The calculation of the collectors, both in the case of Custom Built and Factory Made systems, should be carried out in line with the norm (UNI 477 part 1) based on the data on horizontal surfaces from norm UNI 10349, from the  European Solar Radiation Atlas, or finally from ENEA publications "La radiazione solare globale al suolo in Italia".

In the case of the production of hot water for homes and domestic swimming pools the surface eligible for subsidy must not exceed the minimum able to guarantee, in the month of May, the entire requirement by means of solar power.

In the case of plants for the production of hot water for seasonal use (April to October) and for heating pools in summer, the surface admissible for subsidy must not exceed the minimum needed to guarantee, in the month with the highest radiation, the entire requirement by means of solar power.

Should the user decide to realise plant beyond what is eligible for subsidy, the entire cost of the investment will be borne by the user in person. As regards the orientation of the collectors, orientation in a northerly direction (East, North-East, North- West, West) is not allowed.  Orientation towards the East and the West is allowed only where there are no other options available.

In the case of installations on pitched roofs, excluding industrial premises, in order to respect the correct architectonic parameters, installation of solar collectors with orientation or inclination different from the orientation and inclination of the roof itself is not permitted.

In the case of installation on flat surfaces the following recommendations (adapted to the specific circumstances to obtain optimum effect within the architectonic and environmental guidelines) are indicated:

• To obtain maximum efficiency the solar collector should be orientated towards the south with a maximum tolerance of ± 10°;
• In the case in which the power is roughly constant throughout the year, the best inclination is that equal to the latitude of the location reduced by 5°± 10°;
• In the case in which the power is predominantly produced in summer, the best inclination is that equal to the latitude of the location reduced by15°±20°;
• In the case in which the power is predominantly produced in winter, the best inclination is that equal to the latitude of the location increased by 10°±15°.

For combined solar power systems that integrate heating water and the building itself, the inclination should  be greater than  that outlined above so as to privilege the production of thermal energy in winter.

For “custom built” systems, the calculation of the monthly and annual yield of the solar heating plant, should be carried out in line with the stipulations in the norm UNI 8477 part 2 (f-chart method) or through simulations.

For “factory made” systems the energy yield from the plant should be calculated in line with EN 12976-2, by means of the CSTG method.