The solar resource. Solar energy is the source of all energy on earth, available to us in a number of derivatives. Plant matter for example, which relies on solar energy for nutrition, experiences natural compression and decomposition over millions of years to form the the fossil fuels we use today for electrical generation and transportation. Other examples of this can be seen in use of biomass for fuel or the harvesting of wind energy which is reliant on solar heated air for the formation of currents.
We are also able to utilize the solar resource directly. Solar thermal technologies take advantage of this resource to heat a working fluid that can transfer energy to an air stream or water for domestic or commercial use. Solar Photovoltaic or PV devices exploit various materials (principally Silicon) that experience sub-atomic variations when exposed to solar energy in order to induce an electric current. Both solar PV and thermal technologies provide a useful source of energy with little to no moving parts, no pollution and very little embodied energy.
In order to effectively design a solar energy system, an understanding of the available solar resource at the location of interest is required. All of the energy available on Earth is derived from the sun. We can model the sun's surface as blackbody. At a specific temperature, approximately 5777K for the sun, a blackbody emits energy with a unique radiation spectrum. The spectrum is divided into three broad ranges classified as ultraviolet, visible and infrared which transmit radiation at varying intensities. The highest intensities are found within the visible spectrum, peaking at a wavelength close to 0.5um. Averaged over the entire surface, the power density of the sun is found to be approximately 63 x 10 W/m2.