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: 15.5¢/kWh · Texas · EIA, EIA reference 2025–26". 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 charging loss	EV calculators	×1.12 (about 89% efficient)	DOE / INL
Cooling energy from avoided heat gain	Attic insulation, windows	kWh = BTU ÷ (SEER 14 × 1,000)	Definition of SEER

Fuel prices (natural gas, propane, heating oil, gasoline) are EIA/AAA, 2025–26 reference values (data as of 2026-09-26), like the electricity rates.