SEER2 — Seasonal Energy Efficiency Ratio 2 — is the score that tells you how efficiently an air conditioner turns electricity into cooling across a full season. A higher number means lower running cost for the same comfort. SEER2 replaced the older SEER metric in 2023: the systems are tested under a tougher "M1" procedure with realistic external static pressure (closer to real ductwork), so a SEER2 number runs a few points lower than the old SEER number for the same equipment — they are not interchangeable.
On a replacement, SEER2 first sets the floor. The U.S. Department of Energy enforces regional minimums: split-system central ACs must be at least 14.3 SEER2 in the South and Southwest and 13.4 SEER2 in the North. Installing below your region's minimum isn't allowed and can forfeit warranty and rebate eligibility. Above the floor, SEER2 is a spending decision: mid-tier units sit in the mid-teens, while high-efficiency variable-speed systems reach the high teens to low-20s and unlock most utility rebates.
Two cautions specific to replacement. First, the SEER2 rating is only valid for the AHRI-matched coil-and-condenser pair — a condenser-only swap onto an old coil doesn't deliver the number on the label. Second, a high SEER2 unit that is oversized or poorly installed won't deliver its rated savings, which is why the efficiency tier only pays off alongside a real Manual J load calculation and a quality install.