The water heater operates with electric energy, where the resistance inside the water heater is heated and thus the water is warmed.
On the other hand, the electric boiler, in addition to heating the water through electric energy, has an internal exchanger (coil) with the ability to connect to the central heating system, to assist in the production of hot water. This limits the use of electric energy.
This means that once connected, the hot water produced by, for example, the oil boiler, heats not only the radiators but also the water in the water heater, thus saving money during the winter months.