The study analyzes the evolving role of solar+storage for home backup power during long-duration power interruptions. In particular, it evaluates how required storage sizing is impacted as homes become more efficient, flexible, and electrified. The study relies on NREL’s ResStock building modeling platform to create statistically representative distributions of the existing building stock in ten locations across the United States. It then shows how the amount battery storage required for backup power rises or falls as a series of building envelope efficiency, load flexibility, and electrification measures are applied across the building stock in each region. The study also includes sensitivities to show how backup power requirements are impacted by the timing and duration of power interruptions, and explores variation in backup power requirements across the building stock within each study location. The results demonstrate the value of pairing solar+storage with efficiency upgrades, smart home controls, and (in mild winter climates) efficient heat pump retrofits. That value comes in the form of reducing the amount of storage required and/or extending the range of interruption conditions over which a given system can provide backup power (i.e., more extreme weather and/or longer interruptions). Heat pumps in cold-weather climates can pose a challenge for solar+storage backup power, given the amount of storage required, though are a vast improvement over electric-resistance heating. Retaining existing fossil-based heating systems for occasional use during power interruptions, as either the primary or supplementary source of heat, can mitigate this challenge. Other forms of building electrification (e.g., cooking and water heating) generally have marginal impacts on backup battery sizing, given their relatively small energy demand.

This study explores tradeoffs between the use of home solar+storage systems for backup power versus day-to-day utility bill savings. The study focuses specifically on the “reserve setting” available with most home battery storage systems, which allow the customer to maintain some minimum level of storage in reserve in case of an unforeseen power interruption. The more capacity that is held in reserve, the greater the customer’s ability to ride-through possible power interruptions, but less capacity is then available to manage utility bills on a day-to-day basis. This study evaluates this operational tradeoff across a diverse set of locations and residential electricity tariff structures, relying on Berkeley Lab’s PRESTO model to stochastically simulate power interruption events, and exploring a range of sensitivities, including variations in customer value of lost load (VoLL), interruption frequency, and other key drivers. The results show that, in most circumstances, the opportunity cost of holding storage capacity in reserve, in terms of foregone bill saving, tends to outweigh any gains in reliability value associated with mitigated power interruptions. This finding is robust across tariff structures and across most of the sensitivities considered, including those related to rate level, customer load level, and storage sizing. There are a limited set of circumstances where raising the reserve setting improves the overall customer value (comprised of bill savings plus reliability value). Specifically, that exception occurs when all of the following conditions apply: (a) the customer resides in a location with exceptionally poor reliability, (b) the customer has exceptionally high VoLL; (c) the customer is on a net billing rate or on a TOU rate that allows grid discharging but not grid charging; and (d), depending on the location, the price arbitrage differential on that rate is relatively small. In all other circumstances analyzed, total customer value declines with reserve level.

Estimates of the economic impact of widespread, long duration (WLD) power interruptions can be used to prioritize and justify significant investments in power system resilience. This report presents estimates of this type for WLDs originating within the Commonwealth Edison (ComEd) service territory. The intended audience for this research includes utility executives and technical staff, regulators, and government agencies. This project involved surveying ComEd customers to understand how they might respond when confronted with a WLD power interruption. The research team used the survey responses to calibrate a state-of-the-art regional economic model (“POET”) to estimate economic impacts to households and 38 industry sectors across 17 impacted micro-regions (individual counties or aggregations of counties) within ComEd’s service territory and beyond. We ran one-day, three-day, and 14-day interruption duration scenarios each with varying geographic extents as well as estimated the benefits of deploying additional backup generation across the service territory. The results were then compared to a “business as usual” scenario assuming that no interruption occurred. There are six key findings from this analysis: -There may be significant losses to gross output (business revenue), gross domestic product, and household consumption during WLD interruptions, especially multi-day interruptions that occur across all of ComEd, Cook county, or the suburbs of Chicago. -The wholesale trade and transportation sectors appear to be highly sensitive to power interruptions—losses to these sectors are large relative to the losses observed across the entire economy. -Several sector-region combinations—e.g., the transportation sector in Cook county—are very sensitive to interruptions. -High-income households experience proportionately larger losses to consumption during a one-day power interruption, but low-income households experience proportionately larger losses during the longest power interruptions. -Increasing the amount of backup generation deployed across ComEd’s service territory provides significant net benefits to system-wide gross domestic product, gross output, and household consumption relative to the existing amount of backup generation already being used. -Some micro-regions (e.g., Dekalb and Kendall counties), sectors (e.g., wholesale trade, transportation), and low-income households in Cook county may especially benefit from targeted resilience interventions. We recommend that decision-makers consider running cost-benefit analyses using each of the economic metrics presented in this report independent of one another to evaluate the robustness of the insights that each of these estimates may provide. In addition to this report, we developed a tool that will allow ComEd staff and other decision-makers to visualize the full suite of results using an easy-to-interpret, user interface. We hope that the findings from this research effort will provide valuable insights to ComEd, policymakers across Illinois, and other stakeholders who have an interest in the resilience of the power system.