Conversion efficiency: the percentage of solar energy shining on a device that is converted into electrical energy. Our eyes are most sensitive to green light. Green is the middle wavelength which hits the maximum sunlight. Three or four solar cell types stacked on top of each other - made form "exotic" materials: Power generation may be reduced by cloud cover. Generation techniques requiring direct sunlight (csp) are ineffective without optimum conditions. Can only maintain a temperature difference; uses a lot of power to do so, which produces heat. The reverse works too: a temperature difference produces a voltage (this is how the curiousity rover is powered) Heat lost through the crust can be harnessed. No wastes; no greenhouse gases or air pollution. Hot water is pumped and used to heat buildings. Geothermal is dominant energy source in some areas. Wind drives a large turbine to produce electricity. Along the coast and in the center of the us.