[larkery]

A COP of 5 is quite optimistic for a domestic scale heat pump, at least in the climate where I work (UK). Sensible values here would be more like 2 - 3.5 depending on the temperature gradient required.

This is a function of how big the radiators are and how cold the heat reservoir is, so if you plug an air->water device into some normal radiators its performance won't be good on a really cold day, whereas if you plug a ground->water or water->water device into underfloor heating it'll probably be pretty good.

The big question (in the UK) for per-dwelling heat pumps is whether the distribution network has enough capacity to meet the winter peak load - if everyone gets a heat pump the cost might have to include replacing a lot of substations and distribution wires.

I am interested in the question of whether you could have PV cells on your roof with a coolant loop that goes into a heat pump, and a thermal store in the house for buffering. Then when it's sunny you can dump some electrical heat into your thermal store whilst also use your heat pump to chill the PV cells, keeping them at high efficiency. When it's not sunny you can draw off your thermal store giving a high efficiency for the pump.

If the numbers came out right you might be able to be self-sufficient for heat using something like this, just by making good use of the radiation already falling on the house.