SEER2 Rating Explained: What Changed and What It Means for Your Next Quote
SEER2 is the seasonal efficiency rating used on every central air conditioner and heat pump tested under the AHRI 210/240-2023 procedure that became mandatory on January 1, 2023. It measures the same thing SEER always measured (cooling delivered over a season, divided by the electricity it took to deliver it), but under a stiffer test. That's why the number on a 2023-or-later nameplate reads lower than the SEER number the same physical machine would have earned under the old procedure. Nothing about the equipment changed. The test did.
What SEER2 Actually Measures
SEER and SEER2 are both a ratio: total cooling output in Btu over a cooling season, divided by total electrical energy input in watt-hours. The formula didn't change. What changed is the external static pressure the lab holds against the equipment while it runs the test. The new procedure, developed by AHRI and adopted into the U.S. Department of Energy's test procedure at 10 CFR 430, tests the machine against a higher static pressure than the old one did, closer to what a unit connected to real ductwork actually fights against. A system working harder against more resistance in the test moves less air per watt on paper, which is why the reported number drops even though the compressor, coil and refrigerant charge are identical.
That's the whole story on why a SEER2 number looks like a downgrade. It isn't one. It's a different test on the same equipment.
The Conversion Factor Isn't One Number: It's Equipment-Class-Specific
This is the part that gets flattened into a single rule of thumb almost everywhere it's explained, and it's the part that actually matters when you're comparing a legacy-rated trade-in against a new SEER2-rated quote.
AHRI developed conversion factors between the two rating systems, and they are published by equipment class, not as one universal multiplier. The California Energy Commission's Rating Conversion Table lists them in the direction SEER = SEER2 × factor:
| Equipment class | Factor (SEER = SEER2 × factor) |
|---|---|
| Split air conditioner, under 45,000 Btu/h | 1.049 |
| Split air conditioner, 45,000 Btu/h or more | 1.051 |
| Split heat pump | 1.049 |
| Packaged air conditioner or heat pump | 1.045 |
| Space-constrained air conditioner | 1.026 |
| Space-constrained heat pump | 1.008 |
| Small-duct, high-velocity system | 1.000 |
Read the spread across that table: 1.049 at one end, 1.000 at the other. Nearly five percentage points separate a small-duct high-velocity system, which needs no conversion at all, from an ordinary split air conditioner under 45,000 Btu/h. That's a wider gap than the difference between two adjacent models in the same product line. A single flat conversion factor applied across all of it is wrong for every class except the one it happened to be calibrated on.
RESNET publishes the same relationship from the opposite direction in MINHERS Addendum 71f, more coarsely rounded, and the two sources reconcile to rounding on every class. The table above reflects the factors as published and current as of 2026; if AHRI issues a revised conversion for a future equipment generation, check the California Energy Commission's Rating Conversion Table directly rather than treating this page as the permanent record.
Why "SEER2 = SEER × 0.95" Is Only Sometimes Right
The rule of thumb in circulation (multiply the old SEER number by 0.95 to get SEER2, or divide the SEER2 number by 0.95 to get the old rating back) isn't invented out of nowhere. It's exactly what RESNET's Addendum 71f publishes for a ducted split system, and it's the reciprocal of AHRI's 1.049 factor to within half a percent. For that one equipment class, it's correct.
The mistake is applying it everywhere. RESNET's own table, on the same page, gives 1.00 for a ductless system, 0.97 for a ducted space-constrained air conditioner, and 0.99 for a ducted space-constrained heat pump. Use 0.95 on a ductless mini-split and you've shifted its rating by five percent for a system that needed no adjustment at all. An even looser version circulates too: "divide by 1.05," which works out to multiplying by roughly 0.9524, and that number doesn't correspond to any published equipment class. It's a rounding of a rounding, not a sourced conversion.
The practical rule: know the equipment class before you convert. A split system, a packaged unit, and a small-duct high-velocity system are not interchangeable for this purpose even when they're rated at the same SEER2 number.
Worked Example: One SEER2 Rating, Three Different Legacy Numbers
Take a system rated 15.0 SEER2 and run it back through three of the conversion factors above.
