This chapter reviews some of the basic magnitudes of solar radiation and some of the basics of PSpice. A brief description of a photovoltaic system is followed by definitions of spectral irradiance, irradiance and solar radiation. Basic commands and syntax of the sentences most commonly used in this book are shortly summarized and used to write PSpice files for the AM1 SG and AM0 sun spectra, which are used to plot the values of the spectral irradiance as a function of the wavelength and compare