The energy consumed for water heating is a significant fraction of the total national use. The most common water heater design, particularly at the residential level, incorporates thermal storage capacity to accommodate the typically high energy withdrawal rates. For electric water heaters, the only significant loss mechanism is ambient heat exchange. In the present paper, three different configurations consisting of a fully-mixed storage vessel and an electric heating element are analyzed. Compared on the basis of equal performance, defined as a sustained draw at or above a minimum energy discharge rate, the configurations are shown to require significantly different storage volumes for identical energy input rates. Accounting for ambient heat loss, these differences in storage volume are shown to translate into appreciably higher thermal efficiencies for the configuration with the smallest required storage volume.