EER measures cooling efficiency at a single peak condition (95 degrees F outdoor), while SEER measures cooling efficiency averaged across an entire season (65-104 degrees F). SEER is always a higher number than EER for the same unit because it includes efficient part-load operation during milder weather. A central AC with 12 EER might have 17 SEER — same unit, different measurements capturing different aspects of performance.
Use SEER when estimating annual cooling costs. Use EER when you need to know how the unit performs on the hottest days. Both have been updated to SEER2 and EER2 since January 2023.
The Core Difference
Why SEER Is Always Higher Than EER
This confuses many homeowners. The same air conditioner might be rated 12 EER and 18 SEER. That's not contradictory — they're measuring different things.
EER captures the hardest working moment. At 95 degrees F outdoor, the system runs at maximum capacity. The compressor works hard, consuming significant electricity. This gives you the lowest efficiency number.
SEER captures the whole season. Most cooling hours occur when outdoor temperatures are 75-90 degrees F, not 95 or higher. During those milder conditions, the system cycles on and off (single-speed) or runs at reduced capacity (two-stage/variable-speed), achieving much higher efficiency. When the seasonal average is calculated, those efficient mild-weather hours pull the SEER number well above the EER.
The gap between SEER and EER reveals how much a system benefits from part-load operation:
The larger the SEER/EER ratio, the more the system benefits from running at partial capacity. Variable-speed units with ratios of 1.7+ are efficiency champions during mild weather but only modestly better than two-stage units on the hottest days.
Which Should You Prioritize?
Prioritize SEER (or SEER2) When:
You live in a moderate climate where most cooling happens at outdoor temperatures below 90 degrees F. Regions like the mid-Atlantic, upper South, Midwest, and Pacific Northwest see limited time above 95 degrees F. The seasonal average tells the real story of your energy costs.
Your electricity rate is flat (same $/kWh regardless of time). If you don't face peak-demand charges, total seasonal kWh consumption (which SEER captures) drives your bill.
You care about annual cost estimates. When comparing AC units for total yearly savings, SEER/SEER2 is the right metric.
Prioritize EER (or EER2) When:
You live in an extreme heat climate like Phoenix, Las Vegas, Houston, or southern Florida. When your AC runs at full capacity for 6-8 weeks straight, EER determines your peak-month costs.
Your utility uses demand charges or time-of-use pricing. Peak electricity rates coincide with peak cooling demand (hot summer afternoons). High EER means fewer watts drawn during the most expensive hours.
You're choosing a supplemental cooling unit (window AC, portable AC) that runs at full capacity whenever it's on.
You want to evaluate true peak-day performance. A 22 SEER/12 EER unit and a 17 SEER/13 EER unit are very different on the hottest day of the year.
For Most People
Use SEER2 as the primary comparison metric for annual cost estimates. Use EER2 as a secondary check, especially if you're in a hot climate. The ideal unit has strong numbers in both categories.
Approximate EER-to-SEER Relationship
There's no exact conversion because the relationship depends on the compressor type. However, these general rules apply:
For single-stage systems: SEER is roughly 1.2-1.3 times EER.
For two-stage systems: SEER is roughly 1.4-1.5 times EER.
For variable-speed systems: SEER is roughly 1.6-1.9 times EER.
These ratios apply similarly to SEER2 and EER2.
Real-World Impact Example
Consider two 3-ton AC systems in Houston (2,200 cooling hours, $0.15/kWh):
System A: 22 SEER2 / 11.5 EER2 (variable-speed)
System B: 17 SEER2 / 13.0 EER2 (two-stage)
Annual cost based on SEER2: System A costs $368, System B costs $476. System A saves $108/year on the seasonal average.
Peak-month cost (July, 350 hours near full load): System A costs $110, System B costs $97. System B saves $13 in the peak month because it has better full-load efficiency.
Over the full year, System A wins on total energy cost due to its massive part-load advantage. But System B delivers lower electricity bills during the hottest month.
Key Takeaways
- EER = peak-day efficiency at 95F. SEER = seasonal average across 65-104F.
- SEER is always higher than EER for the same unit, because seasonal averages include efficient mild-weather operation
- Variable-speed systems have the biggest SEER/EER gap (ratio of 1.6-1.9x). They dominate in part-load but are only moderately better at full load.
- Use SEER2 for annual cost estimates. Use EER2 for peak-month or peak-day analysis.
- Hot climates should check EER2 alongside SEER2 to avoid surprises on August electricity bills
- Both were updated in 2023: SEER became SEER2, EER became EER2 (roughly 4.7% lower numbers)
- There's no exact EER-to-SEER conversion — the ratio depends on compressor type
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