Good questions. A heat pump is 3-5 more efficient than any fossil fuel furnace. Fossil fuel furnace is 70-96% efficient, heat pump is typically between 300-500% 'efficient'. COP varies depending on the outdoor conditions, but you can infer COP at various temperatures by looking at the HSPF2 (heating season performance factor) which is the weighted average of the btu output/watt input at various outdoor/indoor conditions.