EER, EER2, CEER, and IEER Ratings Explained (With Charts)

"I'm comparing two window ACs. One lists EER 12 and the other lists CEER 11.5. What's the difference — and which one is actually more efficient?"

If you've ever shopped for an air conditioner and run into a wall of efficiency acronyms, you're not alone. EER, EER2, CEER, IEER — they all measure cooling efficiency, but they measure it in different ways, under different conditions, for different types of equipment.

We're going to break down every one of these ratings in plain English. You'll find comparison tables, conversion charts, DOE minimum requirements, ENERGY STAR thresholds, and 5 worked examples that show exactly how to use these numbers when shopping for an AC.

Quick answer: EER measures how efficiently an air conditioner cools at one specific test condition (95°F outdoor, 80°F indoor, 50% humidity). CEER adds standby power consumption to that measurement, and EER2 uses a stricter test procedure. Higher is always better for all of them.

What Is EER? Energy Efficiency Ratio Definition and Formula

Energy Efficiency Ratio (EER) is the most fundamental cooling efficiency metric in HVAC. It tells you how many BTUs of cooling output you get for every watt of electricity consumed — measured at a single, specific test condition.

Here's the formula:

EER = Cooling Output (BTU/h) ÷ Power Input (Watts)

Simple enough to understand. If a window AC produces 12,000 BTU/h of cooling and draws 1,000 Watts, its EER is 12.0. That means you're getting 12 BTUs of cooling for every watt of electricity.

EER Formula: How to Calculate Energy Efficiency Ratio

Calculating EER is straightforward. You need two numbers from the unit's spec sheet — both are always listed:

1. Cooling capacity in BTU/h (found on the nameplate or product listing).
2. Power input in Watts (the electrical draw during cooling operation).
3. Divide cooling capacity by power input. That's your EER.

The higher the resulting number, the more efficient the unit. An air conditioner with an EER of 12 uses 20% less electricity than one with an EER of 10 to produce the same amount of cooling.

EER Test Conditions: 95°F Outdoor, 80°F Indoor, 50% RH

All EER ratings are measured under identical laboratory conditions. This is what makes them useful for apples-to-apples comparison:

These conditions simulate a hot summer afternoon — the exact scenario where you need your AC to perform its best. That's why EER is sometimes called the "peak load" efficiency rating.

Keep in mind: real-world performance varies. If it's 105°F outside, your unit's actual efficiency will be lower than its rated EER. If it's 85°F, it'll likely perform better.

What Is EER2? The New EER Rating System Explained

EER2 is the updated version of EER that became mandatory for all cooling systems manufactured on or after January 1, 2023. The formula is identical — BTU/h divided by Watts — but the testing procedure changed significantly.

The U.S. Department of Energy introduced a new test method called the M1 procedure (Appendix M1). The biggest change: the external static pressure (ESP) used during testing increased from 0.1 inches of water column to 0.5 inches.

Why does that matter? Because 0.5" ESP better simulates real-world ductwork resistance — the blower motor has to work harder, which means it draws more electricity during the test. The result: EER2 values are lower than EER values for the same equipment.

EER vs EER2: What Changed With the M1 Test Procedure

Here's the deal: if you're comparing a new unit rated in EER2 against an older unit rated in EER, the numbers aren't directly comparable. A system rated at 11.7 EER2 is roughly equivalent to one rated at 12.2 EER under the old test. Don't let the lower number fool you.

EER2 Minimum Requirements by Region

The DOE sets EER2 minimums only for specific regions and equipment sizes. Here's the current standard for residential split-system central air conditioners:

*The 11.7 EER2 minimum applies to units with a certified SEER2 below 15.2. Units with SEER2 ≥ 15.2 only need 9.8 EER2.

Important note: the DOE does not set a mandatory minimum EER2 for central AC systems in the Northern region. The primary enforced metric nationwide is SEER2. EER2 minimums apply only in the Southeast and Southwest.

