This question has come up a couple of times lately.

Did some rough guess on it based on the fuel an Onan uses for similar power.

Figure about 180 amps average, which appears to be what we are seeing due to thermal output reductions. About 2250 watts depending on the voltage maintained. That is about 80% output of an Onan.

IIRC, a 2800 Onan uses .5 gal/hour at full output, so .8 X .5 = .4 gal/hour

Van at 60 mph, getting 15 mpg would use 4 gallons. Add .4 gallons for the generator and it would use 4.3 gallons. 60 miles divided by 4.4 gallons = 13.6 mpg.

Substantially more than I would have guessed. It will get better quickly as the amps to the batteries decrease, but it could easily be at 180 amps for 1-2 hours depending on battery bank size and SOC.

If you ran the coach air off the inverter to get more cooling, you would probably lose about 1/2 that much so about .7mpg just for the AC.

Math check is encouraged

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