Well carried out in a good insulated house with heat recovery ventilation the most efficient method to save energy at heating and warm water production.
Once upon a time
The usual efficiency of a power plant had been 3 kWh thermic energy into 1 kWh electric power. The usual efficiency of a heat pump had been 1 kWh electric power into 3 kWh heat. For what all the effort? The most modern power plants are close to 2 kWh electric power from 3 kWh heat. At solar roofs using the underground as seasonal storage, it's because of the more warm undergeround possible to make out of 2 kWh electric power 12 kWh heat.
This is comparing the average technology from 1980 with the best possible technology from 2006 a 1:4 difference. 1:4 for making electric power in a power plant and using the electric power in a heat pump for heat production.
Optimal corporation with the electricity / heat roof
With our published minimum requirements for a roof, produce electric power, produce heat, let snow slide down, is the heat pump the only system matching the total concept.