one building. The utility's system also has back-up capacity to protect against sudden outages. The cost of such "insurance" is also markedly lower than what it would be for an individual structure. The use of utility supplied chilled water is most cost effective when it is designed into the building's infrastructure or when chiller/cooling tower equipment must be replaced. Commercial customers often lower their air conditioning costs from 10-20% by purchasing chilled water. Chilled water storage Water can also be chilled at night, where electricity is available at off-peak rates, then stored in a large, insulated tank until needed, the next day, for cooling. References External links Chilled Water Plant Design and Specification Guide Cooling technology Heating, ventilation, and air conditioning Mechanical engineering