Hour-by-Hour Solar Production per Roof Face: Inside the Model

• the panel — its power and specifications;
• every face with panels — its pitch and direction, and how many panels sit on it; each face becomes its own array;
• where each panel sits — for the shade at that exact spot.

Faces without panels don't enter the production calculation, though they still shape the fire setbacks. A panel is not counted when it sits off every roof face, on a face without a pitch, or on top of another panel, and the calculator says how many panels aren't counted and why. A panel shown in red — inside a setback or on an obstacle — is still counted, which is why Cali-Graf asks for it to be fixed before the proposal. Details: how the roof gets into the calculation.

Because each face is its own array, an east face and a west face on the same house are modeled with their own sun and shade, and the way a microinverter system differs from a string inverter — a shaded panel holding back the others, or not — is reflected in the result.

Shade analysis is on by default. For every panel, Cali-Graf works out when through the year trees, chimneys and nearby buildings shade that exact spot, and takes that out of the production hour by hour. If a seller turns it off — only when the shade on the image is gone, such as a tree that has been cut down — a standard 3% is taken for shade instead. Cali-Graf's own warning: on a shaded roof, the standard 3% is far less than the real loss, so production and savings would come out higher than the home will see. Details: turn shade analysis on or off.

In the roof editor, the sun map shows the same thing at a glance. It colors the roof by how much sun each spot gets in a year, from about 700 to 2,050 kWh per kW of panels — yellow sunniest, purple shaded or facing away — and each face shows its average as Sun on this face. Two faces with the same direction can be worlds apart if a tree shades one of them all afternoon. The sun map comes from Google Solar's yearly calculation for every 10 cm of the roof; Cali-Graf's own production model is separate and hour by hour, but the two agree on which spots are worth panels. Details: sun map.

The total is 7.77% rather than 8% because losses compound rather than add. Many solar tools instead apply a single default figure — the PVWatts calculator's default total system losses are 14% — which includes a fixed allowance for shade rather than a measured one. That difference in method is one reason two proposals for the same roof disagree; see why two solar proposals disagree.

• Yearly production sits next to the home's usage in the result, with production by billing period in Numbers by period.
• Shade loss per face is listed in All numbers, so a seller can see how much each face loses to shade.
• Details, at the bottom of the result, lists every face with panels — how many, the tilt and which way it faces — and the engine's notes on the weather and equipment models used. If production looks off, checking each face's tilt and direction against the design is the first step.

Panels also lose a little output each year; the 25-year forecast applies the degradation rate from the panel's datasheet, and the line above the 25-year chart names it. For background on panel aging, see solar panel degradation in California.

• A typical year is not next year. The weather input is a typical year; real years are sunnier or cloudier.
• The roof as traced. A pitch or direction off by a few degrees, a face traced too big, or an aerial image older than a new tree or addition changes production. The model does not replace a site visit.
• Data availability. Google has no 3D roof data for some homes, mostly new construction and some rural areas, and those can't be designed in Cali-Graf for now.

How a per-face model compares with a city-level calculator on the same home is shown in why an online solar estimate differs from a roof-specific design.

Product facts on this page come from Cali-Graf's own help center and the pages linked below; figures in examples come from the stated inputs. Features, limits and supported utilities change; each was verified October 16, 2026.

1. Cali-Graf Help — How production is calculated
2. Cali-Graf Help — How the roof gets into the calculation
3. Cali-Graf Help — Turn shade analysis on or off
4. Cali-Graf Help — Sun map
5. Cali-Graf Help — Data sources
6. Cali-Graf Help — How accurate are the numbers
7. NLR (formerly NREL) — National Solar Radiation Database
8. NLR (formerly NREL) — System Advisor Model (SAM)
9. NLR (formerly NREL) — PVWatts Calculator
10. California Energy Commission — Solar Equipment Lists
11. Google — Solar API overview