upsnake,
I don’t know of any portable generators that idle, or change speeds… they all run at 3600 RPM whether-or-not there is load. Mine doesn’t have any sort of throttle lever, or knob, or whatever… it runs at 3600 RPM continuously, even with nothing hooked to it. Actually it can damage the generator head to run it a less than rated RPM, even with no load.
But… a small 4-cycle gasoline engine running at 3600 RPM is not at “wide-open-throttle”. The throttle plate is controlled by the governor, which opens the throttle only as much as is needed to maintain the RPM’s. With no load applied to the engine, at 3600 RPM, the throttle plate barely moves from idle position… only under load does the throttle plate open further. The amount of fuel used is in direct proportion to how much air is flowing through the carburetor (or past the throttle plate… or how far the throttle plate is open).
The generator head, turning at 3600 RPM is making full AC voltage (120 or 240 depending)… but if nothing is plugged in to pull amps there are no watts being produced (i.e. zero load on the generator head and near zero load on the engine). Watts are a measure of power (amps x volts), and if nothing is using power (pulling amps) there are no watts being produced by the generator.
When something is plugged into the generator it places a load on it, which places load on the engine, which causes the throttle to open in order to maintain RPM. As you increase the load (amps) at any given volts, there will be more load placed on the engine, causing it to use more fuel, and the more heat created by the generator head. That heat is hard on the generator head… and it is lost energy (watts) that is reducing the relative efficiency of the generator/engine combination.
A 13-HP engine/5500w generator, running 3600 RPM, at 20% load (1100-1200 watts) will use a whole lot less fuel than a 4-HP engine/1500w generator, running 3600 RPM, at 80% load (1100-1200 watts)… The larger generator will hardly make any damaging/wasteful heat and would take near forever to wear-out, but you’d likely burn up the 1500w generator in well under 100-hours.