An integrated microwaveable heat storage device that absorbs substantial amounts of microwave energy in a microwave oven in a relatively short period of time (one to 2 minutes) and releases this energy as heat (infrared radiation) over an extended period of time (typically 20 to 30 minutes The device includes three rigid elements, the upper element usually in the form of a container (10) such as a dish or cup cooperating with a base (14) to form a sealed enclosure or cavity. This enclosure is transparent to microwaves and encloses a mass of solid microwave absorbing material heater (12) in minimum contact with the walls of the cavity. This receptacle (enclosure) and the microwave absorbing material (the heater (12)) can have different cross sections, shapes, volumes and masses according to the specific intended use of the heat-storage device.