The SEER formula is: SEER = Total Cooling Output (BTU) divided by Total Electrical Energy Input (Wh) over a full cooling season. For practical purposes, you can estimate your annual cooling cost using: Annual Cost = (Cooling Capacity x Cooling Hours) / (SEER x 1,000) x Electricity Rate. A 3-ton, 16 SEER AC running 1,500 hours at $0.16/kWh costs approximately $540 per year to operate.
This guide walks through the SEER formula step by step, shows you how to calculate cooling costs, and provides worked examples so you can compare any two AC units on an equal basis.
The Basic SEER Formula
SEER = Total Cooling Output (BTU) / Total Electrical Input (Wh)
Both values are measured over an entire standardized cooling season. In lab testing, this involves running the system across eight temperature bins from 65 to 104 degrees F, weighted by the typical hours at each temperature.
You won't do this lab test yourself. Instead, you'll use the published SEER or SEER2 rating to calculate practical things like energy costs and savings.
Formula 1: Estimate Annual Cooling kWh
Annual kWh = (Cooling Capacity BTU/h x Annual Cooling Hours) / (SEER x 1,000)
This formula tells you how many kilowatt-hours your AC uses per year for cooling.
Example: A 3-ton AC (36,000 BTU/h) rated at 16 SEER runs for 1,500 hours per year.
Annual kWh = (36,000 x 1,500) / (16 x 1,000) = 54,000,000 / 16,000 = 3,375 kWh
Tonnage to BTU/h Conversion
| System Size (Tons) | Cooling Capacity (BTU/h) | Typical Home Size |
|---|---|---|
| 1.5 | 18,000 | 600-1,000 sq ft |
| 2.0 | 24,000 | 1,000-1,300 sq ft |
| 2.5 | 30,000 | 1,300-1,600 sq ft |
| 3.0 | 36,000 | 1,600-2,100 sq ft |
| 3.5 | 42,000 | 2,100-2,400 sq ft |
| 4.0 | 48,000 | 2,400-2,800 sq ft |
| 5.0 | 60,000 | 2,800-3,500 sq ft |
Formula 2: Estimate Annual Cooling Cost
Annual Cost = Annual kWh x Electricity Rate ($/kWh)
Or combined into one formula:
Annual Cost = (BTU/h x Hours) / (SEER x 1,000) x $/kWh
Example: Same 3-ton, 16 SEER unit, 1,500 hours, at $0.16/kWh.
Annual Cost = 3,375 kWh x $0.16 = $540
Formula 3: Calculate Savings Between Two Units
Annual Savings = Annual Cost (Old Unit) - Annual Cost (New Unit)
Or directly:
Annual Savings = (BTU/h x Hours x $/kWh) x (1/SEER_old - 1/SEER_new) / 1,000
Example: Upgrading from 12 SEER to 18 SEER. 3-ton, 1,500 hours, $0.16/kWh.
Old cost: (36,000 x 1,500) / (12 x 1,000) x $0.16 = $720/year
New cost: (36,000 x 1,500) / (18 x 1,000) x $0.16 = $480/year
Annual savings: $720 - $480 = $240/year
Or using the direct formula: Savings = (36,000 x 1,500 x 0.16) x (1/12 - 1/18) / 1,000 = 8,640 x (0.0833 - 0.0556) = 8,640 x 0.0278 = $240/year
Formula 4: Simple Payback Period
Payback (years) = Additional Cost of Higher-SEER Unit / Annual Savings
Example: The 18 SEER unit costs $2,000 more than the 12 SEER unit.
Payback = $2,000 / $240 = 8.3 years
Pre-2026 (with the now-expired federal 25C credit): Through Dec 31, 2025, a $600 AC 25C credit reduced the upgrade premium to $1,400, shortening payback to ($2,000 − $600) / $240 = 5.8 years. The 25C credit expired for installs after Dec 31, 2025 under the OBBBA (PL 119-21, signed July 4, 2025), so for 2026 installs the payback returns to 8.3 years before any state/utility rebates. State and utility rebates ($100–$500 typical) plus IRA HOMES (performance-based) can still shorten 2026 payback by 1–2 years. (Sources: IRS OBBB FAQ; Congress.gov CRS IN12611.)
