SEER (Seasonal Energy Efficiency Ratio) is the U.S. standard for rating central air conditioners and heat pumps. It divides the cooling output of a system (in BTU) by the electricity it consumes (in watt-hours) across a typical summer cooling season. Higher SEER = more cooling per watt = lower bills.
The number changed meaning in 2023. The Department of Energy introduced SEER2, which uses stricter testing conditions (higher external static pressure to better simulate real ductwork). A system rated SEER 15 before 2023 typically drops to about SEER2 14 under the new test. The equipment didn't change — the measurement did. When shopping, only compare SEER2 to SEER2 and SEER to SEER.
Federal minimums in 2026 are SEER2 14.3 in the southern US (FL/TX/AZ/CA/GA/NV/NM and similar climates) and SEER2 13.4 in the north. Most utility rebate programs require SEER2 15.2 or higher to qualify, so a system that just meets the federal minimum will not earn rebate money.
The payback comes from lower power use. Because SEER2 measures cooling delivered per unit of electricity, raising it cuts your cooling energy in direct proportion. Moving from SEER2 14.3 to SEER2 18, for example, uses about 20% less electricity for the same cooling. The higher-rated equipment costs more up front, so the payback period depends on your local electricity rate and how long your cooling season runs — fastest in hot climates and when stacked with utility rebates.