- Split air conditioner under 45,000 Btu/h: 15.0 × 1.049 = 15.73 SEER
- Packaged air conditioner or heat pump: 15.0 × 1.045 = 15.67 SEER
- Small-duct, high-velocity system: 15.0 × 1.000 = 15.00 SEER, unchanged
Same nameplate number, three different answers, because the factor belongs to the equipment class, not to the rating itself. A calculator, or a contractor doing this by hand, that applies one flat factor to all three gets two of the three answers wrong.
What This Means When You're Writing a Quote
The comparison that comes up constantly in a residential HVAC quote is a legacy-rated trade-in against a new SEER2-rated system, and it's easy to make that comparison look worse or better than it actually is depending on which way you convert.
If a homeowner's existing unit carries an old SEER rating and the proposed system is SEER2-rated, converting one side onto the other's scale, using the right factor for that equipment's class, is what makes the two numbers mean the same thing. Skip that step and a genuine efficiency gain can look smaller than it is, or a wash can look like an upgrade, purely from rating-scale confusion rather than anything about the equipment. If the trade-in's nameplate is illegible or the model number is all that's left to go on, decoding tonnage and equipment class from the outdoor unit's model number is the starting point. An AC tonnage calculator that reads manufacturer nomenclature directly handles that faster than a manual lookup.
Once both systems are expressed on the same scale, the number a homeowner actually wants is the payback period: how many years of lower electricity bills it takes for the higher-efficiency option to earn back what it costs extra. That's a separate calculation from the rating conversion. It needs the equipment's capacity, the local electricity rate, and the number of cooling hours in the season, not just the two ratings. A SEER2 savings calculator that applies the equipment-class-specific factor and shows the payback math line by line is built to answer exactly that, rather than a flat percentage claim on a spec sheet.
None of this replaces sizing the equipment correctly in the first place. A SEER2 rating describes efficiency, not whether the tonnage is right for the house. If sizing hasn't been nailed down yet, that's a separate calculation with its own rules; see our piece on why square-footage HVAC sizing rules get it wrong.
Frequently Asked Questions
Is a 14 SEER2 unit the same efficiency as a 14 SEER unit? No. Because the SEER2 test holds the equipment against a higher external static pressure, a 14 SEER2 rating represents a physically more efficient machine than a 14 SEER rating did under the old test. Converting one onto the other's scale, using the factor for that equipment's class, is the only way to compare them honestly. Reading the numbers as equivalent understates how much more efficient the SEER2-rated unit actually is.
How do I convert SEER to SEER2, or SEER2 back to SEER? Divide the old SEER number by the equipment class's conversion factor to get SEER2, or multiply a SEER2 number by that same factor to get the equivalent legacy SEER figure. The factors come from AHRI and are published in the California Energy Commission's Rating Conversion Table: 1.049 for a split air conditioner under 45,000 Btu/h, 1.051 at or above that, 1.049 for a split heat pump, 1.045 for a packaged unit, 1.026 for a space-constrained air conditioner, 1.008 for a space-constrained heat pump, and 1.000 for small-duct high-velocity equipment.
What is HSPF2, and does the same conversion logic apply? HSPF2 is the heating-season equivalent of SEER2: heat delivered by a heat pump over a heating season, divided by the electricity it used, under the same M1 test procedure that produced SEER2. It replaced HSPF on the same January 2023 timeline, for the same reason: a stiffer static-pressure test produces a lower number on paper for an unchanged machine. It carries its own AHRI-published conversion factors rather than sharing SEER2's.
Does a higher SEER2 rating always mean a lower electricity bill? It means lower cooling energy use per ton of capacity delivered, assuming the equipment is installed, charged, and commissioned correctly. Whether that translates into a bill low enough to justify the extra equipment cost is a payback question, not a ratings question. It depends on the local electricity rate, the length of the cooling season, and how much more the higher-rated system costs. Those numbers belong on the sheet handed to the customer alongside the rating comparison, not folded into it.
Why does the SEER2 number on some heat pumps barely change from the old SEER number? Because the conversion factor for that equipment's class is close to 1.000. A small-duct high-velocity system converts one to one, with no adjustment at all, and a space-constrained heat pump moves by less than one percent (1.008). Equipment in those classes was already tested closer to real-world static pressure conditions before 2023, so the new procedure didn't change its result by much. A standard split system, by contrast, moves by roughly five percent, which is the gap that causes most of the comparison confusion in the field.