The rational justification for a low-temperature thermal system derives from eco­nomic and environmental reasons. The reduction of environmental pollution and the saving of energy which can be obtained using solar energy represent a solid advantage for the community.

The Sun can normally give us 80-95% of the hot water that we use daily to wash our hands, take a shower, do the dishes or also wash clothes, if we manage to connect the solar system to the dishwasher’s or washing machine’s hot water entrance. If the products are of good quality and the system is well sized, the outcomes are very good even in northern Italy. While in southern Italy the out­comes are excellent in winter also, in northern Italy we can achieve good results only if one more panel is installed or if a capacious tank is provided to make up for the lack of sun during cloudy days, since a good tank keeps water at a constant temperature for a few days. It is not easy to know the exact saving that could be obtained from a sanitary water solar system, as it depends a lot on the habits of families. So it depends on how much water is used for personal hygiene or for dishwashing and also on the kind of central-heating boiler or water heater the family possesses. Using an approximate calculation, which is sufficiently reli­able, we can indicate a yearly methane saving of 100-180 m3 per person, with a yearly lack of carbon dioxide emission (the main cause of greenhouse effect) of 230-400 kg per person. To these quantities we have to add the wastes generated by the accumulation of the central-heating boiler and water heater, for example, with a simple pilot flame or with intermittent running of the water heater to keep the temperature of a huge quantity of water constant during the more unthinkable hours of the day and night. These methane wastes will be 150-200 m3 per year and they have to be added to the real savings calculated on the basis of the number of persons in a family. The methane cost per m3 or the cost for the fuel used by our own central-heating boiler has to be taken into consideration to calculate cor­rectly the saving in conformity with taxes, the additional tax when exceding the permitted use of energy, the IVA, the inflation, the rise which will take place in the next few years, etc. Since solar systems are capable of producing more than 90% of the hot water requirements between April and October, the central-heating boiler or the water heater could be turned off during this period, resulting in a significant reduction of wastes.

It is important to determine the regeneration time of the investment, which may be important to justify a solar system installation. The number of years taken to regain the investment can be obtained by dividing the cost paid by the yearly max­imum money saving obtained by the use if sanitary hot water produced by solar energy. On average, if we talk about a single family system, the hot water cost alone will be about 1500-2600 €, amortizable within 3-5 years, while the sys­tem’s useful technical life span is about 20 years and the maintenance costs are 1% of the system’s original cost.

A traditional water heater (electrical or methane one) never regains its cost because there is always a bill cost, whereas the more a solar energy system is used, the more it is convenient.

Only when the use of water is reduced, it is more convenient to have a small electrical or gas water heater [16, 18].