Tank storage can have a considerable effect on the estimated energy costs for a system. As tanks fill or drain, they also act as an energy (and therefore cost) storage element. If a tank is full when a simulation begins and empty when it ends, there is an energy deficit-at some point the pumps will need to operate again in order to replenish the tank. Likewise, if a tank begins empty and fills over the course of a simulation, that represents an energy credit when the total daily cost is calculated.