Energy from sources other than fossil and nuclear fuels is to a large extent free of the concerns about environmental effects and renewability that characterize those two sources. Each alternative source supplies energy continuall, whether or not we use it, and most have their origins in energy generated outside the Earth, yet the potential of each is limited by its total supply set against its rate of use. The Sun will radiate energy until it ceases thermonuclear fusion, in around 5 billion years. This unit explores the Sun as a potential source of usable energy. This study unit is just one of many that can be found on LearningSpace, part of OpenLearn, a collection of open educational resources from The Open University. Published in ePub 2.0.1 format, some feature such as audio, video and linked PDF are not supported by all ePub readers.