The solar resource is the average quantity of solar energy that a specific site (or a large area) receives during a specified period, such as a year. The larger the solar ersource, the more energy will be produced by any solar system. The difficulty that all solar system designers face is to obtain reliable and accurate solar resource data for a specific site under scrutiny, or a specific area in the world.
Furthermore, the resource is not constant from day to day, and also varies seasonally and from one year to the next. Finally, diferent types of solar collectors use different components of the resource. For intance, fixed flat-plate collectors use global radiation (the sum of direct radiation from the sun and of diffuse radiation from the sky and ground reflections), whereas concentrating collectors use only direct radiation.
The resources corresponding to a fixed tilted collector, a vertical wall, or a tracking collector are all different, and are not the same as the radiation incident on a horizontal surface. All these geometrical characteristics complicate the evaluation of the solar resource potentially usable by a specific collector.
Solar resource measurements are scarce in the world, so that, in most cases, one must rely on modeled data. No model is perfect (and no measurement either!), so that one also has to evaluate the uncertainty in solar resource data. All this delicate process is conveniently described as "Solar Resource Assessment".
Some sources of data are indicated in the Links section of this website.