What Is CEER Rating? Combined Energy Efficiency Ratio for Room ACs

CEER stands for Combined Energy Efficiency Ratio. The DOE introduced it in June 2014 specifically for room and window air conditioners, and it replaced EER on EnergyGuide labels for these products.

Here's what makes CEER different: it accounts for standby power consumption — the electricity your window AC draws when it's plugged in but not actively cooling. EER only measures efficiency during active cooling. CEER combines both.

The DOE's analysis found that a typical room AC draws about 1.4 Watts in standby mode and spends roughly 5,115 hours per year in that mode. That's a lot of phantom power draw that EER completely ignores.

CEER vs EER: Why Standby Power Matters

Because CEER accounts for standby power, the CEER rating is always slightly lower than the EER for the same air conditioner. The difference is typically 0.1 to 0.3 points. A window AC with EER 12.0 might have a CEER of 11.8.

When shopping for a window AC or any room air conditioner, the CEER is the number you should compare. It's the more accurate representation of what that unit will actually cost to run.

CEER Rating Chart for Window Air Conditioners

What's a good CEER rating? Here's how to think about it:

The best inverter-driven window ACs on the market achieve CEER ratings above 14.0. For context, the BTU output of these units typically ranges from 8,000 to 14,000 BTU/h.

What Is IEER Rating? Integrated Energy Efficiency Ratio for Commercial HVAC

IEER stands for Integrated Energy Efficiency Ratio. It's designed for commercial HVAC systems with cooling capacities of 65,000 BTU/h and above — think rooftop units, commercial packaged ACs, and large split systems.

The core problem IEER solves: EER only measures efficiency at 100% full load. But commercial systems spend less than 2% of their operating hours at full load. Most of the time, they're running at 50-75% capacity.

IEER measures efficiency at four different load points and weights them based on how much time a typical commercial system actually operates at each level:

How IEER Weighs Part-Load Performance

IEER = (0.02 × A) + (0.617 × B) + (0.238 × C) + (0.125 × D)

Notice that 75% load gets 61.7% of the weight. That's because commercial buildings rarely need full-blast cooling. Technologies like variable-speed compressors and modulating condenser fans dramatically improve part-load performance — and IEER captures that advantage in a way EER never could.

A commercial unit with an EER of 9.5 might have an IEER of 11.8 or higher if it has strong part-load efficiency. The IEER is defined by AHRI Standard 340/360 and is the primary cooling efficiency metric for commercial equipment in DOE and ASHRAE 90.1 standards.

EER vs SEER: Which Efficiency Rating Matters More?

This is one of the most common questions we get — and the answer depends on where you live. Both EER and SEER measure cooling efficiency, but they measure it under very different conditions.

When EER Matters More Than SEER (Hot Climate Performance)

Here's the thing: SEER's weighted average outdoor temperature works out to about 82°F. If you live somewhere where the AC runs at full blast for months — Phoenix, Houston, Miami, Las Vegas — the SEER rating overstates real-world efficiency.

EER is measured at 95°F. That's much closer to what your AC actually faces during a Texas or Arizona summer. A unit with a high SEER but mediocre EER will save you money in the spring and fall but cost you more during the peak heat — which is exactly when your electricity bills are highest.

Bottom line: If outdoor temperatures regularly exceed 95°F where you live, prioritize EER (or EER2). If you're in a moderate climate with wide seasonal swings, SEER gives you a better picture of annual efficiency.

EER to SEER Conversion Chart

You can't convert EER to SEER exactly because they measure different things. But the DOE provides an acceptable approximation. Here are both formulas:

Simplified: EER ≈ SEER × 0.875

DOE quadratic: EER = −0.02 × SEER² + 1.12 × SEER

These conversions are most reliable for single-speed systems with EER values below 14. For variable-speed and inverter equipment, the SEER-to-EER relationship varies significantly because these units perform much better at partial loads.

