SEER stands for Seasonal Energy Efficiency Ratio, and it’s calculated by dividing an air conditioner’s total cooling output over a typical cooling season (measured in BTUs) by the total electrical energy it uses over that same period (measured in watt-hours). A higher SEER number means the system uses less electricity to produce the same amount of cooling. Most residential central air conditioners sold today fall between 14 and 25 SEER.
The Basic SEER Formula
At its simplest, SEER equals cooling output divided by energy input:
SEER = Total BTUs removed ÷ Total watt-hours consumed
This looks similar to another efficiency metric called EER (Energy Efficiency Ratio), but there’s an important difference. EER measures efficiency at one specific outdoor temperature, typically 95°F. SEER averages performance across an entire cooling season, accounting for the fact that your air conditioner doesn’t always run at full blast. On mild 75°F days it cycles on and off, and on scorching 100°F days it runs continuously. SEER captures that real-world variation by weighting performance across a range of outdoor temperatures from 65°F to 104°F.
Because SEER includes those lighter-load conditions where the unit is more efficient, a system’s SEER rating is always higher than its EER rating. A unit rated at 16 SEER might have an EER of only 12 or 13.
How to Estimate SEER From Your Equipment
If you want a rough SEER calculation for a system you already own, you need two numbers: the unit’s cooling capacity in BTU per hour and its electrical power draw in watts. Both are printed on the equipment’s nameplate or spec sheet.
Start with the EER-style snapshot. Divide the BTU/h rating by the wattage. For example, a 36,000 BTU/h unit (that’s a 3-ton system) drawing 3,000 watts gives you an EER of 12. To approximate the seasonal rating from that single-point EER, a commonly used shorthand is:
Estimated SEER ≈ EER × 1.12
So an EER of 12 translates to roughly 13.4 SEER. This multiplier isn’t exact for every system, but it gets you in the ballpark. Variable-speed and two-stage systems tend to have a larger gap between EER and SEER because they perform especially well at partial loads.
Converting Between BTUs and Watts
SEER math sometimes requires converting between thermal units and electrical units. The key number: 1 watt equals 3.412 BTU/h. So if you know your system’s cooling capacity in watts and need it in BTU/h, multiply by 3.412. Going the other direction, divide BTU/h by 3.412 to get watts.
This conversion is useful when calculating annual energy costs. Say you have a 24,000 BTU/h system with a SEER of 16. Divide 24,000 by 16 to get 1,500 watt-hours of electricity per hour of equivalent full-load cooling. Over a season with roughly 1,000 cooling hours, that’s 1,500 kilowatt-hours. Multiply by your local electricity rate (say $0.15/kWh) and you get about $225 per cooling season.
Using SEER to Compare Annual Costs
The practical reason most people care about SEER is to compare operating costs between two systems. The formula for estimated annual electricity use is:
Annual kWh = (BTU/h × cooling hours) ÷ (SEER × 1,000)
Here’s a concrete comparison for a 3-ton (36,000 BTU/h) system running 1,000 hours per season at $0.15/kWh:
- 14 SEER: 36,000 × 1,000 ÷ 14,000 = 2,571 kWh → about $386/year
- 18 SEER: 36,000 × 1,000 ÷ 18,000 = 2,000 kWh → about $300/year
- 22 SEER: 36,000 × 1,000 ÷ 22,000 = 1,636 kWh → about $245/year
Jumping from 14 to 18 SEER saves roughly $86 per year in this scenario. Whether that justifies the higher equipment cost depends on how many years you plan to run the system and how hot your climate is. In Phoenix, where cooling hours can exceed 2,000, the savings double. In Seattle, where you might only run air conditioning 400 hours, the payback period stretches considerably.
What Changed With SEER2
Starting January 1, 2023, the Department of Energy introduced a revised testing method called SEER2, based on a new test procedure known as Appendix M1. The biggest change is how ductwork resistance is simulated during testing. The new protocol requires higher external static pressure on the indoor unit, which mimics real-world duct conditions more accurately. Because the test is harder, SEER2 numbers come out slightly lower than the old SEER numbers for the same equipment.
The conversion is roughly: SEER2 ≈ SEER × 0.95. So a system previously rated at 16 SEER might now be labeled around 15.2 SEER2. The equipment didn’t get less efficient; the test just became more realistic. New federal minimum standards are now expressed in SEER2, with most regions requiring at least 14.3 or 15.2 SEER2 for central air conditioners depending on where you live.
If you’re comparing a new unit rated in SEER2 to an older unit rated in SEER, multiply the SEER2 number by roughly 1.05 to get an apples-to-apples comparison. Or multiply the old SEER by 0.95 to convert it to the SEER2 scale.
Why Published SEER Differs From Real-World Performance
SEER is measured under controlled lab conditions with clean filters, properly charged refrigerant, and correctly sized ductwork. Your actual efficiency will be lower if any of those factors are off. Dirty filters alone can reduce cooling performance by 5 to 15 percent. Leaky ducts, which lose conditioned air into attics or crawlspaces, can waste 20 to 30 percent of the energy your system uses. And a system with too much or too little refrigerant won’t hit its rated efficiency no matter how high the SEER sticker says.
The SEER rating also assumes the system is correctly sized for your home. An oversized unit short-cycles, turning on and off frequently without running long enough to dehumidify properly or reach its most efficient operating point. This is especially relevant for high-SEER variable-speed systems, which achieve their best efficiency during long, steady runs at low speed. If the unit is too large for the space, it never gets the chance to settle into that efficient low-speed mode, and your real-world performance falls well short of the published number.