We started with the fact that the sun gives the earth 1 kW/m2, enough energy in 1 hour to supply the earth for a whole year. Now we understand why it is so hard to capture that energy. The atmosphere absorbs part of the sunlight. The sun does not shine at night and does not rise high in the winter. There are cloudy and stormy days. There is little sunlight at high latitudes, where the power is most needed. Solar cells can capture only part of the solar spectrum and are not efficient at that. The peak efficiencies quoted apply only when the sun is directly overhead. The color of sunlight changes near sunset and no longer matches the color the solar cells are optimized for. Solar panels cannot economically be turned to follow the sun as it moves across the sky. We are lucky to capture a few percent of solar energy, but even that is a lot of energy that should not be wasted.

Local solar panels on rooftops and exterior walls should be popularized and widely accepted as standard for new structures. These can contribute a few percent to the grid’s power, but no more because solar power is intermittent and cannot economically be stored. Selling excess power back to the power station is just a gimmick; the utilities could care less about this small disturbance.47

The advances in thin-film technology have made photovoltaic solar power com­petitive with conventional power sources. The energy payback time will fall below one year, which is short enough, though not a short as for wind power. To use this technology for large solar farms to provide central-station power, however, is fraught with problems. The main problem is that the sun does not shine at night, the time when people turn on their lights. There is no cheap, proven method for storing that much energy. Alternatively, one can build high-tech transmission lines to carry the electricity across time zones from daylight to moonlight, but this will take many decades to implement.

Solar power is an important supplement to grid power, but it is not suitable as a primary central-station power source. Fifty years from now, only coal, fission, and fusion are capable of supplying the dependable, steady backbone power that the civilized world can count on.