Abstract: There is disclosed storage apparatus for storage of energy in the form of heat or cold, preferably over a prolonged storage period. The apparatus comprises a container enclosing an energy storage zone and an insulating zone around the storage zone. The storage zone is filled with a mass of heatable or coolable storage material forming a storage core, and the insulating zone with a porous mass of granular or fibrous material forming a thermal insulating layer. Energy can be supplied to the storage core by, for example, a solar-powered electrical resistance heater and extracted from the core at a desired time by, for example, a heat exchanger. The insulating effect of the insulating layer is enhanced, particularly from the viewpoint of long-term energy storage, by creation of a vacuum in the cellular structure of the insulating layer.