Photovoltaic modules there are three main types: the features, advantages and drawbacks:

Photovoltaic amorphous silicon:
The photovoltaic module cheaper, but also with the reduced efficiency and, unfortunately, also subject to a degradation of performance over time.
This type of photovoltaic panel looks like a sheet of glass gray / bluish color uniform, the thickness is a few millimeters and usually has an aluminum frame to give extra strength and manageability to the module itself.
In practice, a special glass is coated on one side, with several steps of the state amorphous silicon and various other products, in order to create an optimum level of water resistance and electrical insulation.
The transparent side is what you expose to the sun, while in the opaque layer of aluminum profiles are set for fixing the roof. The same side apart from the two wires that carry the current generated by the solar panel plant.
The voltage produced by each PV module is approximately 24 - 40 Volt and, once connected in parallel with each other, the various currents add up and are routed to the inverter, which is an electronic device that converts direct current generated by panels into alternating current at 220 volts used in the plant home or for placing on the network Enel for compensation.
The performance of these photovoltaic panels ranges from 6 to about 10%, but, in the first two months of life, the yield decreases by about 20%, then remained stable with a performance degradation that must be guaranteed, and should not exceed 20% in the first 20 years of operation.
In any case, the power of these modules is calculated by considering its immediate initial loss of 20%, and then, during the first months of life, the surrender of power sold with a panel of 40 watts, in reality is 48 watts, up actually about 40 W to stabilize after the first months of operation.

From a standpoint of 'energy cost for nature', the amorphous silicon photovoltaic panel is the product that defends better, since, require a relatively low amount of energy to be produced, can return in a few years l ' energy that was used to produce it, and can generate up to 10-12 times more in their lifetime. This is truly important from a standpoint 'ecological', as it often is in danger of producing and using products that have consumed more energy to be produced, than they are able to return, and this nature may not to please ....

The downside is that unfortunately, having a low yield compared to other models of solar panels, install a fairly high (This obviously does not affect the speech energy cost just done), but still, having a roof ample the problem is solved, but it also saves cost, since in practice the cost per watt produced with this technology is 25-40% lower than other PV technologies.
Another major advantage of amorphous silicon modules is related to the fact that during the cloudy days, shade, or in the evening and early morning, you get even higher yields of 8 and 15% compared to the mono-and poly-crystalline technology because this technology can take advantage of these special moments.
It therefore follows that the amorphous silicon solar panels are particularly suited for areas where often there is the presence of clouds or physical obstacles that create shadows.

Amorphous silicon photovoltaic panels can be supplied without a frame of aluminum, useful option if you want to make a single external frame that encompasses all the panels, or just to save money, since the presence of the frame has no effect on performance or electrical isolation of the module itself.
Attack of the various modules is through the roof for aluminum in each case on the back of the modules: in fact, the aluminum frame is useful almost exclusively for aesthetics and to improve the manageability of the various modules.

Solar Panels in multicrystalline silicon O MONOCRYSTALLINE:
These two types of photovoltaic modules are aesthetically as many cells be square or rectangular, side by side under a pane of glass in a frame of aluminum.
In practice, the solar cell is composed of approximately 30-70 individual photovoltaic cells side by side, electrically joined through special materials and arrange for one or more layers of glass in a frame usually made of aluminum, in order to give the whole a certain toughness, manageability, and of course isolation from the elements.

The overall performance of a solar panel monocrystalline silicon is around 13-17%, while that of a multicrystalline silicon solar panel is about 12-14%.
So, for the same space compared to amorphous silicon solar module, you have the yield double or nearly triple, but the cost per Watt of mono-producible multicrystalline remains higher.
Besides the fact that, to produce these types of mono-multicrystalline photovoltaic modules, is spending a lot of energy, so each module also takes 3-6 years (compared with about 2-3 years Product amorphous silicon) to return the only power that was used to be produced, while in their lifetime will produce 4-8 times more, in particular, this problem is the major failing of the single crystal form.

Another defect of the latter quite annoying photovoltaic technology, is linked to a substantial reduction, or even killing performance in the event of shades that cover a small portion of the form, or in case of clouds, or during evening hours or early morning.

Fact is, however, these two types of photovoltaic panels are still very good quality products and stability of returns, which in fact remains constant over time and guaranteed, even for 25 years, producing more energy for the same space occupied optimize space, perhaps not too exploitable part of the roof which is placed in the South

recommend the purchase of monocrystalline or multicrystalline photovoltaic modules if you have state or regional contributions for the construction of photovoltaic roofs (10,000 photovoltaic roofs program, or the like), but normally just because certain parameters required for stability of performance over a period 20-25 years, and this is practically guarantees only by monocrystalline or multicrystalline modules.
This advice applies even if you have a roof is not too large, or you want to minimize the visual impact against the outside of the solar home, or you want to entrust to a technology of high quality and durable.

recommend the purchase of amorphous silicon photovoltaic modules in case we want to create as little environmental impact with regard to nature, so try to waste less energy as possible for the production of photovoltaic modules. In this regard we recall that it takes years of operation so that a photovoltaic module generate enough power to compensate for what was used to be produced.
This choice is also suitable if you live in areas where often there are clouds, mist, or there are buildings or trees that create shade during the day: in this case the overall yield is much better with this type of solar installation.
The choice of amorphous silicon photovoltaic panel we feel is valid even if one wants to reason in this context: constructing a solar module with amorphous silicon obtained excellent results for the first 20 years of practice so we have a good service with fewer costs than the choice of the photovoltaic panel mono-mutlicristallino. In 20 years of solar photovoltaic technology is very advanced, mature and above all much cheaper (we will strive to make it so!) and then consider that when the plant will decrease the his performance we will have economic alternatives to replace him with yields much higher.
Finally we quote a remark still important in our view, however, namely that The technology of household cleaning products leads to ever more savvy with consumption, so even though our plant will make 20% less in 20 years, will also true that our appliances will consume too much less than now, and then we can cover our electric fabbisgno however with our solar energy system.