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High efficiency heat pumps with built-in defrosters (or "heating strips") can often provide full heating capacity (ie: 48,000btu for a 4 ton unit) down to -12f, with reduced output at even lower temps. The COP efficiency drops quite a bit though and it will eventually get to a point where it is essentially as energy efficient as a resistive heater. Whether it's more efficient than using gas or not is a difficult calculation based on your gas price, heat pump COP temperature curve and electricity price. If you expect to see <-20f temperatures regularly, you'll want/need a backup heat source anyway.