An electric heater (1, 1′) for heating a substance inside a tank (100), the electric heater (1, 1′) comprising: —at least one resistive wire (4) adapted to be connected to a source of electricity, said at least one resistive wire (4) being provided with a sheath (41) made of electrically insulating material; —at least a first thermally conductive sheet (5) made of metal fixed to the at least one resistive wire (4); —at least two protective layers (2, 3) made of polymer material, fixed to each other; wherein the at least one resistive wire (4) and the at least a first sheet (5) are arranged between the at least two protective layers (2, 3).