We receive energy from the sun in two main forms: heat and light;

Solar electric systems convert light energy into electricity using photovoltaic cells made from layers of semi- conducting material, usually silicon.

The conversion of light into electricity is based on three important principles:

• Excitation of free mobile charge carriers due to light absorption;
• Separation of the charge carriers and;
• Collection of the charge carriers at the contacts.

We call photovoltaic process this conversion of light into electricity.

Assembly of cells are used to make up solar panels, also called solar modules. Modules or panels connected together constitute photovoltaic arrays.

Solar PV systems can be connected to the grid or can work as isolated systems for the use of single household or institutions in area where the grid is unavailable or just not reliable.

Solar PV systems are on grid when they are connected to the grid and off grid when they are not.

On grid or grid-tied PV systems export excess energy to the grid. In case the sun is not available, the connected loads get power from the grid.

An off grid or stand alone PV system involves the use of storage batteries.

The batteries store the excess energy generated by the PV array and, in it turn, provide electricity to the connected load when the sun is not available.

Photovoltaic cells are categorized based on the type of semiconductor material used to build them.

The three main type of PV cells in use are mono-crystalline silicon; poly-crystalline silicon and thin films.

The most dominant PV technology is based on the the use of silicon wafers that are typically 150-200 microns (one fifth of a millimeter) thick. This technology represents around 90% of the current PV market and belongs to the first generation PV technology

Another PV technology based on silicon is thin film silicon. In this case no silicon wafers are used but very thin layers of silicon, which are deposited on glass or flexible substra. This technology belongs to the second generation.

Third generation PV technology covers a wide range of new novel and innovative ideas. Most of them are still under testing.

Apart from their visual aspect the most obvious difference between PV cell technologies is in its conversion efficiency.

• Mono-crystalline silicon: 12.5-15%
• Poly-crystalline silicon: 11-14%
• Copper Indium Gallium Selenide (CIGS): 10-13% Cadmium Telluride (CdTe): 9-12%
• Amorphous Silicon (a-Si): 5-7%

Cells efficiency impact on the size of the modules: under standard test conditions (STC), and an irradiance of 1000W/m², with an efficiency of 12.5% a mono-crystalline panel sizing 1m² will produce 125W; under the same conditions, an amorphous silicon panel of the same size but having 5% efficiency will generate only 50W

Another important differentiator in solar PV performance, especially in hot climates, is the temperature coefficient of power. PV cell performance declines as cell temperature rises. Compared to crystalline technologies thin film technologies have a lower negative temperature coefficient In other words, they tend to lose less of their rated capacity as temperature rises.

It's important to understand the relationship between temperature and voltage.

While temperature does have a slight impact on current, it has a large impact on voltage.

For design purpose, you need to understand the information delivered on the modules' specification sheet.

• Pmax represents the maximum power a solar module can produce;
• Vpmax is the voltage at which the maximum power is produced;
• Ipmax, the current at which maximum power is produced;
• Voc, the maximum possible voltage across the solar module when current is zero and;
• Isc: The maximum possible current flowing through the solar panel when no load is connected and voltage is zero.

Modules performances are tested under predetermined conditions. These are the standard test conditions (STC). To meet the test conditions, cells, not ambient, temperature of 25 degree Celsius and an irradiance of 1000w/m² must be respected.

In practice, PV modules are connected together in series making PV strings and strings are connected together, forming PV arrays.