Solar power involves using solar cells to convert sunlight into electricity, using sunlight hitting solar thermal panels to convert sunlight to heat water or air, using sunlight hitting a parabolic mirror to heat water (producing steam), or using sunlight entering windows for passive solar heating of a building. It would be advantageous to place solar panels in the regions of highest solar radiation.[26] In the Phoenix, Arizona area, for example, the average annual solar radiation is 5.7 kW·h/(m²·day),[27] or 2.1 MW·h/(m²·yr). Electricity demand in the continental U.S. is 3.7×1012kW·h per year. Thus, at 20% efficiency, an area of approximately...