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Sunlight, the most ubiquitous of all the renewable resources, is also the driving force behind wind, biomass, and hydroelectric. Solar energy directly powers many of the technologies of Table 5.1, including photovoltaics, solar concentrators, and active and passive solar thermal. However, for energy purposes, not all sunlight is equal. Solar concentrators can only concentrate direct sunlight; they cannot focus indirect sunlight that has been scattered by aerosols in the atmosphere. Thus, concentrating solar systems are most viable in unpolluted desert regions with high insolation. Non-concentrating solar energy systems work better in these regions also, but do not suffer the same performance penalties in regions where there is relatively more indirect sunlight.

The total amount of solar energy reaching the earth's atmosphere is 170000 terawatts, or about 13 000 times current world energy requirements. While the amount of solar power available worldwide is extremely large, the power incident per unit area is small because it is spread over the entire surface of the earth. Figure 5.1 shows the total (direct and indirect) insolation worldwide over the course of an average year as measured in terms of kWh/m2 per day. Peak values are just over 1 kW/m2 at the earth's surface, with daily maximums in the best locations of about 8 kWh/m2 per day.

While Figure 5.1 shows the spatial variation of the insolation resource, it does not show the temporal variation. Obviously, the resource is not available at night or during cloudy periods. This means that any application for solar must either require energy only when the sun shines, or be able to convert the sunshine to some form of stored energy, or have another form of energy available as backup. This issue of dealing with the intermittency of solar resources is discussed further below in the "Market issues" section.

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