air conditioning and the engine revs up to meet the demand and starts sucking down the fuel. Check the specs for the generator you are looking at. They will all give you a fuel consumption figure both at idle and max output. Real world you will never get either number, it will be somewhere in the middle. For normal use (lights, battery charger, tv, etc. plus the occasional use of the microwave and refrigerator) you can figure about 125% of the idle consumption, and more if you are running the a/c but all that is a shot in the dark until you get out in the boonies and let it run for a weekend. But that 125% figure should be close enough to figure tank sizing. Also keep in mind nobody runs the thing 24 hours a day, so think about how long you will realistically run it. If you are not running the a/c, most people just use it to keep the batteries charged and run the lights and tv and stuff off the batteries, and just run the generator as needed.
While I'm on my political soapbox, that E10 gasoline/ethanol our wise government has foisted upon us takes 5.5% more gallons to produce the same energy as compared to 100% pure gasoline. That's right folks, the Uncle wants you to pay an extra 5% every time you drive your car, all in the name of the farm lobby paying off politicians so they can force us to buy more corn, and not so coincidentally driving up the cost of everything you eat because we are diverting our foodstuffs to burn up in our cars, artificially raising the demand for and therefore the price of corn. So you say "but I only eat corn at picnics", ah, but did you ever eat a cheeseburger? Or chicken? Or bacon? They all eat corn. Or bread? More acres in corn for ethanol equals less acres in wheat and higher prices for bread.
Sorry, you got me started on one of my pet peeve issues. Always a danger with me.
