A compact solar system for heating a first heat transfer fluid, said solar system comprising an external frame, inside which are contained a main tank for containing a heat transfer fluid, collecting means for collecting the solar radiation, said collecting means being in heat exchange with said main tank for transferring the collected thermal energy to the heat transfer fluid contained in said main tank, and a first hydraulic circuit for the circulation of said first heat transfer fluid, comprising an inlet portion for the inlet of the first heat transfer fluid and an outlet portion for the outlet of the first heat transfer fluid, said solar system further comprising inside said frame an additional tank for containing a heat transfer fluid, said additional tank being thermally isolated from said collecting means, the volumetric ratio between the volume (V 4) of the additional tank and the volume (V 3) of the main tank being defined by the following relationship: 0.3 ≤ V 4 V 3 ≤ 1.3 wherein said first hydraulic circuit provides a first heat exchange portion (4, 94') in heat exchange with the heat transfer fluid contained in said additional tank and a second heat exchange portion (3, 94) in heat exchange with the heat transfer fluid contained in said main tank, said first (4, 94') and second (3, 94) heat exchange portions being in fluid connection with each other through a connecting conduit characterised in that said first (4, 94') and second (3, 94) heat exchange portions are further in fluid connection with each other through a first return conduit, and in that it provides flow adjusting means (5, 6; 5, 6; 8; 6") arranged in said first hydraulic circuit for allowing the circulation of said first heat transfer fluid from the second heat exchange portion (3, 94) to the first heat exchange portion (4, 94') through said first return conduit, when a threshold temperature (T s) of a heat transfer fluid contained in said main tank is reached. a main tank for containing a heat transfer fluid, collecting means for collecting the solar radiation, said collecting means being in heat exchange with said main tank for transferring the collected thermal energy to the heat transfer fluid contained in said main tank, and a first hydraulic circuit for the circulation of said first heat transfer fluid, comprising an inlet portion for the inlet of the first heat transfer fluid and an outlet portion for the outlet of the first heat transfer fluid, said solar system further comprising inside said frame an additional tank for containing a heat transfer fluid, said additional tank being thermally isolated from said collecting means, the volumetric ratio between the volume (V 4) of the additional tank and the volume (V 3) of the main tank being defined by the following relationship: 0.3 ≤ V 4 V 3 ≤ 1.3 wherein said first hydraulic circuit provides a first heat exchange portion (4, 94') in heat exchange with the heat transfer fluid contained in said additional tank and a second heat exchange portion (3, 94) in heat exchange with the heat transfer fluid contained in said main tank, said first (4, 94') and second (3, 94) heat exchange portions being in fluid connection with each other through a connecting conduit characterised in that said first (4, 94') and second (3, 94) heat exchange portions are further in fluid connection with each other through a first return conduit, and in that it provides flow adjusting means (5, 6; 5, 6; 8; 6") arranged in said first hydraulic circuit for allowing the circulation of said first heat transfer fluid from the second heat exchange portion (3, 94) to the first heat exchange portion (4, 94') through said first return conduit, when a threshold temperature (T s) of a heat transfer fluid contained in said main tank is reached.