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Electric heat pumps extract heat from the environment, this is only really efficient above freezing temperatures. A quick google gives me "-10 C" as a minimum operating temp, or 14 F

My house uses an electric heat pump but has an "emergency heat" function that falls back to resistive heat at much higher cost.

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That’s misleading. Yes efficiency drops as the ambient temperature drops, but they are still quite efficient below freezing.

For example here’s one of the units I run at my home: https://ashp.neep.org/#!/product/56518. It has a COP (coefficient of performance) ranging from around 4 at higher ambient temperatures, down to 1.8 at 5f. So it’s just over half the electricity input per watt of heat output for me to run my heat pump at 5f.

There are newer units and units designed to run in cold environments that achiever much higher COP below freezing than my example above.

And to be honest with my home below 30 F mine struggle and the aux heat will have to kick on.

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