The detection device for measuring the quantity of hot water remaining inside a storage reservoir and more particularly inside a tank (2) of a water heater (3, 30), comprises a capillary tube (10) containing a heat-carrying fluid which is placed either inside, or outside the tank (2) of the water heater (3, 30) and an indicator device (12) coupled to said capillary tube (10), said indicator device (12) comprising an amplifying device (15) disposed outside said tank (2) and making it possible to transmit under the effect of the expansion of the fluid contained in the capillary tube (10) either a motion to a member (16), or a variation in resistance to an electronic measurement card (24) in such a way as to continuously indicate the quantity of hot water available remaining in the tank (2) of the water heater (3, 30).