A heat pump moves heat instead of making it, so efficiency is COP (steady state) or HSPF2 (seasonal). Cold-climate models hold usable capacity to 5 F and below; check the NEEP cold-climate table for capacity at 17 F and 5 F, not just the headline tonnage.
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Heat Pump Guide: Sizing for Cold Climates
COP, HSPF2, balance point, and dual-fuel switchover explained.