A spec sheet that lists a 12 EER and a competing spec sheet that lists a 16 SEER are not telling you which unit is more efficient — they’re not even answering the same question. EER, SEER, and COP get treated as interchangeable efficiency numbers because they all claim to describe “how efficient” a system is, but each one is measured under different conditions, for different purposes, and in some cases different units entirely. This guide explains what each one actually measures and how to compare across them without making an apples-to-oranges mistake.
Why These Numbers Don’t Compare Directly
The short version: EER is a snapshot at the worst-case condition, SEER is a seasonal average, and COP is the underlying physical ratio both of the others are built from. None of them is simply “more accurate” than the others — they’re answering different questions, and the right one to look at depends on what you’re actually trying to evaluate.
EER — Energy Efficiency Ratio
EER measures cooling output against electrical input at a single, standardized, high-stress test condition — commonly 95°F outdoor / 80°F indoor for the current AHRI test standard. The formula:
EER = Cooling output (BTU/hr) ÷ Electrical input (W)
Because it’s tested at one specific hot condition, EER tells you how a unit performs on the hardest day of the year — which is exactly why it matters more in consistently hot climates, where a system spends a large share of its runtime near that condition rather than in mild shoulder-season weather. EER is also the rating that applies to equipment SEER doesn’t cover, including most commercial and chiller-adjacent unitary equipment.
SEER — Seasonal Energy Efficiency Ratio
SEER measures total cooling output over an entire simulated cooling season against total electrical input over that same season, incorporating a range of outdoor temperatures rather than one fixed point. The formula, conceptually:
SEER = Total seasonal cooling output (BTU) ÷ Total seasonal electrical input (Wh)
Because most modern equipment uses variable-speed or multi-stage compressors that run more efficiently at partial load than at full load, SEER numbers are typically significantly higher than EER numbers for the same piece of equipment — the SEER calculation gets credit for all those efficient partial-load hours that EER’s single-point test never sees. This is the root of the “SEER 20, EER 12” style listings that look inconsistent until you know they’re measuring different things. As of the current AHRI test procedure (SEER2/EER2), test conditions were revised to reflect more realistic external static pressure than the older standard, which generally produces somewhat lower numbers than the same equipment would have shown under the previous test — worth knowing if you’re comparing an older spec sheet against a current one.
COP — Coefficient of Performance
COP is the odd one out in a useful way: it’s the only one of the three that’s genuinely unitless, because both the output and input are expressed in the same units (watts of cooling or heating output, divided by watts of electrical input). There’s no BTU-to-watt conversion baked in the way there is with EER. This is also why COP is the standard efficiency term for chillers, heat pumps, and international/scientific contexts — and why it works equally well for heating as for cooling, since it’s just a ratio of useful energy moved to energy consumed, regardless of which direction the heat is flowing.
COP = Useful output (kW) ÷ Electrical input (kW)
Converting Between Them
Because EER is COP expressed with a BTU numerator instead of a matching-units ratio, the conversion is a fixed constant:
COP = EER ÷ 3.412
There’s no equally simple universal conversion from SEER to EER or COP, because SEER already bakes in a specific climate’s temperature distribution and a specific equipment’s part-load performance curve — two units with the same SEER can have different EERs depending on how their part-load performance is shaped. Treat published SEER-to-EER comparisons as illustrative for that specific model, not as a general conversion factor.
For commercial and central-plant work, one more metric is worth knowing even though it’s not in this article’s title: kW/ton, the standard efficiency metric for chillers, expressed as electrical input in kW per ton of cooling delivered (lower is better, the reverse convention from COP). It converts directly to COP:
COP = 3.517 ÷ (kW/ton)
Typical water-cooled centrifugal chillers land around 0.45–0.70 kW/ton (COP roughly 5.0–7.8); air-cooled chillers are generally less efficient, often 1.0–1.2 kW/ton or higher. If you’re specifying or evaluating a chiller rather than a unitary AC or heat pump, kW/ton — not SEER — is the number the industry actually compares.
