Why results differ from other software
Two tools can model the same roof and give different numbers. The usual reasons are the rates (which year), the shade (measured per panel or a flat guess), the system losses, the utility’s credit rules, and whether the bills are modeled period by period or as an average.
When a customer shows you another company’s proposal with other numbers, the difference almost always comes from a few assumptions. Knowing them helps you explain yours.
Production
- System losses. Cali-Graf takes off 7.77% and measures shade separately (see How production is calculated). Many tools use the PVWatts default of 14%, which includes a flat 3% for shade — on a roof with little shade that alone gives several percent less production.
- Shade. Cali-Graf measures shade at every panel from Google’s height data. A flat shade guess can be higher or lower than the real one.
- Equipment. Cali-Graf models the exact panel and inverter; a generic model treats every panel the same.
- The roof itself. A pitch or direction off by a few degrees, or a face traced too big, changes the result.
Bills
- Which rates. Cali-Graf uses the utility’s current rates. A proposal made last year used last year’s.
- Credit rules. With LADWP, the solar credit doesn’t pay the adjustment factors or taxes — see How the bill after solar is calculated: LADWP. Tools that net every kWh fully show lower bills with solar.
- Real periods. Cali-Graf follows the customer’s own billing periods and seasons, not a yearly average.
- Tax credit. Older proposals subtract the 30% federal credit, which has ended — see Federal tax credit.
- Rate growth. The 25-year savings depend heavily on the yearly rate increase each tool assumes.
How to compare fairly
Put both tools on the same footing: the same system size, the same usage, the same rate increase. Then compare the first-year production and the first-year bills — those are where real differences show.
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