An electric water tank heater is installable through the drain plug opening of a water tank. The heater includes a power cord interconnected with a heating element through a temperature control device for delivering electrical power to the heating element in a controlled fashion. A housing hermetically seals the interconnection between the heating element, the power cord and the temperature control device. The power cord extends through the drain plug opening of the tank and terminates in a conventional male plug that is configured for insertion into a reciprocal power outlet. A drain plug adapter is secured about the power cord and is mountable through the drain plug opening in a water-tight fashion. The drain plug adapter may be permanently mounted about the power cord or may be removably mountable about the power cord. The power cord may include a detachable connector interposed between the heating element and the male plug. The connector is smaller than the electrical plug, and, as a result, the heater may be installed through drain plug openings that are too small to permit the passage of the electrical plug. The connector can be positioned anywhere along the length of the power cord, and integrally formed with the male plug or the heater housing. The drain plug adapter may be used separately or in combination with the detachable connector.