A solar cell or photovoltaic cell is a device which generates electricity directly from visible light by means of the photovoltaic effect. In order to generate useful power, it is necessary to connect a number of cells together to form a solar panel, also known as a photovoltaic module. There is more about the different types of solar cell here. The nominal output voltage of a solar panel is usually 12 Volts, and they may be used singly or wired together into an array. The number and size required is determined by the available light and the amount of energy required.

The amount of power generated by solar cells is determined by the amount of light falling on them, which is in turn determined by the weather and time of day. In the majority of cases some form of energy storage will be necessary.

In a Grid-connected system, the solar array is connected to the mains. Any surplus power is sold to the electricity company, and power is bought back from them when it is needed.

In a Stand-alone system, however, this is not possible. In this type of system the usual choice for energy storage is the lead-acid battery. The number and type of batteries is dependent on the amount of energy storage needed.

TYPES OF SOLAR CELLS

Solar cells are usually made from silicon, the same material used for transistors and integrated circuits. The silicon is treated or "doped" so that when light strikes it electrons are released, so generating an electric current. There are three basic types of solar cell. Monocrystalline cells are cut from a silicon ingot grown from a single large crystal of silicon whilst polycrystalline cells are cut from an ingot made up of many smaller crystals. The third type is the amorphous or thin-film solar cell.

Amorphous technology is most often seen in small solar panels, such as those in calculators or garden lamps, although amorphous panels are increasingly used in larger applications. They are made by depositing a thin film of silicon onto a sheet of another material such as steel. The panel is formed as one piece and the individual cells are not as visible as in other types.

The efficiency of amorphous solar panels is not as high as those made from individual solar cells, although this has improved over recent years to the point where they can be seen as a practical alternative to panels made with crystalline cells. Their great advantage lies in their relatively low cost per Watt of power generated. This can be offset, however, by their lower power density; more panels are needed for the same power output and therefore more space is taken up.

Crystalline solar cells are wired in series to produce solar panels. As each cell produces a voltage of between 0.5 and 0.6 Volts, 36 cells are needed to produce an open-circuit voltage of about 20 Volts. This is sufficient to charge a 12 Volt battery under most conditions.

Although the theoretical efficiency of monocrystalline cells is slightly higher than that of polycrystalline cells, there is little practical difference in performance. Crystalline cells generally have a longer lifetime than the amorphous variety.

Indian Advantage

India, which has an average of 300 sunny days a year, sees solar power as a potentially vital energy source that could be key to boosting power supplies and reducing greenhouse gas emission in the world's third-worst carbon polluter. India is working urgently to develop alternative power sources and wean itself away from polluting coal-fired generation to power an economy that is expected to grow by at least six percent this financial year.

It has just 551 megawatts of solar capacity installed currently, according to government figures, and some 70 percent of the equipment comes from abroad.

India's National Solar Mission launched in 2010 aims to generate 100 gigawatts of solar power by 2022 -- equivalent to one-eighth of the nation's current installed power base.

The solar energy drive is also part of efforts to tackle frequent power outages especially in rural India that economists say knock an average 1.2 percentage points off annual growth.

But barriers to widespread introduction of solar power remain such as lack of technology and subsidies to consumers. Once installed, however, experts say the costs are low and are mainly confined to maintenance.

The Indian government has unveiled draft legislation that would allow the solar industry to obtain grants covering up to 40 percent of installation costs.