The largest factor for sizing a standby generator is what loads you want to simultaneously support. (Duh) Your heating system is just another load to be considered. Of the two modes, I would guess the heat pump would be the higher of the two and would be the load you would use. Just a caution, one of my friends has a generator in the 26 KW range and at over 130 gal/hr of propane at full load, he upgraded to a 1000 gallon tank from a 500 gallon tank. You would have to run the numbers as to what you think your actual loads would be and the duration of the loads. You can do a lot of this yourself with the equipment specifications and a clock. I don't know how your weather is or how reliable your propane supplier is but where I live you need to prepare for at least a week without power if it all goes to hell.