How we calculate

The steps

1. The sun’s path. For a typical location in your state we follow the sun every 10 minutes through a typical day in each month, using standard solar geometry.
2. Light on the panel. We work out how directly the sun hits a panel at your direction and angle, plus light from the sky and reflected from the ground (20%). We mix clear and cloudy days according to how sunny your state is.
3. State yield. We scale this so a panel in the best fixed position makes what a typical system in your state makes in a year, including normal losses.
4. What your home uses. At every step we compare output with your always-on use. Used power counts at your price; power that leaves counts at what your utility pays ($0 by default).
5. Payback. Year by year, with power prices rising by the rate you choose (3% by default) and panels losing 0.5% a year.
6. Sun hours. Peak sun hours per day are the state’s yearly yield divided by 365, corrected for typical system losses.

What it doesn’t know

• Your exact location: we use one typical spot per state, and big states vary by 10–20%.
• Your exact rate: time-of-use rates, tiers and fixed charges change what each kWh is worth.
• Your habits: shifting laundry or charging into the day raises savings.
• Local shade, dirt and the exact kit you buy.

Treat the results as a good estimate, not a quote.

Sources

• Electricity prices: U.S. Energy Information Administration, Electric Power Monthly, Table 5.6.A (March 2026 data).
• Sun position: declination and hour-angle formulas. Clear-sky light: Meinel air-mass model with an isotropic sky.
• Panel aging of 0.5% per year follows the assumption Google's Solar API uses for US estimates.