HOW THE ESTIMATE WORKS
See the assumptions behind the number.
This calculator starts with your full average summer electric bill. It does not ask for system size: once current spending is estimated from a bill, equipment comparisons use an efficiency ratio. System sizing still requires a professional load calculation before equipment is selected. Results describe a summer month, not an annual monthly average; do not multiply them by 12 to forecast annual savings.
The estimated central-cooling share of your summer electric bill is based on your current duct condition: 55% for Good or recently sealed, 60% for Average, 65% for Poor, and 70% for Severe. These S&S planning assumptions allow for a larger cooling share when deteriorated ducts make the system run longer. They are not measured energy use or a guarantee for your home. Changing the selected condition changes the estimated central-cooling portion of your bill; the full bill you entered stays the same.
Window-unit spending is added to the central-cooling estimate to show combined cooling cost. Increasing the window-unit count increases that total; it does not reduce the central-cooling estimate. Both costs are portions of the full electric bill you entered, so a combined estimate above that bill is rejected. This assumes the selected central-cooling percentage excludes window units. Duct losses are already part of central-system spending. The separate project checkbox adds the savings from improving the ducts, only when that work is included. It does not change the assumed condition of the ducts you have today.
Each window unit uses an assumed 0.9 kW × 12 hours/day × 30.4 days: $39.40 per summer month at 12¢/kWh. Window replacement savings allow for the central system to take over that cooling, using an assumed equivalent efficiency of 9 for the old window unit and the proposed central system’s efficiency and duct delivery. These comparison assumptions do not substitute for measured loads or directly equate certified room-AC and central-AC ratings.
Equipment models and savings
This comparison uses assumed efficiencies of 14.3 for Essential (single-stage), 17 for Comfort (two-stage gas or inverter heat pump/electric), and 20.5 for Premium and Platinum (variable-speed inverter). Actual efficiency depends on the specific indoor and outdoor equipment combination. SEER and SEER2 use different test procedures and are not directly interchangeable; this simplified estimate assumes comparable effective efficiencies. Your advisor can confirm the equipment ratings and refine the comparison for your home.
Equipment savings = current central-system spending × (1 − current effective efficiency ÷ proposed modeled efficiency). A less efficient replacement can increase cost; the calculator shows that rather than inventing savings. Premium and Platinum share an efficiency model, so equal scope produces equal energy savings.
Ductwork savings apply only when included in the proposed project. They reduce the remaining cost after the equipment upgrade, preventing double counting. Existing delivery-loss scenarios are 10%, 25%, 40% and 55%; the proposed repair targets 10% residual loss. Remaining central cost = equipment-only cost × (1 − existing loss) ÷ (1 − proposed loss). Already-good ducts receive no assumed repair savings.
ENERGY STAR discusses typical duct air leakage. Air leakage is not automatically the same percentage of bill savings. The loss factors represent assumed seasonal delivery-energy losses, not measured air-leakage percentages. They and the 10% repair target are simplified S&S planning assumptions requiring inspection. DOE compares cooling cost using equipment efficiency; actual matched equipment ratings, installation and operating conditions must be confirmed.
Total savings = current cooling cost − proposed cooling cost. Proposed cost includes central cooling after equipment and selected duct improvements, plus the electricity needed to replace window-unit cooling. The unrelated portion of the electric bill is held constant. Financing payments are not part of this calculation. Components and totals are calculated at full precision and rounded for display.
A starting point for your home. These estimates depend on assumed cooling use, equipment efficiency and duct performance. An on-site assessment helps us refine the comparison before recommending a system. Actual savings will vary.
What this result cannot tell you
Utility rates, weather, thermostat settings, insulation, occupancy, equipment condition and duct performance all affect actual use. The calculator does not perform a load calculation, select equipment, diagnose a system, calculate financing or predict the non-cooling portion of a bill.