An electrical energy-storage unit has a plurality of electrically coupled and interchangeable storage modules each with a multiplicity of storage cells connected in series or in parallel. The energy-storage unit has storage modules of a first type, which have a certain energy density. The energy-storage unit has storage modules of a second type which have a higher energy density than the storage modules of the first type. At least two storage modules of the second type fit into the spatial volume of a storage module of the first type.