In a freezer, the cabinet air is far below freezing, so it cannot melt frost on its own. Electric defrost adds heater elements, often embedded in the evaporator fins or running beneath the coil and drain tray. The controller stops the compressor and fans, energises the heaters, and ends the defrost when the coil reaches its termination temperature or a maximum time has passed.
It is reliable and simple to control, which is why it is common in upright freezers, freezer display cases and fan-forced chillers working at low coil temperatures. The cost is energy: heater power adds to consumption, and the heat introduced must be removed again when cooling restarts.
Common faults are a burnt-out heater, a failed termination sensor or a frozen drain line. All three show up as a coil that gradually becomes a solid block of ice. In a busy kitchen, a freezer that seems to lose temperature a little more each day often has a defrost heater problem rather than a refrigerant fault.