Earth is being reached by solar rays whose light spectrum is similar to that of a so-called ”perfect black body” having a temperature of approx. 5700 K. Before entering the Earth’s atmosphere, the radiation power is equal to 1367 W per 1 sq.m. of the surface perpendicular to the solar rays. Some of this energy is being reflected and absorbed by the atmosphere, while on a sunny day approx. 1000 W/sq.m. reaches the Earth’s surface.
The amount of solar energy which reaches a given location depends on latitude and weather conditions. The average, annual insolation of Poland is approx. 3500 MJ per sq.m. per year (~1100 kWh per sq.m. per year) for horizontal surfaces, which corresponds to the calorific value of 120 kg of conventional fuel. This energy can be used by solar panels.
The most commonly used material for building semiconductor cells is silicon. Cells made from this material have a maximum electrical efficiency of 16%. Such photovoltaic cells are composed of a semiconductor plate with a P-N connector (P-positive, N-negative). The plate’s structure creates an electric field ( W strukturze takiej płytki występuje pole elektryczne (electrostatic potential barrier). When the cell is reached by solar rays, a pair of carriers with opposite charges emerges. These carriers are then separated by the electrostatic field, creating voltage in the cell. After plugging in a receiver (any device which runs on electricity) the current starts to flow. Such cells are combined to create so-called modules which make up complete systems.
The installation of solar panels on the roof can allow to produce electricity to be used directly in the household, transmitted to the power grid or stored in batteries. For example 1 sq.m. of a photovoltaic module can power an at least 125 Watt lightbulb. Assuming that the average daily energy demand of a single-family household ranges from 8 to 15 kWh (of which 5 kWh regards all uses other than heating) installing 80 sq.m. of photovoltaic modules should produce approx. 10 kWh of electricity.