Formula 5: Convert Between SEER and SEER2
SEER2 = SEER / 1.047
SEER = SEER2 x 1.047
Example: Your old AC is rated 16 SEER. What's the SEER2 equivalent?
SEER2 = 16 / 1.047 = 15.3 SEER2
A new 15.3 SEER2 unit is approximately equivalent in efficiency to your old 16 SEER unit.
Formula 6: Convert SEER to COP
COP = SEER / 3.412
This gives you the seasonal average COP for cooling.
Example: An 18 SEER unit.
COP = 18 / 3.412 = 5.27
This means the unit delivers 5.27 BTU of cooling per BTU of electrical input on average over the season. (This sounds incredibly efficient because SEER heavily weights mild-weather performance.)
Complete Worked Examples
Example A: Florida Homeowner
You live in Tampa, FL. 4-ton system, 2,500 cooling hours, $0.15/kWh. Comparing 14.3 SEER2 vs 20 SEER2.
14.3 SEER2 cost: (48,000 x 2,500) / (14.3 x 1,000) x $0.15 = 8,392 kWh x $0.15 = $1,259/year
20 SEER2 cost: (48,000 x 2,500) / (20 x 1,000) x $0.15 = 6,000 kWh x $0.15 = $900/year
Annual savings: $359. 15-year savings: $5,385.
Example B: Minnesota Homeowner
You live in Minneapolis. 3-ton system, 800 cooling hours, $0.14/kWh. Comparing 13.4 SEER2 vs 16 SEER2.
13.4 SEER2 cost: (36,000 x 800) / (13.4 x 1,000) x $0.14 = 2,149 kWh x $0.14 = $301/year
16 SEER2 cost: (36,000 x 800) / (16 x 1,000) x $0.14 = 1,800 kWh x $0.14 = $252/year
Annual savings: $49. 15-year savings: $735.
Example C: Comparing Old System to New
Your 15-year-old system is rated 10 SEER. You're considering a new 18 SEER2 unit. 3-ton, 1,500 hours, $0.16/kWh.
First convert old SEER to SEER2 for apples-to-apples: 10 / 1.047 = 9.6 SEER2 equivalent.
Old system cost (9.6 SEER2 equivalent): (36,000 x 1,500) / (9.6 x 1,000) x $0.16 = $900/year
New 18 SEER2 cost: (36,000 x 1,500) / (18 x 1,000) x $0.16 = $480/year
Annual savings: $420. 15-year savings: $6,300.
Your old system may perform below its rated SEER. After 15 years, dirty coils, low refrigerant, and worn components can reduce actual efficiency by 10-20%. Your real savings from replacing an old unit may be even higher than these calculations suggest.
SEER2 Savings Calculator
Use this interactive calculator for instant comparisons:
This calculator is being re-verified
We’re auditing all of our calculators against primary-source standards (ENERGY STAR, DOE, ACCA Manual J, NEC) and re-publishing each once its math is confirmed. This one will be back shortly.
Where the SEER Number Actually Comes From (AHRI 210/240)
SEER is not measured at a single operating point. AHRI Standard 210/240 defines outdoor temperature bins at 5 °F intervals from about 65 °F up to 104 °F+. Each bin is weighted by the fraction of cooling-load hours a nationally-representative U.S. residence would experience in that bin, based on DOE-referenced typical-meteorological-year data.
The distribution is heavily skewed toward the milder bins: cooling hours in the 65–77 °F range dominate the annual total, while hours above 95 °F contribute only a small fraction. That skew is why a variable-speed inverter unit (which throttles down to run continuously at low power during the mild bins) rates higher SEER than a single-speed unit of the same nameplate capacity — the variable-speed spends more time near its efficient operating point where most residential cooling hours actually happen.
