Central AC System GuideA good central AC choice starts with correct sizing, then weighs efficiency against cost, and ends with a quality installation. The Department of Energy and ENERGY STAR set the efficiency benchmarks, ACCA Manual J governs sizing, and named cost references show what each efficiency tier costs. The right system is the one sized to the load and rated for the region.
Size to the load, not the square footage
Correct capacity comes first because an oversized system cools poorly and costs more. Energy Vanguard notes the traditional rule of thumb of one ton of cooling per 400 to 600 square feet, then shows that modern homes usually need less, with a 75 home sample in hot climates averaging about 1,200 square feet per ton.
The reliable method is a load calculation. Energy Vanguard points to the ACCA Manual J and Manual S protocols, which account for insulation, windows, and airtightness that a square-footage rule ignores. Ask any contractor to show the Manual J result behind the tonnage they propose.
Understand the efficiency rating
Cooling efficiency is measured by SEER2. Trane explains that SEER stands for Seasonal Energy Efficiency Ratio and that SEER2 is the updated version, defined as the ratio of total heat removed during the cooling season divided by the total electric energy the unit uses over the same period. A higher number means a more efficient system.
SEER2 replaced SEER on January 1, 2023 under a Department of Energy testing update, so a system's SEER2 number is the one that matters when comparing new equipment. Choosing efficiency is choosing where a home sits on the operating-cost curve.
Meet the regional minimum, then decide how far to go
Every new system must clear its regional floor. HVACDirect summarizes that northern states require split system air conditioners to meet 13.4 SEER2, while southeastern and southwestern states require 14.3 SEER2 for split systems, effective January 1, 2023. ENERGY STAR certification, per the Department of Energy Federal Energy Management Program, currently sits at 15.2 SEER2.
Going above the minimum is a payback question. The FEMP finds an ENERGY STAR model pays off in hot-dry states at no more than $869 in 2023 dollars above a less efficient model, saving up to $3,154 over its lifetime, and in northern states at no more than $927 above, saving up to $3,363. Higher efficiency makes the most sense where cooling runs the longest.
Weigh the up-front cost of higher efficiency
Efficiency has a price tag. For a 3 ton unit, Fixr lists $1,725 to $3,600 at 14 SEER, $2,875 to $6,000 at 16 SEER, and $5,750 to $9,600 at 20 SEER. HomeAdvisor confirms the direction, noting higher SEER units cost more up front while lowering long term energy bills.
Balance the premium against how long a home will run the system and local electricity rates. A high efficiency unit rewards a hot climate and a long ownership horizon more than a mild climate and a short one.
Check incentives, with the post-2025 caution
Incentives can tilt the choice toward higher efficiency. ENERGY STAR states that qualifying split systems must meet SEER2 of 17.0 or higher and EER2 of 12.0 or higher, with a credit of 30 percent of project cost up to $600 claimed on IRS Form 5695, for products purchased and installed between January 1, 2023 and December 31, 2025.
The 25C energy-efficient home improvement credit changed for improvements placed in service after 2025, so do not treat any current percentage or cap as settled for a purchase made now. Confirm the live rules at IRS.gov or energy.gov before letting a credit influence the buying decision.