I found a website that was a gold mine of information on generators - regarding operation, selection, usage, etc.
Propane and propane generators
Note this suggestion for any brand per se (principles)
Picking a generator by engine----It takes 2 HP to create 1000watts of power
Anything less is using gearing, pulleys, etc to "cheat" power from a smaller engine thus working it harder taking a toll on engine life. So, check out engine size and output to rated generator output. (PS. Honda gens have belts)
To compare fuel usage use BTU as comparison unit.
Each horsepower consumes 10K BTU per hour
Propane = 92KBTU/Gallon and weighs 4.2lbs/gallon
Gasoline (E10) 111K BTU/Gallon
Natural Gas= 84K BTU/Gallon
How long would a 5000 Watt Generator with a 10 HP engine at 50% load run on a 20# propane cylinder?
10hp at 50% load would be using 5 horse power to generate 2500 watts of energy.
5hp x 10,000 btu would consume 50,000 btu per hour.
Using a 20# cylinder that produces 441,600 total btu, the engine consuming 50,000 btu per hour would run for about 8.8 hour
From looking at the above info of general info for any generator, the 2400isHC is one heck of a generator for someone who just crosses the 2000 watt threshold but does not need 3000 watts on a regular basis. It has a large 5.5 horsepower engine that would technically be able to handle a 2750 watt load without any special gearing, etc. Plus the unit has a backup capacitor to boost output on heavier loads. Someone on the Internet in an article tested the 2400isHC with a load bank and reported that it held 30 amps (around 3400 watts) for around 10 seconds. So, that would mean that anyone should be able to start and run a 13.5K AC with one.
On fuel usage - there is a test by an RV magazine I read that compared Honda, Yamaha and some other brand. The models were similar. Honda 2000, if I remember correctly did better consumption-wise on lighter loads/idle but on 1/2 loads or more, it was working harder (see engine size) and consumed more than the Yammy. There was, I believe, an overall difference of a tenth of a gallon of gasoline consumption between the Honda 2000 and the Yamaha 2400. In this I gave the advantage to the Yamaha with the added backup power.