On the earth’s surface the solar constant measures the power received from the sun by a 1 m2 surface perpendicular to the sun’s rays. It amounts to 1 kW. Practically, in order to estimate the energy available at a specified location, one has to take into account the latitude and the average daily and yearly insolation. Typical annual sunshine ranges from 1000kWh/m2 in northern Europe to 2500kWh/m2 in deserts like the Sahara. In Spain values reaching 1600kWh/m2 are obtained, while in California values as high as 2000 kWh/m2 are observed [32]. Taking, as an example, an insolation of 1800 kWh/m2 and an efficiency for transformation of the solar energy into electricity of 15%, one gets an annual electricity production of 270 kWh/m2, a number approximately valid for photovoltaic as well as thermodynamical systems. To obtain an annual production of 7 TWh, similar to that of a 1 GW nuclear plant, 26 km2 are needed. At current solar cell costs, such a facility would cost around $17 billion, more than ten times that for a nuclear plant of similar power. The corresponding electricity cost would reach about 500 per kWh, compared with the current costs of around 50 per kWh. Such high costs will restrict the use of photovoltaic systems to remote locations not connected to an electricity distribution network. Note, however, that the need to store the energy would increase its cost by at least a factor of two. Thermodynamic systems allow much lower costs down to 120 per kWh and may become competitive for significant energy production in such places as Africa, India and South America, provided the current output of the facility can be fed into a network able to cope with the essentially intermittent form of the solar energy. One should note, in this context, that day/night storage capacities are present in all thermal solar systems, in the form of oil, sodium or molten salt heat storage reservoirs.