June 19, 2026
By Michelle Wan · Reviewed by David Clark, Filterbuy HVAC Solutions · Published 6/19/2026· Updated 6/19/2026
A 13 SEER air conditioner works out to about 11.2 EER, using the quick rule of thumb EER ≈ SEER × 0.875. EER and SEER measure the same system under different conditions, so a conversion is always a close estimate, not an exact match.
Here's what each rating actually measures:
EER — performance at peak heat (95°F): the worst-afternoon snapshot.
SEER — an average across a full cooling season: usually the highest number.
SEER2 — the 2023 DOE standard from a stricter test: about 5% lower than the old SEER. Always compare SEER2 to SEER2.
Got one efficiency number and need the other? A 13 SEER air conditioner works out to roughly 11.2 EER. The quick rule of thumb is EER ≈ SEER × 0.875, but the two ratings measure different things, so a conversion is always an estimate.
Once you know your number, the quiz below matches you to a ductless mini split that fits your space. For reference, Filterbuy's mini split systems are rated 17 SEER2 (about 13+ EER), with variable-speed operation and year-round heating and cooling.
Not sure which system fits your space? Answer 5 quick questions and we'll match you to your mini split.
Most conversion charts stop at SEER. We carried ours one column further, all the way to SEER2, the rating actually printed on systems sold today, so you're not comparing a new unit's number against an old yardstick. If you want SEER numbers on their own, our SEER rating chart breaks down what counts as a good rating by region.
| SEER | EER (approx.) | SEER2 (approx.) |
|---|---|---|
| 13 | 11.2 | 12.4 |
| 14 | 11.8 | 13.3 |
| 15 | 12.3 | 14.3 |
| 16 | 12.8 | 15.2 |
| 17 | 13.3 | 16.2 |
| 18 | 13.7 | 17.1 |
| 19 | 14.1 | 18.1 |
| 20 | 14.4 | 19.0 |
| 21 | 14.7 | 20.0 |
| 22 | 15.0 | 20.9 |
Values are rounded estimates using the U.S. Department of Energy correlation and a 0.95 SEER-to-SEER2 factor for split-system air conditioners. Your exact numbers depend on the equipment and test conditions.
If you'd rather run the math yourself, here's the commonly used approximation between the two ratings, both of which are defined by the U.S. Department of Energy:
EER from SEER: EER = −0.02 × SEER² + 1.12 × SEER
SEER from EER: SEER = (1.12 − √(1.2544 − 0.08 × EER)) ÷ 0.04
For a back-of-the-napkin estimate, EER ≈ SEER × 0.875 gets you close. The multiplier drifts a little by climate, which is exactly why the full formula exists.
They're measuring the same system under different conditions, like timing a runner in a single sprint versus over a full season.
EER is a snapshot at one hot, steady condition (95°F outdoor, peak load). It tells you how the unit performs on the worst afternoon of summer.
SEER is a seasonal average across a range of outdoor temperatures, meant to reflect a typical cooling season rather than one brutal day.
Because SEER folds in milder weather, it's almost always the higher of the two numbers. Neither is "right." They just answer different questions.
Most pages skip this part, and it's the one that trips people up most.
In January 2023, the DOE updated the test procedure behind these ratings and renamed the results SEER2 and EER2. Under the 2023 federal efficiency standards, the new Appendix M1 test pushes air against external static pressure up to five times higher than before, closer to the resistance of real ductwork, so it's a tougher, more realistic measurement. The practical effect: a system's SEER2 number lands roughly 5% lower than its old SEER, even though the equipment is identical. Nothing got worse; the ruler got stricter.
One thing to clear up, because it confuses almost everyone: in the formula above, SEER² (with the little raised "2") means SEER squared, the number multiplied by itself. SEER2 (with a full-size "2") is the name of the new standard. Same look, completely different meaning. When you see "SEER2" on a product page, it's the 2023 rating, not a math operation.
"Down here in Florida, I tell homeowners to pay attention to the EER side of the spec. Our cooling season runs long and our design temperatures are high and humid, so the system spends real time at exactly the condition EER measures: full load on a hot, sticky afternoon. SEER is a useful season-long average, but in a climate like ours it can flatter a unit that struggles when it's actually 95°F out. In milder parts of the country, SEER tells the more honest story. Match the rating to your climate, not the other way around."
David Clark, Filterbuy HVAC Solutions
A simple way to decide:
Hot, humid climates (think Florida, the Gulf Coast, the desert Southwest): weight EER / EER2 more heavily. You'll spend a lot of hours at peak conditions, and that's what EER captures.
Moderate or mixed climates: SEER / SEER2 is the better guide, since your season includes plenty of mild days.
Either way: when comparing two systems, make sure you're comparing like to like, SEER2 to SEER2, not SEER2 to an older SEER number. That single mistake makes a newer, better unit look worse on paper.
For ductless mini splits, the bar is higher than for a lot of older central systems, because their variable-speed compressors ramp up and down instead of slamming on and off. That steadier operation is what pushes efficiency up.
As a reference point, Filterbuy's ductless mini split systems are rated at 17 SEER2, with variable-speed operation, year-round heating and cooling, and a 7-year compressor warranty. On the chart above, a SEER in that neighborhood lands around 13+ EER, strong performance whether you're cooling a hot garage in Phoenix or a back bedroom in Atlanta. For context, ENERGY STAR's federal criteria set the high-efficiency bar for central air conditioners at SEER2 17 and EER2 12, so a system in that range is genuinely efficient, not just marketing. They ship free in 2 days and cover 12,000 to 24,000 BTU, enough for roughly 300 to 1,200 square feet.
If you're weighing an upgrade, the efficiency math is the easy part. The converter and chart above turn any spec into an apples-to-apples comparison in seconds, and if your bills are climbing, it's worth checking what else drives a high summer energy bill.
The DOE correlation is EER = −0.02 × SEER² + 1.12 × SEER. A quick estimate is EER ≈ SEER × 0.875.
Higher is better for both, since each means more cooling per unit of electricity. They just measure efficiency under different conditions.
SEER is a seasonal average that includes mild weather, so it typically reads higher than EER, which is measured only at peak heat.
No. SEER2 is the 2023 DOE rating from a stricter test, and it runs about 5% lower than the old SEER for the same system. Always compare SEER2 to SEER2.
Yes. The EER/SEER relationship is the same. For heat pumps, EER and SEER describe cooling, while heating efficiency uses HSPF/HSPF2 separately.
Sources: U.S. Department of Energy, AHRI 2023 energy efficiency standards, and ENERGY STAR central air conditioner criteria. Reviewed for technical accuracy by David Clark, Filterbuy HVAC Solutions.
