We used our 8000w Onan successfully at well over 9000’. I don’t know if it is because ours is diesel or just the model, but it charged our batteries just fine sitting high in the Bighorn Mountains in Wyoming on a couple of rainy overcast days. I am sure it had less power than at sea level, but it didn’t take any extraordinary time to charge the batteries.
It won't take more time to charge your batteries at a higher elevation, if the genny doesn't keep on dying. What happens is the genny dies under a very heavy load well before you get to that 8KW capacity.
Diesel is more efficient because a carbureted gasoline engine runs rich at altitude in the thinner air. Diesel doesn't care about the air to fuel ratio until it gets rich enough to start "rolling coal", i.e. blowing black smoke (unburned fuel) out the exhaust. This advantage goes away with a fuel injected gas engine that monitors the air-fuel ratio using an oxygen sensor on the exhaust. Both diesel and altitude compensated gas engines lose about 3% of their output per 1000 ft. of altitude.
But Onans do not have any of that stuff in their gas gennys, that makes the diesel gennys better at higher elevations. So if you're at high elevations a lot, with the old 1960's technology of the newest Onan gennys, you want diesel, not my two Onan gasoline gennys.
Thanks for the explanation Lou. I remember years ago having to adjust things for my oil gasoline generator, but I didn’t understand the differences between the diesel and gas. I just knew it worked perfectly now, dumping the appropriate 100 amps into the battery (more when the solar is active) even at high elevations.
