Our boat's Westerbeake is a conventional 1800 RPM "constant" (a/k/a conventional) generator. In the case of the diesel gen sets, the fuel use varies as the engine loads up when the electrical load changes. Think of it as having a car, not changing gears, and maintaining the same speed as you go up am
Ned down hills.

The controls are much simpiler and a properly sized engine reputedly lasts longer. Our fuel use, on our 7.6 KW unit, varies from aboutnan1/4 gallon per hour and 1.5 gallons/hour. At "max" working load, the unit is rated for about 50 amps at 115 VAC.

One issue not to lose sight of is the start-up loads for motors and compressors that you might have on your refrigerator, AC units, etc. The inverter controlled units used to have a reputation for not liking those. The "conventional" constant RPM gen sets just show a slight voltage drop or "surge", the inverter controlled units, which vary RPM have a more complicated task to deal with.

I am not sure of haw the natural gas and propane units function differently as one varies the "demand" for fuel bases an load.