HSPF2 is the heating counterpart to SEER2 — the seasonal heat a heat pump delivers divided by the electricity it uses, measured under the 2023 Department of Energy test update (Appendix M1) that also produced SEER2. Because M1 tests against higher static pressure that better mimics real ducts, the new HSPF2 numbers read lower than the old HSPF for identical equipment (roughly 8.8 HSPF ≈ 7.5 HSPF2), so never compare an old spec sheet against a new one.
The federal minimum for a split-system air-source heat pump is 7.5 HSPF2, applied nationally with no regional split. Unlike a furnace-plus-AC swap, a heat-pump install makes this the rating that drives your winter operating cost, because the heat pump is the heat source. In a heating-dominated home a one- or two-point HSPF2 gain compounds across the whole season far more than the same gain in SEER2 would. The ENERGY STAR threshold for a ducted heat pump is 7.8 HSPF2 — above the federal floor, and the common bar for utility rebates.
One thing HSPF2 does not tell you is how the unit behaves on the coldest days. It is a season-long average: a standard heat pump loses capacity as the temperature drops, while a cold-climate model is engineered to hold output near and below freezing. If winter performance matters where you live, read the HSPF2 alongside the manufacturer's rated capacity at 5°F and the system's balance point, not in place of them.