The impact of the high power requirements of large residential tankless electric resistance water heaters on electric system distribution wiring, transformers, breakers, and other equipment is of concern for both electric utilities and builders. This study was undertaken to analyzehowprobable and worstcase diversified electrical demand of large (28 kW) residential tankless electric water heaters compares to that of 4.5kWstorage (tank-type) water heaters in residential applications as a function of the number of households on a given electrical distribution circuit.

This work developed methods to estimate both most likely (most statistically probable) and 3-sigma (statistically improbable but theoretically possible) worst-case diversified electrical demand for both tankless and tank-type water heaters as a function of the number of households. As expected, single households experience peak water heating electrical demands that correspond to the maximum power rating of the water heaters. However, time diversity of hot-water use increases rapidly as more households are added, resulting in diversified electrical demand of large tankless electric water heaters dropping rapidly from maximum values as the number of households on an electrical distribution system increases. Probable tankless water heater diversified electrical demand drops to levels only slightly higher than for tank-type units when even only a few (2 to 10) households are on a given distribution system, and it continues to drop even further as more households are added.

Importantly, this work computes diversified electrical demand estimates using three totally different methods, showing that there are multiple analysis techniques that all lead to the same conclusions--namely that diversified electrical demand of 28 kW tankless electric water heaters rapidly drops to levels near to that of 4.5 kW storage electric water heaters when even only small numbers of households are considered.