Where Each Rating Actually Applies
- SEER (or SEER2) — residential and light commercial split-system air conditioners and heat pumps, generally under 65,000 BTU/hr capacity. The number most consumer shopping comparisons are built around.
- EER (or EER2) — required alongside SEER for most equipment, and the primary rating for equipment types SEER doesn’t cover, including portable and window units and larger commercial equipment. More directly relevant than SEER in hot, low-swing climates.
- IEER — the part-load-weighted rating for commercial packaged equipment above 65,000 BTU/hr, functionally SEER’s counterpart for that equipment class, using a different weighted-average formula than either SEER or the chiller-world IPLV.
- COP / kW/ton — chillers, heat pumps (especially for their heating-mode performance, where SEER doesn’t apply at all), and any context comparing equipment internationally or across fuel-vs-electric heating sources.
Reasoning Through a Comparison
- Confirm both numbers are actually the same type of rating. A SEER-to-SEER or EER-to-EER comparison is valid; a SEER-to-EER comparison is not, without converting first.
- If you only have EER for one unit and SEER for another, don’t try to force a conversion — request the missing rating from the manufacturer, or compare using whichever rating both units actually publish.
- Weight EER more heavily in consistently hot, low-temperature-swing climates, since the unit will spend more of its runtime near EER’s test condition than a mild climate would.
- For chillers, convert to a common basis (COP or kW/ton) rather than reading EER at face value — chiller spec sheets don’t always lead with EER, and kW/ton is the number the rest of the industry will expect when comparing options.
- For heat pumps, remember COP (or HSPF for the seasonal heating equivalent) covers the heating side — SEER only describes cooling performance, so a heat pump’s SEER tells you nothing about its winter efficiency.
Common Mistakes
- Comparing a SEER number directly against an EER number as if they’re on the same scale — they’re not, and the SEER number will almost always look better for reasons that have nothing to do with which unit is actually superior.
- Assuming a high SEER guarantees low bills in a hot, stable climate. SEER is a seasonal average across a range of temperatures; in a climate that spends most of the year near the EER test condition rather than in mild shoulder weather, EER is the more representative number.
- Applying a generic SEER-to-EER conversion ratio pulled from one specific unit’s spec sheet to a different unit — the relationship depends on that unit’s specific part-load performance curve, not a fixed industry constant.
- Using SEER to judge a chiller or a heat pump’s heating performance. SEER doesn’t apply to chillers at all, and only describes a heat pump’s cooling side.
- Confusing thermal kW with electrical kW when working with tons of refrigeration — a ton is 3.517 kW of heat moved, not 3.517 kW of electricity consumed; conflating the two produces a COP or kW/ton figure that’s off by a large margin.
Frequently Asked Questions
Is a higher COP always better than a lower one?
Yes, unlike kW/ton, higher COP always means better efficiency, since it’s a output-over-input ratio. Just confirm you’re comparing COPs measured at the same or similar operating conditions, since COP itself varies with load and ambient temperature.
Why does the same unit show a much higher SEER than EER?
Because SEER credits the unit for its efficient partial-load hours across a full season, while EER only tests the single hardest condition, where compressors generally run least efficiently. This gap is normal and expected, not a sign of inconsistent data.
Does a higher EER or SEER rating always mean lower total running cost?
Not automatically — correct sizing, ductwork condition, and installation quality all affect real-world energy use as much as the equipment’s rated efficiency. Our AC running cost guide walks through the other variables that determine an actual utility bill.
Do EER, SEER, and COP apply to the actuators and controls that operate the equipment, or just the equipment itself?
Just the equipment — these are thermodynamic efficiency ratings for the refrigeration cycle itself, unrelated to how the equipment is controlled. For how the control side of an HVAC system communicates and signals, see our BMS control signals guide.
