Last year Microsoft outfitted its campus in Silicon Valley with a solar system from Sun-Power, a local company that makes high-efficiency (and, some say, the world's best-looking) solar panels. A few months later Microsoft's arch-rival, Google, began building something on an even grander scale-one of the largest corporate solar installations to date. But all of this may yet be topped by Wal-Mart. In December the retail giant solicited bids for placing solar systems on the roofs of many of its supermarkets. Besides producing favourable publicity, the appeal of using solar power is obvious. Unlike fossil fuels, which produce significant amounts of pollution and enormous amounts of greenhouse gases, the sun's energy is clean and its supply virtually limitless. In just one hour the Earth receives more energy from the sun than human beings consume during an entire year. According to America's Department of Energy, solar panels could, if placed on about 0.5% of the country's mainland landmass, provide for all of its current electricity needs. Yet since they were first invented more than five decades ago, photovoltaic solar cells-devices made of semiconductor materials that convert light into electricity-have generated much publicity but little energy. In 2006 photovoltaic systems produced 0.04% of the world's electricity, according to the International Energy Agency. The thing that has held back the widespread deployment of solar panels is their price. Sunshine is free, but converting it into electricity is not. At w present, solar power is at least two to three times as expensive as the typical electricity generated in America for retail customers. (Because homeowners and businesses generally use solar power in place of electricity bought from utilities, the relevant comparison is with the price of retail electricity, not the lower wholesale prices from power plants.) Even so, many people believe the prospects for solar energy have never looked brighter. Decades of research have improved the efficiency of silicon-based solar cells from 6% to an average of 15% today, whereas improvements in manufacturing have reduced the price of modules from about $200 per watt in the 1950s to $2.70 in 2004. Within three to eight years, many in the industry expect the price of solar power to be cost-competitive with electricity from the grid. In the meantime, some European countries and parts of America have instituted subsidies to support the adoption of solar power. California's "Million Solar Roofs" initiative, for example, will hand out about $3 billion in rebates and other incentives over a decade to encourage the installation of solar panels. In Europe Germany offers producers of solar power generous feed-in tariffs, which have made it the largest market for photo-voltaics in the world. As a result of such incentives, the market for solar power has grown by about 40% a year for the past five years, reaching ab out $11 billion in 2005. In a matter of a few years, solar power has become a big business. This development has not gone unnoticed by America's venture capitalists, "whohave embarked on a ' spending spree. Cleantech Venture Network, an umbrella organisation based in Ann Arbor, Michigan, says vcs invested $2.9 billion in North American clean-technology start-ups in 2006