Right now, renewable energy sources like solar and wind still provide just a small fraction of the world's electricity. But they're growing fast. Solar is growing exponentially. Across the globe, 55 terawatt-hours of solar power had been installed by the end of 2011. That may not seem like much in itself - the United States by itself, after all, needed about one hundred times that much power in 2011. But solar has been growing at a stunning rate, as panels keep getting dramatically cheaper. If these exponential growth rates [continue] solar could provide nearly 10 percent of the world's electricity by 2018. Official agencies keep underestimating the growth rate of renewables. The International Energy Agency is forecasting that solar will catch on much more slowly - providing a mere 4.5 percent of the world's electricity by 2035. But [t]he IEA has almost always underestimated how quickly wind and solar can grow. Forecasters have consistently been too pessimistic. For instance, back in 2000, the IEA's World Energy Outlook predicted that non-hydro sources of renewable energy would make up 3 percent of global energy by the year 2020. The world reached that point in 2008, well ahead of schedule. Using only current technology, renewables could technically provide the vast bulk of U.S. electricity by mid-century.