From GridLAB-D Wiki
SOLAR_TILT_MODEL – Underlying solar position and tilt model used to calculate irradiance on the panel
Synopsis
module generators; class solar { enumeration {DEFAULT=1, SOLPOS=1} SOLAR_TILT_MODEL; } object solar { SOLAR_TILT_MODEL value; }
Remarks
Currently implemented model types include DEFAULT and SOLPOS. Both models are only used when the panel is in FIXED_AXIS mode.
DEFAULT
The incident solar radiation on a tilted array is calculated using algorithms from Solar Energy Thermal Processes by Duffie and Beckman (1974).
SOLPOS
The incident solar radiation on a tilted array is calculated using algoirthms from the [NREL SOLPOS] page and from "Modeling Daylight Availability and Irradiance Components from Direct and Global Irradiance" in Solar Energy volume 44, number 3 by Perez et al.