Methodology & data sources

The core savings model

Heating and cooling energy cost scales inversely with rated efficiency. For cooling: new cost = current cost × (current SEER2 ÷ new SEER2). For furnaces: new gas cost = current gas cost × (current AFUE ÷ new AFUE). For heat pumps in heating: delivered heat is held constant at therms × 100,000 BTU × AFUE, and electricity use is delivered BTU ÷ (HSPF2 × 1,000) kWh. Nameplate SEER converts to SEER2 at ×0.95 and HSPF to HSPF2 at ×0.85 (split-system field corrections for the 2023 DOE test procedure). An optional age adjustment degrades the current unit's effective rating by 0.7%/yr (capped at 40%) to reflect coil fouling and refrigerant drift.

Estimating today's cost — two paths

Bills path (preferred). Annual cooling cost = (peak summer monthly bill − average shoulder-season bill) × equivalent full-load cooling months. Only the seasonal excess is attributed to HVAC — conservative for homes with pool pumps or other seasonal loads riding the same peak. The heating calculator applies the same delta method to gas bills.

ZIP path. kWh = EFLH × tons × 12,000 ÷ (SEER2 × 1,000), then × the local rate. Equivalent full-load hours come from a per-ZIP dataset we built: five years of daily weather for every U.S. ZIP (Open-Meteo historical reanalysis, CC BY 4.0), degree-days computed by the Met Office sinusoid method, with cooling degree-days using a 50% apparent-temperature blend so humid climates carry their real latent load. Degree-days convert to EFLH through power curves fitted by moisture regime (IECC humid/dry) to 231 validated regional anchor points — median error ~8% in humid climates, ~15% in dry. California ZIPs additionally carry CEC Title 24 climate zones.

Prices, projections, and payback

Electricity rates are utility-level residential averages from the NREL “U.S. Electric Utility Companies and Rates by ZIP” dataset (EIA-861); where several utilities serve one ZIP, investor-owned utilities are preferred, then the median, with state medians as fallback. Tiered and time-of-use plans are averaged — adjust the on-screen rate if your plan differs. Gas prices are user-entered from the bill (delivered price = total cost ÷ therms). Simple payback = net installed cost after rebates ÷ first-year savings, undiscounted. Multi-year figures escalate fuel prices at a user-editable rate (default 3%/yr) and apply no discount rate — both choices are visible and changeable.

The keep-vs-replace projection

The keep path compounds three effects: utility-price escalation, efficiency degradation of the aging unit, and an expected-repair curve ($40 + $8·age + $0.8·age², capped at $900/yr, rescaled to the user's on-screen repair estimate). The replace path charges the full net install cost in year zero, near-zero repairs during the 10-year parts warranty, then restarts the degradation and repair curves from age zero. The model is deliberately generous to keeping: the old unit is assumed to survive the entire 15-year horizon with no catastrophic failure, and warranty labor charges are ignored on both sides. Units older than ~16 years get an R-22 refrigerant advisory but no cost penalty.

Carbon estimate

Avoided CO₂ = annual kWh saved × 15 years × the state grid intensity from EPA eGRID state output emission rates (U.S. average when the state is unknown), in metric tons; the gasoline-car equivalency uses EPA's ~4.3 t CO₂e per vehicle-year. Grid intensity is held constant over the horizon — on a decarbonizing grid this makes the figure an upper-range estimate, and we say so on the result.

Known limits — read before relying on the numbers

The models exclude: duct condition and installation quality (which can move real results ±10–25%); time-of-use and demand-charge rate structures; gas-meter fixed charges avoidable by full electrification (understates electrification savings); backup-heat costs for heat pumps in deep cold (overstates cold-climate heat-pump savings — we flag this on the result); AC savings when a heat pump also replaces a cooling system (run both calculators and add them); and home-value effects. HSPF2 is a national-profile seasonal rating; cold-climate results should be verified against the specific unit's capacity and COP at 17°F and 5°F. All outputs are estimates for education, not guarantees or professional advice.

Data sources

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