In my experience, sometimes the heat pump cannot keep up with the rate of envelope heat loss below certain temperatures (think a mid January polar vortex with sustained temperatures under -10F for days in the Midwest). Also, loss of power will mean loss of heat, where at least with a fossil gas furnace, as long as you have enough power to ignite the burners and run the blower, you’ve got heat (I’ve been able to run this with a vehicle inverter on an EV and a “suicide” extension cord with male plugs on both ends, isolating the furnace circuit; would’ve been perfect if it could’ve generated its own power for the blower while consuming fossil gas for heating).

Looking for a self sustaining cogeneration unit that can produce heat and power if needed, if anyone has recommendations, for when I need to recommend it. Heat pump whenever possible, but backups when needed.

If the heat pump is optimally sized, the time over which the backup resistive heater is used will be nonzero (assuming cost of the heat pump scales with its capacity). This is because spending extra to have it cover that last part is not worth it. This ignores its use for A/C; if in a place where the A/C load dominates the sizing then this argument doesn't apply.