An appliance having a storage tank disposed on its back surface wherein the storage tank comprises a front cover and a back cover that matingly engages the front cover to form a liquid tight seal with the front cover. The storage tank further includes a phase-changing material disposed within the storage tank and a heat exchanger containing refrigerant tubing which transfers heat from the phase-changing solution to the refrigerant tubing. The storage tank, when fully charged with cooling capacity, maintains the food storage compartment at a temperature of 45° F. or less for at least 8 hours without activating a compressor.