All levels of society should be sensitive to the environmental issues. The ecological parameters must be taken into consideration as a part of the external costs. But these costs someone has to pay. The sustainable development has to balance power supply and demand respecting both economical and ecological criteria. However, it will be very difficult to secure energy at low cost, with high rates of economy growth and acceptable pollution levels. Having this in mind, renewable energy resources could play a dominant role in the 21st century alone and in combination with other ecologically acceptable fuels.

The price of fossil fuels (coal, oil, natural gas) does not represent their true cost to the society. Besides many factors the ecological impost also has to be included in cost of fossil fuel. The impost consists of:

— Repayment of environment contaminators according to the emission of CO2, SO2, NOx,

— Repayments according to the burden of the environment-communal or dangerous waste,

— Repayments for the environment contamination caused by traffic — motor vehicles.

The example of solar energy utilization project described here shows savings of

thermal energy in the system of space heating and domestic hot water preparation. A yearly value of solar contribution at level of 70 % is expected. The solar contribution in electricity supply of the house is around 50%. Also a temporary excess of electricity produced in PV generator can be delivered to the network. The reduction of the emission of the CO2 at the rate of 6500 kg per year is expected.

The house is also equipped with rain-water collection system, saving almost 55% of overall sanitary domestic water demands.

2. Conclusions

The pilot project "Solar roof Spansko-Zagreb" shown here presents production of both thermal energy and electricity in the family house. In this work the possibility of energy savings in the system of space heating and domestic hot water preparation is presented in the case when solar energy is used, and electricity production from solar energy is shown. On the other side, almost ideal supplementation of the city gas as an additional fuel with solar energy, especially in the reduction of harmful gases emission is shown. The significance of this pilot project is in the usage of renewable energy resources, which are capable to ensure large enough amounts of energy for both economic growth and sustainable development for the well of future generation.