To provide an electric water heater capable of being post-installed to an existing basin for a wheel-chaired user to use hot water with no trouble, and to provide its installation method.

The electric water heater comprises a cylindrical hot water storage tank 21 with an incorporated heater horizontally laid in a casing 10 in parallel to a standing wall face, an inflow side pipe portion including a water supply pipe 24 and a pressure reducing valve 26 arranged in a space ranging from the lower part of the hot water storage tank 21 to the back face of a case body 11, and a tapping/outflow side pipe portion including a tapping pipe 32 or a release valve 31 arranged in a space ranging from the upper part of the hot water storage tank 21 to the back face of the case body 11. The lower corner of a front panel is in a tapered shape with its angle cut off. Thus, the depth of the casing 10 is a smaller size and a distance between the lower face and the floor surface of the casing 10 is sufficiently secured, avoiding the contact of the feet of the wheel-chaired user when using the basin.