Because the RTF's analysis is based on existing evaluations, the impact estimates are backward looking. That is to say that the estimates are based on what has occurred in the past, which may not be indicative of the future residential water heating market. The average demand response impact that a program can expect during an event will largely be determined by the technology and efficiency of the underlying water heating systems enrolled. This mix of underlying technologies is expected to shift meaningful in the coming years as the market share of heat pump water heaters increases, and as heat pump technology progresses. Additionally, household usage of hot water (and coincident peak demand) has shifted over time, and may continue to shift, due to water saving measures, household behavioral changes, and other factors.
For heat pump water heaters specifically, most models have several operational modes for the household to choose from. One primary differentiation between these modes is their propensity and criteria to utilize electric resistance back-up heating. The RTF analysis has no data on the distribution of mode settings in regional households, or how different modes correlate to demand response impacts.
At the time of approval, PNNL was performing additional analysis on the data collected in BPA's CTA-2045 study to possibly generate more granular results based on heating zone, housing type, of other factors. Based on RTF guidance, these additional results may be incorporate into the analysis at a future date if appropriate.