The Energy Efficiency Ratio (EER) is a standardized measurement of how efficiently an air conditioner converts electricity into cooling power. It’s calculated by dividing a unit’s cooling capacity in BTU per hour by its power consumption in watts. The higher the number, the less electricity the unit needs to cool the same space.
How EER Is Calculated
The formula is straightforward: EER = BTU per hour ÷ watts. If a 10,000 BTU air conditioner draws 1,200 watts of power, its EER is 8.3. A more efficient unit with the same cooling output might only draw 900 watts, giving it an EER of 11.1. Both cool the same amount, but the second one uses 25% less electricity to do it.
What makes EER useful is that it’s measured under a single, fixed set of conditions. The standard test runs with the outdoor temperature at 95°F and 75°F wet bulb, while the indoor side is set to 80°F and 67°F wet bulb. That 95°F outdoor condition matters because it represents a hot afternoon when your AC is working hardest and your electricity bill is climbing fastest.
EER vs. SEER
You’ll often see both EER and SEER on air conditioning specs, and the difference comes down to one word: seasonal. EER captures efficiency at a single peak temperature (95°F outdoors). SEER averages efficiency across an entire cooling season, testing at outdoor temperatures ranging from 65°F to 104°F. Because SEER blends in all those milder days when the unit barely has to work, the SEER number for any given unit will always be higher than its EER.
This distinction has real consequences depending on where you live. SEER works well for climates with moderate summers, where your AC cycles through a wide range of conditions. But in hot, dry regions like Phoenix or Palm Springs, the seasonal average can be misleading. In a typical Palm Springs home, hours above 94°F account for more than two-thirds of annual cooling energy use. The SEER formula weights temperatures above 90°F at only about 7.4%, roughly one-eighth of the 68% weighting that would actually reflect Phoenix’s climate. Two units with identical SEER ratings can differ by as much as 10% in real operating costs in those climates, purely because one has a lower EER at peak heat.
If you live somewhere that regularly hits the mid-90s or higher, EER is the number to compare. It tells you how the unit performs precisely when demand and energy prices peak, and when your compressor begins running constantly (typically around 98°F outdoors).
What Counts as a Good EER
For room air conditioners, most units on the market today fall between 8 and 12 EER. A rating around 10 is solid for a standard window unit. Anything above 12 is considered high efficiency, and you’ll typically pay more upfront for it. Units below 9 are on the low end and will cost noticeably more to run over a summer.
Central air conditioning systems are harder to compare directly because their EER depends on the complete system, including the compressor, coil, and air handler. But the same principle holds: a higher number means lower operating costs at peak temperatures. When shopping, comparing EER side by side between models of the same size gives you the clearest picture of which one will cost less to run on the hottest days of the year.
The Shift to EER2
In 2023, federal standards introduced an updated metric called EER2. The key change is how the test accounts for ductwork. The original EER test didn’t fully factor in the energy required to push air through ducts, which every central AC system has to do in real life. EER2 uses a higher external static pressure during testing, simulating the resistance that ductwork creates in an actual home installation.
Because EER2 tests under more demanding conditions, EER2 numbers for the same unit will be lower than the old EER numbers. This doesn’t mean equipment got less efficient overnight. It means the rating now reflects what the system actually does once it’s installed, rather than how it performs on a test bench with minimal airflow resistance. When comparing units, make sure you’re looking at the same metric. Mixing old EER with new EER2 ratings will make newer units look worse than they are.
Using EER to Estimate Energy Costs
Because EER directly ties cooling output to electricity consumption, you can use it to estimate what a unit costs to run. Take the BTU rating, divide by the EER, and you get the wattage the unit draws. Multiply that by your electricity rate and the hours you expect to run it, and you have a rough cost estimate.
For example, a 10,000 BTU unit with an EER of 10 draws 1,000 watts (10,000 ÷ 10). Running it for 8 hours a day at an electricity rate of $0.15 per kilowatt-hour costs about $1.20 per day. The same size unit with an EER of 8.3 draws 1,200 watts and costs about $1.44 per day. That 24-cent daily difference adds up to roughly $22 over a three-month summer, and more in hotter climates where the unit runs longer.
The gap widens with larger systems. For a central AC running 10 or more hours a day in a desert climate, the difference between a high-EER and low-EER system can easily reach $100 or more per cooling season, making EER one of the most practical numbers to check before you buy.