Wiklipedia: Air Source Heat Pumps
Air source heat pumps - Wikipedia, the free encyclopedia
"Efficiency Ratings" / "Risks & Precautions".
Air source heat pump efficiency is measured by the "Coefficient Of Performance".
In very mild weather, the COP of an ASHP can approach or exceed 4.
However, on a cold winters day, as ambient temperatures reduce more work is required to move the same amount of heat indoors.
Heat pump performance is limited by the Carnot cycle and approaches 1.0 as the outdoor-to-indoor temperature difference increases.
In cold weather ASHP's become progressively more inefficient consuming more electricity as ambient air temperatures reduce.
At this point you'd realistically want to turn your ASHP system off and turn on a secondary supplementary heating system like a gas condensing boiler to heat your home cheaply and effectively.
People conventionally install ASHP's in locations where natural gas mains dont exist.
If an inexpensive, economic, supplementary heating alternative doesn't exist, you are lumbered with significantly increased electricity bills over periods of sustained cold weather.
Those that dont get very high ASHP electricity bills have systems installed in extremely well insulated houses, with low heating requirements.
Daikin Altherma:
Daikin Altherma air-to-water heat pumps are currently amongst the most efficient on the market with average COPs between 3 and 5.
www.daikinac.com/content/residential/whole-house/daikin-altherma/#energy-efficiency