How the numbers are computed

This page is the whole recipe. The calculator has no black box: every figure it shows comes from public weather data, the physics described below, and the inputs you give it. Where we assume something, the page says so; where the honest answer is "an upgrade isn't worth it for you," the calculator says that too.

The weather layer

The engine runs on typical-year weather for 2,684 US stations, derived from TMYx typical-meteorological-year files (2009–2023 period) assembled from NOAA/NCEI Integrated Surface Database observations, with ERA5 reanalysis gap-fill. For each station we reduce the 8,760-hour year to a table of hours spent at each outdoor temperature (1°F bins), plus a daily temperature-and-humidity layer used by the comfort model. We publish only these derived hour-count tables — meteorological facts computed from the public record — never the source files themselves.

Your zip code is matched to the nearest station by distance from the zip's Census-defined population centroid. We deliberately show the analysis for "near {your city}" rather than naming the station. If no station lies within 150 miles of a zip (a handful of remote areas), the calculator says so instead of quietly using far-away weather.

The cooling model

Cooling load is computed with the bin-hour method: for every temperature bin above the house's cooling balance point, the load is proportional to the temperature difference, sized so the system you described reaches its full rated output at design conditions. Each bin's load is then divided by the equipment's efficiency at that condition — not the single sticker number. The model applies:

• SEER2 → EER2 conversion and an off-design correction, so hot afternoons are priced at the efficiency the unit actually delivers in them;
• part-load behavior by compressor class (single-stage, two-stage, variable), which is also what drives the humidity results below;
• age-related degradation on the existing unit (compressors and coils lose capacity and efficiency with wear; the estimate halves if you tell us it's been serviced regularly). New-equipment ratings are taken at sticker.

The heating model

Heating uses the same bin-hour framework below the heating balance point, calibrated from your furnace's sticker size and your area's design temperature. Fuel systems are priced by AFUE with a cycling adjustment for oversized single-stage equipment; a furnace's combustion efficiency is held at its sticker rating regardless of age — unlike compressors, heat exchangers don't meaningfully lose burn efficiency, so we don't manufacture phantom savings from an "old" furnace. Heat pumps are priced with temperature-dependent COP and capacity curves (an HSPF2 rating tells us the average; the curve tells us what January costs), including auxiliary resistance heat when the heat pump can't carry the load, and dual-fuel systems switch to the furnace below the economically-correct switchover temperature (bounded by what real dual-fuel controls allow). Where a heat-pump-versus-furnace comparison is on screen, we also show the crossover temperature — the outdoor temperature at which your prices make the two equally cheap to run — because that single number explains the verdict.

The humidity model

"Sticky hours" count the hours a year your indoor air sits above a 62°F dewpoint — a standard comfort ceiling. The model tracks daily outdoor temperature and moisture, the moisture your household adds, your home's air-tightness, and how much moisture each compressor class actually removes at part load. This is why the comfort verdict steps with compressor type rather than SEER: efficiency ratings don't dehumidify; run time and coil behavior do. The latent-load calibration is humid-climate-derived; in arid climates the model correctly reports near-zero sticky hours rather than inventing a comfort benefit.

Prices

Electricity, natural gas, and heating-oil prices default to state residential averages and are always editable — a real bill beats any average. Defaults are kept current from U.S. Energy Information Administration residential price series; each release is dated in the data files. The analysis is held at today's prices over the 5/10/15-year horizons — we don't forecast energy markets, and we say so rather than hiding an escalation assumption inside the math. The numbers also assume standard, unsubsidized rates: some utilities offer discounted heating rates (dual-fuel and off-peak programs) or equipment rebates that can materially change the answer — worth one call to your utility.

What we deliberately leave out

• Equipment and installation prices. Install quotes vary by market, access, and contractor; blending them into operating-cost math would manufacture false precision. This tool answers what a system costs to run.
• Rebates and tax incentives baked into the arithmetic. Where they exist they're real money — but they change yearly and run out mid-year, so they belong beside the math, not hidden inside it.
• Energy-price forecasts. See above.

Sources & credits

• NOAA/NCEI Integrated Surface Database — U.S. government work, public domain. The bedrock observations.
• TMYx typical-year files, climate.onebuilding.org — the assembled typical-year archive our bin tables are derived from.
• ERA5 reanalysis (Copernicus Climate Change Service) — gap-fill within the TMYx files. Contains modified Copernicus Climate Change Service information; neither the European Commission nor ECMWF is responsible for any use of it.
• U.S. Census Bureau ZCTA gazetteer — zip-to-location matching. Public domain.
• U.S. Energy Information Administration — state residential energy prices. Public domain.
• Equipment ratings referenced on this site come from the AHRI certification directory; we are not affiliated with any equipment manufacturer or utility.

The honest fine print

Everything here is an estimate for planning, not a quote or a guarantee. A real home differs from any model — duct condition, sizing, installation quality, and how you actually live all move the number. The calculator's job is to get the physics and the economics right so the comparison is trustworthy; get a load calculation and a real quote before buying anything.

Built and maintained by the team at Liberty Air and Electric, a licensed Florida HVAC and electrical contractor. Weather tables rebuilt from the public archive; model constants documented in the engine source.