What no one tells you about heat pumps that work down to 10 degrees F, zero degrees, minus 10 degrees, is that you probably cannot get anywhere near enough BTUs

What no one tells you about heat pumps that work down to 10 degrees F, zero degrees, minus 10 degrees, is that you probably cannot get anywhere near enough BTUs out of your heat pump, at such an outside temperature, to keep your house above freezing. But it will draw a great deal of power getting what BTUs it can. As outside temperature goes down, the amount of heat input, in BTUs per hour (or kilowatts, really) needed to maintain a comfortable temperature rises linearly with the difference. When the difference is greatest, the heat pump can deliver least. So in practice, well above such temperature, your controller has turned it off, and is burning propane instead. Of course, the better-insulated your house is, the fewer BTU/hr it takes to keep it warm. Spending a lot on a beefy heat pump without making sure your insulation is in good shape would be a mistake.