I am also an engineer and used to have LP backup heat and the only way the LP is cheaper per BTU is if you rapidly deprecate the heat pump asset.
A gallon of propane has 91,000 BTU, and a kWh of electricity has 3400 BTU. 91k/3.4k ~ 26kWh x 0.065 ~ $1.73 for electricity equivalent at 1.0 coefficient of performance. But your COP is likely around 2-3, so the heat pump will be at least 2x cheaper than the LP. It would take like 6000 hours of operation to break even on the bigger heat pump, and that’s ignoring the cost and maintenance of the propane furnace
I am also an engineer and used to have LP backup heat and the only way the LP is cheaper per BTU is if you rapidly deprecate the heat pump asset.
A gallon of propane has 91,000 BTU, and a kWh of electricity has 3400 BTU. 91k/3.4k ~ 26kWh x 0.065 ~ $1.73 for electricity equivalent at 1.0 coefficient of performance. But your COP is likely around 2-3, so the heat pump will be at least 2x cheaper than the LP. It would take like 6000 hours of operation to break even on the bigger heat pump, and that’s ignoring the cost and maintenance of the propane furnace