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 <i>anywhere near</i> 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.<p>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.<p>So in practice, well above such temperature, your controller has turned it off, and is burning propane instead.<p>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.