Q: For homes that already have solar, it seems like heat pumps would be a tougher sell, because they’re already getting the advantage of reduced electricity. How does the ROI of heat pumps change when the solar is already in place and electric bills are already very low?

All solar installers I have ever met install enough solar to offset the current electricity usage. Adding heat pumps will increase the electricity usage and decrease the heating fuel usage. Hence the ROI on a heat pump is going to be based on the cost of the additional electricity needed to run the heat pump not the current cost of electricity (whether from solar panels or just from the utility). Installing heat pumps powered by utility electricity at 23c/kWh will be about breakeven in terms of heating costs vs heating oil and will be more expensive than heating with natural gas. I think you said you were on propane, it will definitely be cheaper than propane, even if you have to use utility electricity. By far the best way to do it is to install more solar panels at the same time as the heat pump. If you don’t have the roof space to do this, the next best option is to get community sourced solar which is 100% solar at about a 12% discount to utility rates or about 20c/kWh. With a mini-split heat pump at COP (efficiency) 2.5 the heat in your house is going to cost you about 8c/kWh or about the same as heating oil. Propane is more like 11c/kWh of heat in your house. With a ducted system the COP is likely to be more like 3.0 and heat in your house would cost under 7c/kWh which is cheaper than heating oil but still more expensive than natural gas (which is about 5c/kWh of heat in the house). And remember, you have to buy the heat pump whereas you already have the furnace. This is why the most cost effective way to add a heat pump is when you need to replace your AC unit. Finance the new heat pump with the 0% interest Heat Loan and the cost difference between replacing two AC units and upgrading to two heat pumps is only about $3,000.