Study shows how required storage sizing changes as homes become more efficient, flexible, and electrified

November 28, 2023

Residential behind-the-meter solar-plus-storage systems are growing rapidly, driven in large measure by customer demand for backup power. At the same time, residential energy consumption patterns are changing as homes become more energy efficient, as smart devices allow for more dynamic control of home appliances and equipment, and as customers switch to heat pumps and other electric end-uses. How do those latter trends in residential energy consumption impact the potential for solar+storage in backup power applications?

That question is addressed in a new Berkeley Lab report, Solar+Storage for Household Back-up Power: Implications of building efficiency, load flexibility, and electrification for backup during long-duration power interruptions. The report is the second in a series of studies developed in collaboration with the National Renewable Energy Laboratory (NREL), analyzing solar photovoltaic and energy storage systems (PVESS) used for backup power. While the earlier study characterized PVESS backup power capabilities in the context of the existing building stock, the latest study explores how those capabilities may evolve as homes become progressively more efficient, flexible, and electrified. Both of these studies focus on the technical potential (rather than the economics) of PVESS backup power during long-duration (>1-day) power interruptions, though other recent and ongoing work at Berkeley Lab has explored PVESS backup power for short-duration (and unpredictable) events.

This latest study leverages NREL’s ResStock building modeling platform to create statistically representative distributions of the existing building stock in ten locations across the United States. The analysis then shows how the amount battery storage required for backup power rises or falls as a series of energy efficiency, load flexibility, and electrification measures are applied across homes in each region.

Key findings from the report are highlighted below and will also be summarized in an upcoming webinar on December 7th at 10:00am PT/1:00pm PT. Please register here:

Providing backup power to heating and cooling loads can significantly increase the amount of battery storage required. When heating and cooling loads are excluded from backup, a PVESS with just 15 kWh of battery storage could provide backup power over 3-day interruption for most homes in each of the locations studied. However, if backup power is also provided to heating and cooling loads, a significantly larger battery may be needed. Among homes in the baseline (present-day) building stock, median battery sizes of 40-90 kWh would be needed in half of the locations, as shown in Figure 1. Those tend to be either hot locations with large cooling loads and/or locations with a significant amount of inefficient electric-resistance heating in the existing building stock. The primary focus of this study is on critical load backup that includes heating and cooling, as those are typically the loads and are significantly impacted by building efficiency, load flexibility, and electrification.

Figure 1. Median required battery size for the baseline (present-day) building stock with backup power provided for heating and cooling loads. The bars represent median values across roughly 1,000 modeled homes in each location, while the open circles represent median values among the subset of fossil-heated homes. The specific results shown here are based on a 3-day power interruption beginning on the 90th percentile net-load day (i.e., the 36th most challenging day to backup during the year). Sensitivities to those assumptions are provided in the report. PV systems are sized to meet 100% of each customer’s annual electricity consumption, subject to available roof-space, and customers are assumed to have fully charged batteries at the beginning of the interruption. As noted in the caption, backup power is provided to heating and cooling loads, as well as refrigeration, nighttime lighting, a limited set of plug loads, and a number of other end-uses (see report for further details). 

Building efficiency and load flexibility measures can significantly reduce required battery sizing. The efficiency upgrades included in the analysis consist primarily of insulation and air-sealing measures, while the primary load flexibility measure is a modest (5-to-6-degree) thermostat set-point adjustment during the power interruption. As to be expected, the efficiency and load flexibility measures reduce required battery sizing, particularly in hot locations and for homes in cold-winter locations with electric heating. Among the ten study locations, the largest effect was observed in Dallas-Forth Worth (DFW), where the efficiency and load flexibility measures together reduce required median battery sizing by roughly 50 kWh (see Figure 2). 

Figure 2. Median required battery size as measure bundles are applied to the baseline building stock. The results shown here are based on the same interruption conditions and critical load definition as in Figure 1. The colored bars show how the median battery size changes (either downward or upward) as measure bundles are sequentially (and cumulatively) applied to the baseline building stock in the order shown, from top to bottom in the legend. See the report for details on all measure bundles, as well as sensitivities cases with alternative measure sequencing.

Heat pump retrofits can either increase or reduce required battery sizing, depending on the climate and on the existing heating and cooling equipment. In hot climates, efficient heat pumps can significantly reduce storage sizing when replacing inefficient air-conditioning equipment units. This can be seen in Figure 2, with the largest effect in Phoenix, where heat pump retrofits reduce median battery sizing by roughly 30 kWh. Though not shown here, heat pumps also dramatically reduce required storage sizing for homes in cold-winter locations when replacing electric-resistance based heating. However, for fossil-heated homes in cold climates, heat pump retrofits can require significantly larger amounts of battery storage for backup power (e.g., about 30 kWh more for the median Boston home and 50 kWh more for the median home in Duluth). Those impacts can be mitigated to some degree with building efficiency and load flexibility, and of course also by retaining existing fossil-based heating systems for occasional use during power interruptions. As discussed in the report, these impacts are also sensitive to the specific heat pump configuration.

The other building electrification measures analyzed have relatively small impacts on backup battery sizing. The study also considered cases where all homes are retrofitted with heat-pump water heaters, induction ranges, electric ovens, and heat-pump dryers (though backup power is provided to the dryers only in whole-home backup scenarios). Those measures may lead to either an increase or decrease in required battery sizing, depending on whether the measures are replacing fossil-based or less efficient electric based appliances. In either case, however, the impacts are relatively small, given the relatively small size of these loads, compared to space heating and cooling. The report does not consider backup power to electric vehicles (EVs), though it does discuss the role of bi-directional EV chargers as a potential enabling technology for PVESS backup power, particularly in cases where relatively large amounts of storage would be required.

Efficiency, load flexibility, and (in mild winter climates) heat pumps significantly expand the addressable market for PVESS backup power. Residential PVESS today typically includes 10-30 kWh of battery storage. A system at the upper end of that size range could provide backup power over a 3-day power interruption to some portion of all homes in each location. For the baseline building stock, that percentage varies widely, from 6% of homes in Phoenix to 90% of homes in LA. Through the combination of thermostat set-point adjustments, building envelope efficiency upgrades, and (in mild winter climates) heat pump retrofits, this addressable market can be raised to at least ~60% of homes in all ten regions.

Figure 3. Percentage of homes for which a PVESS with 30 kWh storage (or less) could provide backup power over a 3-day interruption. The bars show the percentage of the roughly 1,000 modeled homes in each location for which a PVESS with 30 kWh of storage could provide backup power, based on the same interruption conditions and critical load definitions as in Figure 1. The measure bundles are added sequentially and are cumulative; thus the bars for the heat pump measure also include the set-point adjustment and efficiency upgrades.

The full report provides a wealth of additional detail and results, including the full distribution in battery sizing across the building stock in each location, as well as sensitivity cases for alternate interruption conditions (e.g., more or less extreme weather, and interruption durations ranging from 1-7 days) and alternate critical load definitions (including a more limited critical load case and whole-home backup).

We thank the U.S. Department of Energy Solar Energy Technologies Office for their support of this work, as well as members of the external technical advisory group who provided invaluable guidance and feedback on this analysis.