Methodology & sources
The model (degree-day method)
monthly AC kWh = floor area × U × CDDmonth × 24 ÷ (SEER2 × 1000)
- CDD (cooling degree days, base 65°F): how much and how long it's hotter than 65°F. States use the NOAA NCEI nClimDiv statewide series averaged over 1991–2020. Cities use NOAA 1991–2020 station normals.
- U is the heat gain per ft² of floor per °F (BTU/h·°F·ft²). It's a modeling assumption that lumps insulation, windows, sun and air leaks: good 0.25, average 0.33, poor 0.45. It's calibrated so a 2,000 ft² Phoenix home lands in a plausible range. We'll refine it against published consumption data.
- SEER2 converts cooling load (BTU) to electricity (Wh). Real-world efficiency is often lower than the rating.
- Cost = kWh × energy price. Fixed monthly charges are excluded because they don't change with AC use.
These are estimates for comparison and budgeting, not a bill, quote or engineering load calculation (for sizing, ask for an ACCA Manual J). Thermostat setting, shade, duct leaks and occupancy can move results ±30% or more.
Data sources
- U.S. EIA, Electric Power Monthly, Table 5.6.B (year-to-date, July 2026 YTD) and 5.6.A (July 2026), average residential price by state: eia.gov/electricity/monthly
- NOAA NCEI nClimDiv statewide cooling degree days (climdiv-cddcst): ncei.noaa.gov/pub/data/cirs/climdiv
- NOAA NCEI U.S. Climate Normals 1991–2020 (monthly): ncei.noaa.gov
- SRP residential price plans: srpnet.com
- FPL Residential Rates (RS-1), October 2026: fpl.com (PDF)
- NV Energy (Nevada Power) residential rate schedules, Oct 1, 2026: nvenergy.com (PDF)
- APS Rate Schedule TOU-E: aps.com tariff (PDF)
Data refreshes monthly through an automated pipeline. Alaska, Hawaii and DC aren't in the NOAA statewide series, so the calculator omits them for now.
About
AC Cost Map is an independent, free tool. We're not a utility or contractor, and we don't sell your data. Spot an error? Tell us and we'll fix it.