SunThink! is an open-source program that assists in the good design of solar electricity systems, that runs within the INRIA open source platform for numerical computation Scilab. The current version of SunThink! includes modules that allow for the calculation of:
for solar electricity systems with a variety of chosen system characteristics. Its primary purpose is to facilitate the accurate, appropriate sizing of these systems to fit the specific design goals that are held for them.
The computations performed by SunThink! are based on sums of evaluated integrals of direct and diffuse irradiance, involving mathematically expressed sun position, plane orientation, and global and plane sunrise and sunset, and using a fourth-order cosine-polynomial fit of the Young (1992) interpolative formula for airmass with the Meinel experimentally determined expression for intensity to compensate for atmospheric extinction. This is modified for cloud coverage using a second-order fit of specific local historical cloud-coverage behavior with an expression tuned according to available historical NREL insolation data for sites at different geographic and climatic locations.
SunThink! provides several key aspects of functionality that are not present in PVWATTS, the National Renewable Energy Laboratory performance calculator for grid-connected photovoltaic systems. These include, notably, the tailoring of data to the cloud-coverage patterns of a region, the ability to forecast production losses from shading based on sets of angular shade-source data, and the ability to calculate weighted production numbers according to the relative weighting of electricity price in time-of-use utility rate schedules.