A photovoltaic module, or panel, is a device that converts direct or indirect sunlight into electrical energy.
The basic component is the photovoltaic (or solar) cell, which behaves like a battery.
When illuminated, it supplies an almost constant voltage and a current which is proportional to the intensity of the light.
As the voltage of each single cell is small (less than one volt), they are connected in series (just like the batteries in a portable radio) in order to obtain a higher output voltage.
The resulting product is known as a photovoltaic module (or also solar panel).
There are various types of photovoltaic cell, but those based on silicon (the most prevalent material on the Earth's crust, the same used to make glass) are the most widespread and the most reliable. Silicon cells can be monocrystalline or polycrystalline (generically: crystalline silicon cells) that differ in some aspects related to the mechanical strength and efficiency, but which are used similarly in the manufacture of the photovoltaic modules. A crystalline silicon cell generates a voltage of about 0.5 V and a current which depends on the intensity of light (it is zero in the dark) and can exceed 8 amps, depending on the size of the cell and its efficiency. A photovoltaic module therefore behaves as an electric current generator the voltage of which can be easily calculated. Each cell provides about half a volt, so the module voltage will be equal to half the number of cells, while the maximum current will be of the order of a few amperes, depending on the type of cell. By multiplying the voltage by the maximum current we will have what is called the maximum power of the module, expressed in watts. By way of example, consider a module composed of 36 cells, each of which is capable of giving a maximum current of 6 amperes. Since all the cells are connected in series, the current of the module will be equal to that which passes through a single cell, and therefore still 6 amperes, while the voltage will be about 18 volts, therefore such a module may provide as maximum (under bright sunlight), a power of 18x6 = 108 W. What can we do with this 100 or so Watts? We could connect the photovoltaic module to a light bulb with the right voltage and certainly make it shine. However, the direct use of the electricity produced is not the best way to exploit the Sun’s energy. In fact, it is clear that, while a light bulb could work, even with the current variations caused by clouds or the shade of a seagull, the same could not be said of an electronic device, which would probably be damaged by such a variable power source. Not to mention the fact that the light varies during the day and therefore the power from the module varies too. And then, what happens during the night?
The electrical energy produced by a photovoltaic module must be stored. Grid-connected plants can use the electric grid as storage (absorbing or releasing energy depending on the need). Off-grid plants, need batteries with a proper capacity. Therefore, a photovoltaic module to generate power, a battery to store it and release it when needed, and between these two an efficient battery charge controller. This is the essential trio needed for energy independence.