EER stands for Energy Efficiency Ratio, and it measures how efficiently an air conditioner converts electricity into cooling power. It’s calculated by dividing the system’s cooling output (in BTUs per hour) by its electrical consumption (in watts). A higher EER means the unit produces more cooling per watt of electricity, which translates directly to lower energy bills.
How EER Is Calculated
The formula is straightforward: take the cooling capacity in BTUs per hour and divide it by the power draw in watts. A window unit rated at 10,000 BTU/h that draws 1,000 watts has an EER of 10. The same cooling capacity from a unit drawing only 833 watts gives you an EER of 12, meaning you’re getting the same comfort for less electricity.
EER is always measured at a single, fixed outdoor temperature of 95°F. This is a critical detail. It tells you how well the system performs when it’s working its hardest on a hot day, not how it averages out across milder conditions over a whole season. That distinction is what separates EER from SEER.
EER vs. SEER
SEER (Seasonal Energy Efficiency Ratio) measures efficiency across an entire cooling season, averaging performance at a range of outdoor temperatures. It’s the headline number you’ll see on most central air conditioner spec sheets. EER, by contrast, captures performance at peak load, when temperatures hit 95°F and your system is running flat out.
Window and room air conditioners use EER as their primary efficiency metric. They don’t carry SEER ratings at all. Central air conditioners carry both, though SEER gets more attention in marketing materials. The two metrics don’t predict each other reliably. Two central AC systems with the same SEER can have noticeably different EER ratings, and that gap shows up in real-world energy costs depending on where you live.
Why EER Matters More in Hot Climates
If you live in the Southwest or any region where summer temperatures regularly exceed 95°F, EER is arguably the more important number. SEER’s temperature distribution doesn’t match what a system actually faces in hot, dry climates. Homeowners in these regions who shop by SEER alone can end up with a unit that looks efficient on paper but costs roughly 10% more to operate than a competitor with a higher EER at the same SEER rating.
EER is also a peak demand metric. Utilities care about it because air conditioners running during the hottest part of the day drive the highest electrical loads on the grid. A higher EER system draws fewer watts at exactly the moment electricity demand (and sometimes pricing) is at its peak. If your utility charges time-of-use rates, a high EER unit saves you money when rates are steepest.
What Counts as a Good EER
For room and window air conditioners, an EER in the 10 to 12 range is common for standard models. ENERGY STAR certified units will be higher, though exact thresholds shift as standards update. When shopping, a difference of even one or two EER points adds up over years of use, especially if the unit runs for long stretches during summer.
For central air conditioners, EER values are typically expressed as EER2 under the current testing standard (more on that below). As of 2025, split system central air conditioners need an EER2 of at least 12.0 and a SEER2 of at least 17.0 to qualify for federal energy tax credits. Packaged systems need an EER2 of 11.5 or higher. These thresholds represent the high-efficiency tier, not the bare minimum for sale, so meeting them puts a system well above average.
EER2: The Updated Standard
As of January 2023, the U.S. Department of Energy shifted to updated efficiency metrics: SEER2, EER2, and HSPF2. These new ratings use revised test procedures designed to better reflect real-world installation conditions, including more realistic ductwork and static pressure. The goal was to close the gap between lab-tested efficiency and the performance homeowners actually experience.
EER2 ratings are similar to the original EER but not directly comparable. A system’s EER2 number will generally be slightly lower than its old EER rating for the same unit, because the testing conditions are more demanding. When comparing systems, make sure you’re comparing EER2 to EER2, not mixing old and new metrics. Any equipment manufactured or sold after the January 2023 cutover uses the new ratings exclusively.
How EER Affects Your Energy Costs
The relationship between efficiency ratings and energy savings is roughly linear. Each one-point increase in efficiency translates to about a 7% improvement in energy use. So upgrading from a window unit with an EER of 9 to one rated at 12 represents roughly a 21% reduction in electricity consumed for the same cooling output. Over a five or ten year lifespan, that gap compounds into meaningful savings, particularly in climates where you run the AC for several months straight.
To estimate your own costs, you can reverse the EER formula. Divide the unit’s BTU rating by its EER to get the wattage it draws, then multiply by your electricity rate and the hours you run it. A 12,000 BTU unit with an EER of 10 draws 1,200 watts. At a national average of roughly $0.16 per kilowatt-hour, running that unit for 8 hours costs about $1.54 per day. Bump the EER to 12 and the same cooling costs $1.28 per day. That’s a small daily difference that grows to $50 or more over a full summer.
When to Prioritize EER Over SEER
Focus on EER (or EER2) if any of the following apply to you: you live in a hot climate where temperatures frequently hit 95°F or higher, you pay time-of-use electricity rates that spike during afternoon hours, or you’re buying a window or portable air conditioner where SEER isn’t even part of the equation. For central AC in moderate climates where the system cycles through a wide range of temperatures across the season, SEER2 gives you a better picture of overall annual costs, but checking the EER2 as well ensures the system doesn’t underperform on the hottest days when you need it most.