Technically you’d want to assume efficiency of ~150-210% because heat pumps can move more than 1kW of heat energy for every 1kW used. It can be even higher in the fall or spring months when it’s mild outside.

However, you’re right that it depends on individual circumstances for the prices of electricity and natural gas. It also depends whether the user has or wants solar.

We’re in the Chicago suburbs and it made sense for us to go with a heat pump because we were also installing solar. It wasn’t much more expensive to boost our installed capacity and I think we’ll be able to get to net zero utility bills by disconnecting natural gas in the coming years.