Abstract: A heater for use with hot tubs formed from material subject to damage from excessive heat, the heater comprising a heating element arranged within a chamber adjacent a thermally conductive plate with one or more hollow extensions penetrating through a surface portion of the hot tub to permit direct heat transfer between the thermally conductive plate and the hot tub interior. The thermally conductive plate is preferably formed with transparent inserts permitting light from the heating element to enter the hot tub, the heater also preferably including means for communicating additional hot tub accessories with the hot tub interior. The heater may also be employed with other liquid containers, for example, storage tanks which may be alternatively heated by solar energy.