Imperial Electric Booster Water Heaters (S series) rated for 24kW or larger have multiple banks of heating elements, with each bank controlled by a separate thermostat. During periods of use, the lowest bank of heating elements turns on first and stays on until its thermostat reaches the setpoint. If additional hot water is needed (during periods of high demand), the next level of heating elements come on and heats. This step loading, or staging of the heating elements, allows for reduced energy costs when compared to units that have all of their elements controlled by one thermostat and are either ON or OFF 100% of the time.