SEER vs SEER2: What Changed in 2023 (and Why It Matters)

If you're reading two AC spec sheets side by side and one says SEER and the other says SEER2, you're not crazy — they're different ratings. Different test, different number, different thresholds. The unit is the same; the way we measure it changed.

What SEER measures

SEER — Seasonal Energy Efficiency Ratio — is total cooling output (in BTU) divided by total electric input (in watt-hours) over a representative cooling season. Higher = more cooling per watt = lower bills.

It's a seasonal average. EER, by contrast, is a single-point efficiency measurement at 95°F outdoor. SEER folds in the part-load reality of real operation.

What changed in 2023

The Department of Energy updated the test protocol on January 1, 2023. The new standard — informally SEER2 — uses the “M1” test procedure, which raises the assumed external static pressure from 0.1" w.c. to 0.5" w.c.

Why that matters: 0.1" was wildly unrealistic. Almost no installed residential system runs at 0.1" — filters, ducts, and registers all add resistance. The old test made every unit look better than it really performed. The new 0.5" assumption is much closer to a real ducted install.

The rough conversion

SEER2 ratings come out lower than SEER ratings for the same machine — usually by about 4–5%. Quick mental math:

SEER2 ≈ SEER × 0.95

So a unit that was “16 SEER” under the old test is roughly “15 SEER2” under the new one. Same compressor, same coil, same kW pulled at the same conditions. Different test.

New federal minimums

The South and Southwest get higher floors because more annual cooling hours mean the efficiency math compounds harder on energy bills.

The tax-credit angle

The federal 25C residential energy efficiency credit and most state and utility rebates are now written in SEER2 / EER2 / HSPF2 terms, not the old ratings. If you're quoting a tax-credit-eligible system, make sure you're reading SEER2 off the AHRI certificate, not an old spec sheet. Older units may still be in inventory with only SEER printed on the label — the AHRI listing is authoritative.

Where people go wrong

• Comparing a 2020 unit's “16 SEER” against a 2024 unit's “15 SEER2.” They're roughly equivalent — the 2024 is just being more honest about real-world performance.
• Quoting tax-credit eligibility from an old spec sheet. Pull the SEER2 number off the current AHRI listing, not from the carton.
• Assuming the new test “made units less efficient.” It didn't. It made the rating more accurate. The physics didn't change.