For heat pumps, when it gets too cold outside, the heat pump can't "pump" heat so it uses heat strips for electrical resistance heat backup. These stay on as long as necessary to heat the house. You can tell they are on because the heat from the ducts is warmer than usual and there may be an electrical heat smell. The thermostat will say that the backup heat is on.
Per heating unit, this costs 4 times more electricity/ money than heat from the heat pump system.
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