However, over the last twenty years, technical advances have made PV much more practical. The most efficient solar cells made to date convert nearly one-third of the incident light into electricity and production cells have crept past 12 percent. And as efficiency has risen, prices have fallen. Until recently, PV cells have been made from expensive wafers cut from single crystals of silicon, the same highly purified material used in computer chips. More recently, scientists have learned to make them from polycrystalline silicon and even from glassy layers of amorphous silicon. Other materials have also come into use; cadmium telluride and copper indium diselenide offer high efficiency and easy fabrication. Thanks to these less costly materials and to the economies of mass production, the price of solar cells has dropped precipitously. PV cells now cost roughly $1 per watt, or roughly $0. 12 per kilowatt-hour (kwh) over the useful life of the panel. At $0. 10 per kwh, or slightly less, photovoltaic will be competitive with conventional generators in some of the more expensive power markets.

By 2010, we expect commercial PV panels to reach efficiencies of 15 percent or slightly more. Costs will fall to the neighborhood of $0.08 per kilowatt-hour. (One PV manufacturer, Entrant Corp., has already startled the industry by offering to sell power from a 100-megawatt generator in the Nevadan desert for only $0.055 per kWh, about $0.03 per kWh cheaper than the national average cost of electricity. However, the offer counted on substantial tax incentives to help the company meet that price with current technology). At that point, even in the developed lands, power companies will find it cheaper and easier to supply residential customers with PV panels than to try to expand their large-scale generators and distribution grids.

Several developed countries already are counting on PV to meet a substantial part of the growth in their demand for electricity. Japan already pays up to two-thirds of the cost of household PV systems; according to government plans, 70,000such systems will be installed in Japan by the turn of the century. Germany offers a 70 percent subsidy for PV installations. Italy either pays up to 80 percent of the cost or buys power from PV generators at rates up to $0.28 per kilowatt-hour. As a result, the use of solar energy is growing much rapidly in these countries than in other industrialized nations with less favorable government policies.

Yet even fifteen or twenty years from now PV still will have its greatest impact on less developed lands, where substantial populations are spread out over large distances or where building conventional power lines is impractical for other reasons. South Africa has committed itself to bringing electricity to 2.5 million households by 2000.Virtually all of them live so far from the national electric grid that it will be cheaper to give them PV panels than to string new cables to them. More than 11,000 schools and many remote medical clinics also will receive electricity for the first time, thanks to PV generators. Kenya, Zimbabwe, India. And other Third World countries also have ambitious rural electrification programs that depend largely on PV power.

These efforts will bring relatively modern technologies to millions of people who have never had them. Electric lights are only the beginning. Over the nest twenty years, many of the world's poor will receive refrigerators, both to keep food fresh and to preserve heat-sensitive medicines; radio and television to provide contact with the outside world; and satellite receivers to bring educational broadcast from distant schools. Small, inexpensive, self-contained PV generators will power many of those advances. Even beyond the environmental benefits of clean power, we believe that bringing the world's poor and neglected rural populations into modern society will prove to be the most important contribution made by alternative-energy systems over the next two or three decades.