Photovoltaics (PV) is a way of converting sunlight directly into electrical energy. Photons in sunlight excite electrons from the valence bands into the conduction bands of a semiconductor, enabling the generation of electric current. This photoelectric effect was discovered in the nineteenth century, and the theoretical explanation won a Nobel Prize for Albert Einstein. The cost of early units was high because of the expense of producing high-quality silicon ingots and manufacturing the silicon wafer that is at the heart of this technology. As the price of silicon wafers has fallen with improvements in the manufacturing technology, the efficiency of the commercial units has increased to 15 percent with the use of crystalline silicon. Amorphous silicon wafers are cheaper to manufacture, but they are less durable than crystalline wafers, and they have a 5–10 percent lower efficiency that would require a larger solar panel for the same power.
Solar photovoltaic systems suitable for rural households usually consist of several components. They include a PV module containing the silicon cells to be mounted on the roof or another sunny spot, a battery for storing electrical energy for use at night, a charge controller, wires and structural frames, and outlets for lights and other appliances. Such a system can operate several fluorescent lamps (often four), a radio, a black and white television, and perhaps a fan. The system normally operates on 12 volts DC. Long-lasting deep-cycle batteries, which can discharge 80 percent of their charge during extended overcast weather, are best, but automobile batteries, commonly available in Nigeria, may be used as well. The charge controller prevents damage to the system in case of overcharging by the solar module or prolonged battery discharge from overuse. The cost for a 40–peak watt system is about $350–$500 worldwide, depending largely on the input duties on the solar panel. Other requirements are the cost of installation, periodic battery replacement (once every five years), and training for the user, all of which are often part of a service contract for maintenance. But without special arrangements, that price is out of the reach of most Nigerians in a country in which the annual per capita income is about $250.
One of the more successful enterprises selling, installing, and servicing solar home systems (SHSs) is the Solar Electric Light Company of India (SELCO). SELCO Solar Light Pvt. Ltd., with a registered office in Bangalore, Karnataka, was founded in 1995 with initial financing from the Rockefeller Foundation. It is the first rural solar company in India to