Hi huntindog,
I have (small between 25 and 100 watt) electric heaters as freeze prevention devices. I do not know the duty cycle at particular temperatures. They are set to cycle at just above freezing. Two are controlled by TC-3 thermocubes and two are controlled by mechanical thermostats. I don't know the duty cycle on them--and it would, to some extent, change with ambient temperatures.
My block heater draws 540 watts. That translates to 54 amps of load. I wished to be able to run the block heater for three hours at -40.
Obviously I can run the 9 amp load for the furnace for many hours since I have at least 162 amp-hours of capacity available at -40. Of course, even here, it rarely gets to -40, so I may have even more amp-hours available.
My total cost for generators since I started RV'ing is over 4500.00. I have nothing to show for that, and I probably ran for less than 400 hours in total over a ten year time frame. That is one reason I don't want one. I'm sure you may agree that changing the oil at -20 F would be less than a pleasant job. Then there is gasoline, which might "grow wings", leaving me without a way to run the generator without going to a gas station.