A Case Study of Electric Utility Demand Reduction With Commercial Solar Water Heaters

The City of Austin is studying the impact of solar water heaters on summer peak electric demand. One passive and two active solar water heating systems were installed on city-owned commercial buildings which had electric water heaters in 1985 and have been monitored for two years. A method has been developed to determine the peak demand reduction attributable to the solar systems. Results show that solar water heating systems are capable of large demand reductions as long as there is a large hot water demand to displace. The average noncoincident demand reduction (during the water heater’s peak output) ranged from 0.8 to 5.8 kilowatts per system. However, the coincident demand reduction during the utility peak demand period was 0.3 to 0.8 kilowatts per system. Thus, a critical factor when assessing the benefit to the electric utility is the time of hot water demand.