COP (Coefficient of Performance) measures how much heat a heat pump delivers for the electricity it uses. A COP of 3 means the unit puts out three units of heat for every one unit of electricity it draws. Because it moves existing heat rather than burning fuel, a heat pump can be far more than 100% efficient.
This is why heat pumps cut heating bills. A gas furnace can never beat 100% — it only converts fuel to heat. A heat pump at COP 3 effectively gives you 300% efficiency, which is why the Department of Energy says a modern heat pump can use up to 75% less electricity for heating than electric baseboard or resistance heaters.
The catch is that COP falls as it gets colder, because there is less outdoor heat to gather. Most heat pumps deliver roughly two to four units of heat per unit of electricity in mild weather, but the figure drops in deep cold. That is what gave older heat pumps a bad name in northern winters.
Newer cold-climate models fix much of that. To earn the ENERGY STAR cold-climate label, a heat pump must still hit a COP of at least 1.75 at 5°F and hold at least 70% of its rated heating capacity at that temperature. If you live where winters bite, those two figures — not the mild-weather rating — tell you whether the system will keep up when you need it most.