For the exact bin weightings, consult AHRI Standard 210/240 (paywalled, purchasable from ahrinet.org) or the DOE test procedure at 10 CFR Part 430 Subpart B Appendix M (SEER) and Appendix M1 (SEER2). Those documents are the authoritative source; the ratings on any manufacturer spec sheet trace back to test data compiled per those bin weightings.
Single-Stage vs Two-Stage vs Variable-Speed Testing
For single-stage compressors, testing is simpler — one capacity point, one power point. The unit cycles on and off at part-load hours, which is captured in the seasonal weighting via a Part-Load Factor (PLF) accounting for cycling losses.
Two-stage compressors are tested at both stages. Variable-speed inverter compressors are tested at multiple operating points and the seasonal weighting picks the most-efficient stage the unit would actually use at each outdoor temperature.
Hypothetical Worked Example
For a representative 3-ton (36,000 BTU) single-stage central AC with these test-condition results:
| Point | Outdoor °F | Capacity (BTU/hr) | Power (W) | EER |
|---|---|---|---|---|
| A (rated) | 95 °F | 36,000 | 3,200 | 11.25 |
| B (mid bin) | 82 °F | 37,500 | 2,900 | 12.93 |
Applying the AHRI 210/240 seasonal weighting to this single-stage unit produces SEER in the mid-14 range. The 82 °F point matters more than the 95 °F point because 82 °F represents a much larger fraction of typical cooling hours than 95 °F does. Even a unit rated for extreme heat is efficient mostly in mild conditions — a couple of hot afternoons each year contribute far less to the seasonal average than the many mild days.
(Numbers above are representative for illustration; actual manufacturer-tested SEER values are on the AHRI Directory for specific model matches.)
The SEER-to-SEER2 Change
In 2023, the DOE replaced SEER with SEER2. Same formula, same bins, but a different test condition: external static pressure increased from 0.1 in. w.c. to 0.5 in. w.c. The change reflects real-world duct pressure drop that the 0.1 test underrepresented. The same physical unit will rate about 4.7% lower on SEER2 than on SEER — approximately, SEER2 ≈ SEER ÷ 1.047. This is a rule-of-thumb approximation, not an exact conversion; the AHRI-certified rating is the authoritative figure for any specific unit.
Climate Adjustment (Why the Number Underpredicts in Some Places)
SEER's weighting is a national average. In climates skewed away from that average, the actual seasonal ratio differs:
- Phoenix — much more time above 95 °F than the national-average AHRI weighting assumes. Real-world seasonal EER is closer to the rated 95 °F EER than to the SEER number.
- Portland — much more time in the 65–77 °F range than the average. Real-world seasonal EER is closer to the low-temperature bin performance, often exceeding the SEER rating.
For rough site-specific estimates, use EER for hot climates (Zone 1–2) and SEER for mild climates (Zone 4–6). For a precise calculation, look up the AHRI-certified capacity + power at your local design temperature and compute your own weighted average from local cooling-degree-hour data (available from NOAA climate normals).
Key Takeaways
- Core formula: SEER = Total BTU Output / Total Wh Input over a full cooling season
- Annual cost = (BTU/h x Hours) / (SEER x 1,000) x $/kWh
- Savings = Old Cost - New Cost using the formula for each SEER rating
- Payback = Extra Cost / Annual Savings (subtract tax credits from extra cost)
- SEER to SEER2: divide by 1.047. SEER2 to SEER: multiply by 1.047.
- SEER to COP: divide by 3.412 for the seasonal cooling efficiency ratio
- Key variables: system size (tons), cooling hours, electricity rate, and SEER rating. Change any of these and the economics shift.
Frequently Asked Questions
This varies by climate. Northern US: 600-1,000 hours. Mid-Atlantic: 1,000-1,500. Southeast: 1,500-2,500. Deep South: 2,000-3,000+. You can estimate from your utility bills by counting months with cooling usage and assuming 8-16 hours of runtime per day during those months.
Sources & References
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