Methodology

Almost every cost-to-run calculator uses the same three numbers:

• kWh per day = watts ÷ 1,000 × hours per day
• Cost per month = kWh per day × days in the month × electricity rate
• Cost per year (constant use) = kWh per day × 365 × electricity rate

Where an appliance's power isn't printed as watts, we convert it:

Appliance	Conversion	Source
Window AC	watts = BTU ÷ CEER	AHAM/DOE definition of CEER
Central AC	watts = tons × 12,000 ÷ SEER2	AHRI 210/240-2023
Mini split, cooling	watts = BTU ÷ SEER2	AHRI 210/240-2023
Mini split, heating	watts = (BTU ÷ 3,412) × 1,000 ÷ COP; COP = HSPF2 ÷ 3.412	AHRI 210/240-2023
Water heater	kWh/day = gallons × 8.34 × temperature rise ÷ 3,412 ÷ UEF	DOE, 10 CFR 430
Well pump	watts = HP × 746 ÷ 0.75; hours = gallons ÷ 10 gpm ÷ 60	746 W/HP; 0.75 efficiency and 10 gpm are our assumptions
Gas furnace	$/month = BTU/h ÷ 100,000 × $/therm ÷ AFUE × burner hours × 30	1 therm = 100,000 BTU
Ceiling fan vs AC	AC cost × (1 − 0.03 × 4 °F)	DOE: about 3% of cooling cost per °F

Per-load appliances use fixed energy per cycle: electric dryer 3.0 kWh, heat pump dryer 1.2 kWh (DOE); washer 0.4 kWh plus 2.0 kWh for a hot wash; refrigerators and freezers use the yearly kWh from the FTC EnergyGuide label. Christmas lights use watts per bulb: C9 incandescent 7 W, C7 incandescent 5 W, mini incandescent 0.4 W, C9 LED 0.15 W, mini LED 0.07 W (manufacturers' spec sheets).

Data	Source	Refreshed
Residential electricity rate by state (¢/kWh)	EIA, Electric Power Monthly table 5.6.A, residential sector (EIA API v2, retail sales)	Monthly
Residential natural-gas price by state	EIA natural gas prices, residential, $/Mcf ÷ 10.37 = $/therm	Monthly
Average monthly residential bill by state	EIA, "Average monthly bill" (table 5.a)	Yearly
Heating and cooling degree-days	NOAA Climate Prediction Center, base 65 °F, 1991–2020 normals, population-weighted	Yearly
Typical appliance wattage	energy.gov "Estimating Appliance and Home Electronic Energy Use"; ENERGY STAR product finder	Yearly

Every calculator shows the rate it used and its month: "Rate used: 15.9¢/kWh · Texas · EIA, EIA, August 2026". You can replace it with the rate from your bill.

For heating and cooling appliances, the calculator spreads the yearly cost over 12 months using a typical monthly share of heating degree-days (heaters, electric blankets) or cooling degree-days (pool pumps, dehumidifiers), from NOAA 1991–2020 normals. Months outside the season show $0. The current version uses one typical US pattern for all states; per-state monthly patterns are planned.

Each calculator has three worked cases (inputs and expected outputs) computed by a separate reference implementation of the same formulas. The live page must return exactly those numbers for those inputs before it's published.

• Fixed customer charges, taxes, and tiered or time-of-use pricing on your bill.
• Standby power, except where a calculator asks for it (gaming PC idle watts).
• Heat pump capacity loss in very cold weather (below about 25 °F).
• Reheating a hot tub from cold, and variable-speed well pumps. These are estimates. For an exact number, measure the appliance with a plug-in power meter and use the rate printed on your bill.

2026 note: The federal 25C and 25D tax credits ended for installations after Dec 31, 2025. Our numbers use 2026 rules. State rebates, HEAR/HOMES programs and utility programs may still apply.

If a number looks wrong, email [email protected] with the page and your inputs. Every correction to a constant or formula is recorded in the changelog with its date.

These values come from the reference implementation (specs/*.py). Each calculator page shows the ones it uses.

Value	Used in	Figures	Source
Heat loss per sq ft (UA)	Heating, smart thermostat	good 0.25 · average 0.35 · poor 0.50 BTU/h·°F	Simplification of ACCA Manual J
Seasonal heat pump COP by IECC climate zone	Heating, smart thermostat	zone 1 3.6 · 2 3.4 · 3 3.1 · 4 2.8 · 5 2.5 · 6 2.3 · 7 2.0 · 8 1.8	NEEP cold-climate heat pump data; the smart thermostat calculator uses the same values as Home Kilowatt model values
Full-load cooling hours by climate zone	SEER2 savings, smart thermostat	zone 1 2,400 · 2 2,000 · 3 1,500 · 4 1,100 · 5 800 · 6 600 · 7 400 · 8 300 h	ENERGY STAR calculator for zones 1–6; zones 7–8 and the smart thermostat values are Home Kilowatt model values
Cooling BTU per sq ft	BTU and mini split sizing	20 BTU/sq ft × zone factor 1.3 (zone 1) → 0.8 (zones 7–8)	ENERGY STAR room AC sizing, zone factor from cooling degree-days
99% design temperature	Heat pump and furnace size	per state	ASHRAE Fundamentals / ACCA Manual J table 1A
Solar production per kW	Solar calculators	per state, kWh per kW DC per year	NREL PVWatts v8 (tilt = latitude, south, 14% losses)
Net metering factor	Solar payback, output, lease vs buy	retail 1.0 · net billing 0.6 · limited or utility-dependent 0.5 · other 0.4	Home Kilowatt model values from each state's rule (DSIRE)
Generator fuel use at 50% load	Generator calculators	by size and fuel, scaled by (load ÷ 0.5)^0.8	Manufacturer spec sheets (Generac, Champion, Honda)
Diesel price	Generator fuel cost	heating oil price + $0.30/gal	EIA heating oil; no diesel column in our state data
EV energy per mile	EV calculators	EPA kWh/100 mi is wall-to-wheels; no extra loss applied	EPA, fueleconomy.gov (measured from the energy drawn at the wall, charging losses included)
EV efficiency and range	EV charging cost (30 models), EV calculators	EPA label kWh per 100 miles and EPA combined range, one EPA record per model	U.S. EPA via fueleconomy.gov vehicle data (vehicles.csv), data as of 2026-09-18
Cooling energy from avoided heat gain	Attic insulation, windows	kWh = BTU ÷ (SEER 14 × 1,000)	Definition of SEER

Electricity rates are the EIA's August 2026 residential prices and natural gas the EIA's 2025 annual average residential price ($/Mcf ÷ 10.37 therms), pulled 2026-10-25 and updated monthly. Propane, heating oil and gasoline are EIA/AAA 2025–26 reference values (data as of 2026-09-26).