EER, SEER, and SEER2 all measure how efficiently an air conditioner turns electricity into cooling — but they measure it under different conditions, which is why the same unit can carry three different numbers. Confusing them leads to bad comparisons and overpaying for efficiency you do not need.
The short version: EER captures efficiency at a single brutal-hot moment, SEER averages efficiency across a whole cooling season, and SEER2 is the 2023 version of SEER measured under a harder, more realistic test. Get those three ideas straight and every spec sheet suddenly makes sense.
What EER Measures
EER stands for Energy Efficiency Ratio. It is a snapshot: the ratio of cooling output to electrical input at one fixed, demanding operating point.
EER = BTU/hr ÷ watts
The test holds the outdoor temperature at 95°F and the indoor temperature at 80°F, then measures steady-state performance. Because the conditions are fixed and severe, EER tells you how the unit behaves when it is working hardest — a hot summer afternoon at peak demand.
There is no averaging and no credit for milder hours. A higher EER means more cooling per watt at peak load. Typical central air units land around EER 9–12, with high-efficiency equipment reaching 13 or more.
What SEER Measures
SEER stands for Seasonal Energy Efficiency Ratio. Instead of one point, it estimates efficiency across an entire cooling season that includes a spread of outdoor temperatures, not just the 95°F worst case.
SEER = total cooling output (BTU) ÷ total electrical energy used (watt-hours) over a cooling season
Because most of a season is milder than peak — and modern equipment runs more efficiently at part load — SEER numbers are always higher than the same unit’s EER. SEER rewards equipment that throttles down gracefully on mild days, which is why variable-speed systems post high SEER figures even when their peak-load EER is unremarkable.
SEER answers a different question than EER: not “how efficient at the worst moment,” but “how efficient over a typical summer.” That makes it the better predictor of your seasonal cooling bill.
What SEER2 Measures
SEER2 is not a new metric — it is SEER measured under an updated, tougher test that took effect January 1, 2023. The efficiency concept (cooling BTU ÷ watt-hours over a season) is identical. What changed is the test condition.
The old SEER test (procedure M1) ran at just 0.1 inches of water column (in.w.c.) of external static pressure. The new SEER2 test (procedure M2) runs at 0.5 in.w.c. — five times higher — to reflect the resistance real ductwork, filters, and grilles impose on a blower. Working against more realistic resistance, the same hardware measures lower, so:
SEER2 ≈ SEER × 0.95
Nearly every unit’s number dropped about 4.5–5% when re-expressed as SEER2. The machine did not get worse; the yardstick got more honest. For the full breakdown of the transition, conversion table, and what it means at purchase time, see our SEER vs SEER2 guide.
EER vs SEER vs SEER2 at a glance
How the Three Compare
The cleanest way to keep them straight is to line up what each one measures, the condition it is tested under, and what a good value looks like today.
| Rating | What it measures | Test condition | Typical “good” value |
|---|---|---|---|
| EER | Efficiency at one peak point | Steady state, 95°F outdoor / 80°F indoor | 11–13+ |
| SEER | Efficiency averaged over a cooling season | Legacy M1 test, 0.1 in.w.c. static | 15–18 (old scale) |
| SEER2 | Same seasonal average, realistic test | New M2 test, 0.5 in.w.c. static | 15–18 (current scale) |
EER and SEER are not interchangeable, but there is a loose rule of thumb relating them for central equipment: at typical efficiency tiers, EER ≈ SEER × 0.85 to 0.9. Treat any EER-to-SEER conversion as rough — they are measured under genuinely different conditions, so the relationship drifts by equipment type. When precision matters, use the manufacturer’s published EER and SEER2 figures rather than converting one into the other.
How to Calculate EER
EER is the one rating you can reasonably compute yourself, because it is a single steady-state ratio rather than a seasonal model.
Take the unit’s cooling capacity in BTU/hr and divide by the power it draws in watts at rated conditions:
Step 1 — Capacity: a 2-ton unit delivers 24,000 BTU/hr (1 ton = 12,000 BTU/hr).
Step 2 — Power draw: suppose it pulls 2,000 watts at 95°F.
Step 3 — Divide: 24,000 ÷ 2,000 = 12 EER.
That is the whole calculation. SEER and SEER2, by contrast, cannot be hand-calculated from a single reading — they require modeling performance across a range of outdoor temperatures and part-load conditions, which is why they come from standardized lab testing rather than a tape measure and a clamp meter.
