Expertise & Methodology
How JouleIO energy calculators work — data sources, methods, editorial standards, update cadence.
Why this page exists
Energy decisions — solar install, heat pump upgrade, appliance replacement — can swing tens of thousands of dollars over a system's lifetime. When a calculator influences those decisions, you deserve to know where the numbers come from.
Primary data sources
- EIA Electricity Data Browser
Average residential electricity rates ($/kWh) by state, monthly. Foundation for /electricity-cost/ and /energy-cost/ city-adjusted estimates.
- EIA Residential Energy Consumption Survey (RECS)
Household energy use by appliance, climate zone, and home characteristics. Powers per-appliance kWh estimates.
- NREL PVWatts Solar Calculator
Solar generation estimates by ZIP, panel orientation, and tilt. Source for /solar/ city-level production estimates.
- DOE Energy Efficiency Data
Heat pump COP ratings, appliance efficiency standards, weatherization savings benchmarks.
- EnergyStar Product Database
Certified appliance efficiency ratings (refrigerators, water heaters, HVAC). Used for /appliance/ comparison output.
- NOAA Climate Data
Heating Degree Days (HDD) and Cooling Degree Days (CDD) by city for HVAC load estimation.
Calculation methodology
Appliance kWh per year
kWh = wattage × hours/day × 365 / 1000. We adjust standby vs active draw for electronics. RECS-weighted average use frequency by appliance type.
Solar payback period
System cost after state rebates and any verified tax credit divided by annual electricity savings (PVWatts production x local kWh rate). For new 2026 homeowner-owned systems, federal residential credit defaults to 0% unless eligibility is documented.
Heat pump operating cost
(HDD × home heat load BTU/HDD) / COP / 3412 = kWh/year for heating. Same for cooling using CDD. Higher COP = lower cost. We disclose COP varies with outdoor temp (cold-climate units perform better).
Electricity cost city-adjustment
State avg rate × city utility-specific multiplier when available, else state avg. Tier rates (CA, AZ) shown when applicable.
Editorial standards
- All calculators run client-side. No address, ZIP, or utility account data transmitted off-device.
- Electricity rate data shows EIA publication month on every calculator output.
- Solar estimates show PVWatts model assumptions (panel orientation, tilt, derate factor).
- Heat pump COP ratings cite specific test conditions (47°F vs cold-climate 17°F) per AHRI standard.
- Articles cite primary sources (EIA, NREL, DOE, EnergyStar) — not utility marketing material or solar installer sales pitches.
- We do not accept solar installer or utility company sponsorships that affect calculator output.
Update cadence
| What | When |
|---|---|
| EIA electricity rates | Monthly (EIA-861 monthly state release) |
| EIA RECS appliance data | Quadrennial (every 4 years, last 2020 release) |
| NREL PVWatts model | Annual (NREL TMY3/4 weather data updates) |
| EnergyStar certifications | Quarterly (EPA refresh) |
| Federal residential energy credits | Annual + ad-hoc legislative changes (current 2026 default: no new residential 25D credit after Dec. 31, 2025) |
| Article fact-checks | Quarterly + within 14 days of major utility rate change >5% |
Corrections and feedback
Email [email protected]. We respond within 14 days.
Who builds JouleIO
See /about/team/ for team backgrounds.