Scale is the hard deposit of hardness salt crystals on heating elements.
Salt deposits significantly impair thermal conductivity of metal.
As a result of additional thermal insulation the electric heater has to use more energy to heat the medium.
Thermal conductivity of scale is dozens of times, and often hundreds of times lower than thermal conductivity of steel used for production of heat exchangers.
That is why even the thinnest layer of scale creates high thermal resistance and may lead to deformation and damaging of tubes and other parts of the equipment.