Calculating EER
When EER Matters More Than SEER
If SEER is the better predictor of seasonal cost, why does EER still exist? Because the two answer different questions, and in some situations the peak-load number matters more.
EER matters more when:
- You live in a hot, dry climate (think Phoenix or Las Vegas) where the system spends most of its run time near peak conditions. There the season looks like the EER test most of the time, so peak efficiency dominates the bill.
- You care about peak electrical demand — utilities with demand charges or time-of-use rates penalize the watts you pull on the hottest afternoons, exactly what EER measures.
- You are sizing for a utility rebate program, many of which set EER thresholds rather than SEER thresholds for cooling-dominated regions.
SEER (and SEER2) matters more when:
- You live in a mixed or humid climate with a long shoulder season of mild cooling days, where part-load efficiency drives most of the savings.
- You want the best estimate of total annual cooling cost, which is what SEER2 was designed to model.
In hot-dry regions, do not let a high SEER2 number distract you from a mediocre EER — the unit may shine on paper but spend its life running at the one condition where it is only average.
What Counts as a Good Rating Today
Two numbers anchor the answer: the legal minimum and the practical sweet spot.
Federal minimums (SEER2, since 2023):
- South and Southwest: 14.0 SEER2 minimum for central air conditioners.
- North: 13.4 SEER2 minimum.
Those are floors, not targets — the cheapest legal equipment, nothing more. For most homeowners the practical sweet spot sits at 15–18 SEER2, where the upfront premium over a base unit typically pays back within a reasonable number of years through lower bills. Above roughly 20 SEER2, the additional cost climbs faster than the savings, so it rarely pencils out unless you have very high electricity rates, very long cooling seasons, or utility rebates and federal tax credits to offset the price.
What counts as a good SEER2 rating
A Note on CEER and IEER
Two cousins show up on equipment outside standard ducted central AC:
- CEER (Combined Energy Efficiency Ratio) is the standard for window and room air conditioners. It works like EER but also folds in the energy the unit draws in standby mode, so it slightly penalizes phantom load. When you compare two window units, compare CEER, not EER.
- IEER (Integrated Energy Efficiency Ratio) is the commercial counterpart to SEER. It blends performance at multiple part-load points (100%, 75%, 50%, and 25% capacity) into one weighted number, which suits large rooftop and packaged units that spend most of their hours at part load. Residential shoppers will rarely see it; facilities managers will see it constantly.
For a typical home with ducted central AC or a heat pump, EER and SEER2 are the only two numbers you need to weigh.
Use the Free Calculator
SEER Savings Calculator — get your exact answer in seconds.
Enter your current rating, a target rating, system size, annual runtime, and your electricity rate, and the SEER Savings Calculator returns annual savings and a simple payback estimate. If you only have an old SEER number on an existing unit, run it through the SEER2 Converter first so you are comparing the same scale.
FAQ
What’s the difference between EER and SEER?
EER measures efficiency at a single peak condition — 95°F outdoor, 80°F indoor — as BTU/hr divided by watts. SEER measures efficiency averaged across a whole cooling season that includes milder days, so it is always the higher number for the same unit. EER tells you how the system performs at its worst moment; SEER predicts your typical seasonal cost.
How do you calculate EER?
Divide the cooling capacity in BTU/hr by the power draw in watts at rated conditions. A 24,000 BTU/hr (2-ton) unit drawing 2,000 watts at 95°F has an EER of 24,000 ÷ 2,000 = 12. SEER and SEER2 cannot be hand-calculated the same way because they model performance across many temperatures and part-load points.
What is a good SEER2 rating?
The federal minimum is 14.0 SEER2 in the South and Southwest and 13.4 SEER2 in the North. For most homeowners, 15–18 SEER2 is the practical sweet spot, balancing upfront cost against energy savings. Above 20 SEER2 the extra cost usually only pays back with high electricity rates, long cooling seasons, or rebates and tax credits.
Is higher SEER worth it?
It depends on how much you run the system and your electricity rate. In warm climates with long cooling seasons and rates above about $0.12/kWh, stepping from a base unit to 16–18 SEER2 typically pays back within several years. In mild climates with short seasons, the savings shrink and a mid-tier unit is often the smarter buy. Run the numbers for your usage rather than assuming higher is always better.
Should I look at EER or SEER2 in a hot, dry climate?
In hot, dry regions like the desert Southwest, EER deserves more weight than usual because the system spends most of its run time near peak conditions — exactly what EER measures. SEER2 still matters for the overall season, but a unit with a strong EER will serve you better on the hottest afternoons and under demand-based utility rates.