How we calculate payoff

• Electric rate: residential average revenue per kWh from U.S. EIA Form 861 (2024), mapped to ZIP via OpenEI/NREL utility-rate ZIP files.
• Sunshine: NASA POWER 22-year annual all-sky surface irradiance (kWh/m²/day), on a 1.5° grid, ×1.1 tilt gain.
• Output: sun hours × 365 × 0.8 system derate.
• Size: matches your usage (default 863 kWh/mo, EIA U.S. average), 2–20 kW.
• Cost: $2.75/W installed. The federal 25D homeowner credit ended for systems placed in service after Dec 31, 2025, so it is not applied. Local rebates and SRECs can shorten payoff.
• Savings: assumes full net metering and 3%/yr rate increases. States with net billing (e.g. California NEM 3.0) pay off slower without a battery.