Systematic approach and sustainable use of water and energy is the key point for human existence. Irrigation systems are one of the main directions for the development of the economy. Renewable energy sources, especially solar photovoltaic (PV) energy are suitable as input energy for the irrigation systems. By using the original and innovative scientifically sizing method named Critical Period Method, the irrigation systems are sustainable, given the economic, environmental and social indicators. This method includes design elements of the solution by subsystems: Photovoltaic generator + invertor – PV, pump station – PS and water reservoir – WR based on the critical period of operation of each one. Critical Period Method is different from the usual sizing methods because instead of a single critical period, which relates to the maximum daily water consumption, this method considers three critical periods (for subsystems PV, PS, and WR) due to the different balancing days. Each of the critical periods is determined with regards to a certain balance period. Critical day/period for subsystem PV is determined by statistical minimization, with regards to the difference between over pumped and demand quantity of water. For the subsystem, WR critical day/period is the day with maximum water demand and the shortest duration of solar radiation suitable for the operation of the pump station. A critical day/period for the subsystem PS also coincides with this critical day. Pumps use electric energy produced by using solar photovoltaic energy and causing water pumping into the water reservoir, which with its usual hydraulic role of storing water also has the function of energy reservoir. This concept is adaptable and can be implemented in the previously constructed systems, as well as for the new ones. Application of Critical Period Method replaces the installation of long energy supply lines and associated energy losses, and additionally reduces greenhouse gas emissions. Application of Critical Period Method is suitable for rural areas because of locations where classical power network is not available or has limited availability, i.e. remote areas and on islands. In order to prove this innovative method, a practical application is predicted to perform in irrigation of agriculture field in Deçan, Kosovo.