EER to COP Conversion (COP = EER ÷ 3.412)

The Coefficient of Performance (COP) is the international efficiency metric used in SI units. Converting between EER and COP is simple and exact — unlike EER-to-SEER, this is a direct unit conversion.

COP = EER ÷ 3.412

The 3.412 factor comes from the conversion between BTU/h and Watts (1 Watt = 3.412 BTU/h). You can also learn more about this relationship in our COP calculator.

EER, COP, and SEER Conversion Table

kW/Ton is another commercial metric: kW/Ton = 12 ÷ EER. Lower is better.

EER Rating Chart by Equipment Type

Not all air conditioners are created equal. A "good" EER depends entirely on what type of equipment you're looking at. Here's what to expect across the major categories:

What Is a Good EER Rating for an Air Conditioner?

For window and room ACs, an EER of 12.0 or above is very good. Anything above 14.0 CEER puts you in ENERGY STAR territory. Budget units typically land in the 9.5–10.5 range.

For central air conditioners, the EER2 number is more relevant now. A good EER2 is 12.0+. For reference, the 25C tax credit requires at least 17 SEER2 and 12 EER2 for split systems.

For mini splits, EER ratings above 13.0 are common. The best variable-speed models achieve even higher. These units are also rated by SEER, which is usually the headline number.

DOE Minimum EER Requirements Table

DOE Minimum CEER for Room Air Conditioners (Current Standards — Effective 2014)

Updated DOE Minimums for Room ACs (Updated DOE Minimums)

The updated standards represent 20-36% energy savings depending on the product class. The DOE estimates these updated requirements will save homeowners approximately $1.5 billion per year in electricity bills and reduce carbon emissions by 106 million metric tons over 30 years.

These higher minimums will effectively require variable-speed compressors for units above 8,000 BTU/h. You can learn more about how AC capacity relates to room size in our BTU sizing guide.

ENERGY STAR EER Requirements for Room and Window ACs

ENERGY STAR-certified room ACs significantly exceed DOE minimums. Here are the current Version 5.0 requirements:

ENERGY STAR is updating to Version 7.0 alongside the updated DOE standards. The proposed Version 7.0 requires CEER 10% above DOE minimums for standard room ACs and 5% above for room heat pumps (reverse-cycle units).

For central AC and heat pump efficiency, ENERGY STAR has separate requirements based on SEER2 and HSPF ratings.

Why EER Matters More in Hot Climates

We touched on this in the EER vs SEER section, but it's important enough to expand on. If you live in the Sun Belt, EER is arguably the more important number.

SEER's seasonal calculation averages efficiency across a temperature range of 65°F to 104°F. The energy-weighted average works out to about 82°F outdoor temperature at 52% load. That's fine for Chicago or Charlotte — but it dramatically understates the cooling demand in Phoenix, where summer highs regularly exceed 110°F.

EER measures at 95°F and 100% load. That's much closer to the conditions your AC faces during peak summer in hot climates. A unit with high SEER but mediocre EER has good part-load efficiency in mild weather but struggles when you need it most.

The DOE recognized this, which is why the Southeast and Southwest are the only regions with mandatory minimum EER2 requirements for central air conditioners. If your local utility bills spike during July and August, EER should be a priority factor in your purchasing decision.

EER Worked Examples

Example 1: Comparing Two Window ACs by EER

You're shopping for a window AC for your 350 sq ft bedroom. Two options:

• Unit A: 10,000 BTU/h, draws 833 Watts. EER = 10,000 ÷ 833 = 12.0
• Unit B: 10,000 BTU/h, draws 1,000 Watts. EER = 10,000 ÷ 1,000 = 10.0

Both deliver the same cooling output, but Unit A uses 16.7% less electricity for the same cooling. At $0.16/kWh running 8 hours a day for 120 days, Unit A saves about $25.60/year. Over a 10-year lifespan, that's $256 in electricity savings.

