produces nearly 800 megawatts, 24 hours a day, 365 days a year for a quarter of the cost. Solar power has shown that it can play a valuable role in specific locations where the sun is plentiful, especially in the developing world, in areas remote from the electrical grid. The issue is whether renewables can be scaled up to generate the massive amounts of energy that coal and nuclear do, whether they can one day produce the trillions of watts the world's cities need, whether they can ever replace fossil fuels. MARTY HOFFERT, New York University: Most people want to turn on the lights at night, and the sun isn't shining at night. Renewables tend to be very episodic. That is, they're not always there. And the power density is low. That is, the number of watts per square meter is pretty low. NARRATOR: And this may be a crucial weakness. Because the energy density of solar, wind and biomass is low, very large areas of land will be needed to produce significant amounts of power. MARTY HOFFERT: There's going to be a certain inevitable amount of land use issues associated with renewable energy, in that you're going to need a lot of area. If you wanted to supply 10 terrawatts of power and you wanted to do it with biomass energy, you would need an area of the Earth equal approximately 10 percent of the Earth's surface, land surface area. And that's a huge amount of land. That's all