Electronically controlled instant water heater for energy efficient hot water supply at the wash basin: the unit is simply installed directly under the wash basin and will operate while the hot water tap is opened. The smart electronic calculates the necessary heating power due to inlet temperature and water flow, in order to heat the water to the exact temperature which is needed. The user is able to choose his individual preferred temperature from three different options (35, 38 and 45 °C). The instantaneous water heater avoids heat losses and only heats up the water, which is actually needed for washing hands.