How production is calculated

Production is the starting point for every bill and saving, so Cali-Graf builds it from measured inputs and public, well-tested models.

Step by step, for each roof face

• Weather. A typical year of hourly sun and temperature for the home’s ZIP code, from NREL’s National Solar Radiation Database (NSRDB).
• Sun on the panels. NREL’s PVWatts engine (in its SAM software) works out the light reaching the face every hour from the face’s pitch and direction.
• Shade. For every panel’s spot, the shade of trees, chimneys and neighbors from Google’s height data is taken out hour by hour.
• Heat and the panel model. Hot panels make less. The output of one panel is modeled with the exact model’s parameters from the California Energy Commission list, then multiplied by the panels on the face.
• Inverter. The exact inverter’s efficiency curve and power limit (Sandia model). Microinverters are modeled panel by panel; string and hybrid inverters share the whole array.

System losses

On top, Cali-Graf takes off 7.77% for real-world losses: dirt on the panels 2%, nameplate tolerance 1%, early light-induced degradation 1.5%, connections 0.5% and system availability 3%. With Shade analysis off, a standard 3% allowance for shade is added instead of the measured shade.

What you see

The yearly production is in the result next to the usage, and by period in Numbers by period. Shade loss in All numbers shows how much shade takes from each face. How close the model is to real systems: How accurate are the numbers.