Heat pumps are great when things are mildly cold. While do work down to -20C or so, efficiency drops off, so you need bigger and bigger units. It often makes sense to have a smaller heat pump that will only work down to -5C (random temperature, a real analysis of your house is required to give a better answer), and then a gas furnace backup for those really cold days. Once you have the gas furnace backup you can run that off of a small generator on the few days you are without power.