Cooling delivered. One ton of cooling is 12,000 Btu per hour. So the cooling a system delivers in a year is its tons times 12,000 times its full-load hours.
Electricity used. SEER2 is the heat a system removes over a cooling season, in Btu, divided by the electricity it uses, in watt-hours. So the kilowatt-hours used are the cooling delivered divided by the SEER2 times 1,000. In short: kWh = tons × 12,000 × hours ÷ (SEER2 × 1,000).
Savings. We run that for both ratings, take the difference in kilowatt-hours, and multiply by your electricity price. This follows the form of the savings equation in the Uniform Methods Project's protocol for unitary and split-system cooling equipment (NREL, published on energy.gov), which multiplies capacity by the difference of 1/SEER for the baseline and installed units and by equivalent full-load hours. That protocol assumes more efficient equipment of the same capacity runs the same number of hours as the baseline equipment, and this tool makes the same assumption for your current and new systems. The protocol's baseline is a code-minimum unit, not the unit being replaced, and it does not cover early replacement, so this tool borrows the equation's form, not the protocol's scope.
What the estimate leaves out
- SEER2 is a seasonal rating measured under DOE test conditions (Appendix M1). Real use in your home will differ, so results are estimates.
- Full-load hours are your input. For Phoenix only, the tool offers SRP's modeling assumption of 2,141 cooling hours a year as an example; SRP does not say whether these are full-load hours, and it is not a measured figure for any home.
- The default electricity price is a statewide average for July 2026. Tiered and time-of-use rates make the value of each saved kilowatt-hour different.