Example 2: Cost Difference Between EER 10 and EER 12

Let's say we have a 12,000 BTU/h window AC running 8 hours per day, 120 days per year, at $0.16/kWh. Here's how EER affects running cost:

The difference between EER 10 and EER 12? $30.72/year — or $307 over 10 years. That often pays for the price premium of the higher-efficiency unit within 2-3 years.

Example 3: Converting EER to COP

A ductless mini split is rated at EER 14.0. What's the COP?

COP = EER ÷ 3.412 = 14.0 ÷ 3.412 = 4.10

A COP of 4.10 means the unit delivers 4.10 units of cooling energy for every 1 unit of electrical energy consumed. That's excellent efficiency — well above the COP of most residential systems.

Example 4: CEER vs EER When Shopping for a Room AC

You find a window AC with these specs:

• EER: 12.5 (active cooling only)
• CEER: 12.2 (includes standby power)

The difference (0.3 points) comes from standby power draw. Over the DOE's assumed usage pattern (750 hours cooling + 5,115 hours standby at 1.4W), that standby consumption adds about 7.2 kWh per year — roughly $1.15 at national average rates.

The CEER is the more honest number. When comparing two room ACs, always compare CEER to CEER — not EER to CEER.

Example 5: Choosing Between High-SEER and High-EER in Arizona

You're in Scottsdale, Arizona. Two 3-ton central AC systems to choose from:

• System A: SEER2 18.0, EER2 11.0. Great seasonal average, average peak performance.
• System B: SEER2 15.5, EER2 13.0. Lower seasonal average, excellent peak performance.

In Scottsdale, the AC runs at or near full load for 4-5 months per year with daytime highs routinely exceeding 100°F. System B's higher EER2 means it's more efficient during those peak months — exactly when your electricity bills are highest.

For a typical 3-ton AC running 10 hours/day at $0.14/kWh during a 150-day cooling season, System B saves roughly $85-$120/year during peak months compared to System A, despite the lower SEER2 headline number. Over a 15-year system lifespan, that advantage adds up.

EER Rating FAQ

Is a Higher EER Rating Better?

Yes. A higher EER means the air conditioner produces more cooling per watt of electricity. An EER 12 unit uses 20% less electricity than an EER 10 unit for the same cooling output.

What Is the Minimum EER Rating for Air Conditioners?

For room/window ACs, the current DOE minimum CEER ranges from 9.3 to 11.0 depending on capacity and type. These minimums increase significantly under the updated DOE rule. For central AC, the DOE requires minimum EER2 only in the Southeast and Southwest regions (11.7 EER2 for systems under 45,000 BTU/h).

What Is the Difference Between EER and CEER?

CEER includes standby and off-mode power consumption; EER does not. CEER is always slightly lower than EER for the same unit (typically by 0.1-0.3 points). CEER replaced EER on EnergyGuide labels for room ACs in 2014.

Can You Convert EER to SEER?

Approximately, yes. Use the simplified formula SEER ≈ EER ÷ 0.875 or the DOE quadratic formula EER = −0.02 × SEER² + 1.12 × SEER. These are estimates — accurate conversion requires measuring efficiency at multiple load points, which only manufacturers do during testing.

Does EER2 Replace EER?

For all new equipment manufactured after January 1, 2023, yes. EER2 uses the more realistic M1 test procedure with higher static pressure. EER2 numbers are lower than EER for the same equipment, but they more accurately reflect real-world installed performance.

What EER Rating Should I Look for in a Window AC?

Aim for a CEER of 12.0 or above for good efficiency. ENERGY STAR-certified models start around 12.8-14.7 CEER depending on capacity.

If you're in a hot climate where the AC runs most of the day, it's worth paying extra for a unit with CEER above 13.0 — the electricity savings pay for themselves within a few years. Check our CADR rating guide for related equipment metrics.