How the numbers are made
Methodology
Every Home Kilowatt result is an estimate built from a published formula, public data and the inputs you can see and change. This page lists all of them.
1. The core formula
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).
2. Data sources
| 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.
3. Seasonal curves
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.
4. How we test
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.
5. What we don't model
- 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.
6. Incentives in 2026
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.
7. Corrections
If a number looks wrong, email hello@homekilowatt.com with the page and your inputs. Every correction to a constant or formula is recorded in the changelog with its date.