HOW THE ESTIMATE WORKS
See the assumptions behind the number.
Without an electric bill, central cooling cost = system tons × 12,000 × annual equivalent cooling hours ÷ (effective SEER × 1,000) ÷ 12 × electricity rate. The starting model uses 12¢/kWh, 2,500 annual equivalent full-load cooling hours for Southwest Louisiana and 9 effective SEER for an older system. Annual cooling cost is divided by 12; actual summer costs can be higher.
With a summer bill, central cooling is estimated as 55% of that bill. The worksheet scales both equipment and ductwork savings to that baseline. A blank bill or 0 uses the equipment model. The 55% share is an assumption, not a measurement of your home.
Each window unit is estimated at 0.9 kW × 9 hours/day × 30.4 days × your electricity rate: $29.55/month at 12¢/kWh. The headline adds window-unit cost and, when the ductwork box is checked, modeled duct-loss energy cost to the central-cooling baseline. This combined expense is a website display subtotal; the worksheet savings formulas and caps are unchanged. When a bill is entered, some window-unit use and duct losses may already be in the 55% estimate, so the subtotal can overlap. Without a bill, it combines a 12-month central-cooling average with summer window-unit use. Do not multiply it by 12 to predict annual spending.
Equipment models and savings
The v11.2 worksheet uses 14.3 for Essential (single-stage Airquest R-series), 17 for Comfort (two-stage gas or inverter heat pump/electric), and 20.5 for Premium and Platinum (variable-speed inverter). These are comparison assumptions, not a certified rating for every equipment combination. The worksheet notes 14 for an Essential Performer N-series substitution; this calculator uses the current 14.3 default. Confirm the proposed equipment and matched-system rating with your advisor.
Equipment savings = cooling baseline × (1 − current effective efficiency ÷ proposed modeled efficiency), with a minimum of $0. The modeled new-equipment costs use the same tons, hours and electricity rate. Premium and Platinum use the same efficiency, so their energy savings are equal when the selected ductwork and window-unit scope are equal.
Ductwork savings appear only when you include ductwork in the proposed scope. The estimate uses the selected loss factor and the extra runtime associated with that loss. Window-unit savings use the number of units and the worksheet’s starting operating assumptions.
Duct-loss assumptions are 10% for good/recently sealed, 25% for average, 40% for poor and 55% for severe condition. The worksheet estimates extra runtime as 1 ÷ (1 − loss), and potential duct savings as cooling baseline × loss ÷ (1 − loss), only when ductwork is included. ENERGY STAR describes typical duct air losses of about 20–30%; the higher selections are worksheet scenarios requiring an inspection, not measured losses or guaranteed energy savings.
Total monthly savings are the smallest of: equipment + selected ductwork + retired window-unit savings; 85% of the entered summer bill (or the central-cooling baseline if no bill is entered); and 60% of the central-cooling baseline + window-unit savings. The cards identify when a cap lowers the total below the sum of its parts. Calculations keep full precision; displayed dollars are rounded.
From the estimate sheet’s calculation note: “Estimates, not guarantees” — the model uses approximate electricity rates, Southwest Louisiana cooling hours and duct-leakage assumptions. Entering a real summer bill scales the equipment and duct calculations; leaving it at 0 uses the modeled baseline. Source: v11.2, PRESENT TO CUSTOMER rows 15–24 and Assumptions rows 8–29 and 76. The worksheet’s financing-payment comparison is separate from this energy tool.
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.