SEER2 measures the seasonal cooling efficiency of a heat pump — total cooling delivered over a season divided by the electricity used — on the Department of Energy's 2023 test scale (Appendix M1). The M1 update tests at a higher external static pressure that better represents installed ductwork, so SEER2 numbers run a little lower than the old SEER for the same equipment (about 15 SEER ≈ 14.3 SEER2). Compare SEER2 to SEER2, never across the two scales.
The national minimum for a split-system air-source heat pump is 14.3 SEER2. Unlike split air conditioners — which carry a regional minimum (13.4 SEER2 in the North, 14.3 in the South and Southwest) — heat pumps use a single national floor. In a cooling-dominated market like Arizona or South Texas, SEER2 drives most of the year-round energy bill, which is why state utility rebates such as SRP Cool Cash and Austin Energy set their entry bar at 15.2 SEER2, above the federal minimum.
SEER2 is a seasonal average at moderate conditions; a companion number, EER2, rates efficiency at a fixed 95°F design day and matters more in desert climates where peak-afternoon performance drives the demand. For a heat-pump buyer the practical takeaway is that SEER2 and HSPF2 describe the same unit's two jobs — read both, and weight them by whether your bills are cooling- or heating-dominated.