INTRODUCTION The sun is essential for life on earth; plant and animal life in their current form would not exist without its power. Radiation from the sun ultimately drives all the forms of renewable energy discussed in this book (the only exception being tidal energy). On earth, the sun is a diffuse energy source with a power at the earth's surface of less than 1000 W/m 2 and the natural processes that create the earth's wind and waves, rainfall and biomass act to concentrate the energy from the sun so making its subsequent conversion into mechanical or electrical power easier. An understanding of the solar energy resource is essential for the study of both photovoltaic electricity generation and solar thermal processes. This short chapter gives a simple engineering description of the solar resource and its content is a pre-requisite for the study of Chapters 5 and 6. In Sections 4.1 and 4.2 the solar resource is described with examples. Section 4.3 presents a simple view of the sun–earth geometry. Section 4.4 discusses the orientation of solar panels. Section 4.5 describes the solar spectrum and air mass. Finally in Section 4.6, a brief introduction is given to the wave–particle description of light. The Solar Resource The sun is a spherical collection of hot gases with a diameter of 1.4 × 10 6 km and an internal temperature of up to 20 × 10 6 K. The earth orbits the sun in a slightly elliptical orbit at a distance of approximately 1500 × 10 6 km. The centre of the sun can be thought of simply as a nuclear fusion reactor converting hydrogen into helium and so releasing energy. The energy generated by the fusion reaction at the centre of the sun is transmitted through its hot gases to the surface with increasing wavelength. Finally radiation is emitted from the surface of the sun with a spectrum similar to that of a black body at a temperature of 6000 K. This radiation then travels through space to arrive at the outer surface of the earth's atmosphere. At the outer surface of the earth's atmosphere the intensity of the radiation is around 1367 W/m 2 .