Behind-the-meter solar-plus-energy storage systems (PVESS) can generally provide at least minimum levels of backup power during power interruptions, according to a new report by Lawrence Berkeley National Laboratory (LBNL).
The report, Evaluating the Capabilities of Behind-the-Meter Solar-plus-Storage for Providing Backup Power during Long-Duration Power Interruptions, found that backup performance of PVESS can vary depending on a variety of circumstances.
The best performance observed in the report, which included both simulations and historical analysis of how PVESS would have performed during a sample of actual historical events, was for residential buildings. If heating and cooling loads are excluded from those residences, a small PVESS with 10 kilowatt hours (kWh) of storage, the lower end of sizes currently in the market, can fully meet basic backup power needs over a three-day outage in virtually all U.S. counties and in any month of the year, the report found.
If critical loads include heating and cooling, a 10-kWh PVESS would meet 86 percent of critical load on average across all counties and months, while a 30-kWh PVESS, the upper end of sizes currently in the market, would meet 96 percent of critical load.
The report’s authors noted, however, that the results showed considerable performance within individual regions, based on variations in building stock. Performance declines for higher-usage homes but, more significantly, performance is affected by heating technology, building infiltration or “leakiness,” air-conditioner efficiency, and temperature set-points.
The single biggest impediment to backup performance is the presence of electric space heating, which is currently mostly electric resistance, and is most prevalent in the Southeast and the Pacific Northwest, the report said.
The authors also noted that backup performance for homes with electric heat or high cooling loads is quite sensitive to weather variability. For example, in counties with high penetration of electric heat, between 53 percent and 96 percent of critical load is served during winter months, depending on which specific day the outage begins. A similar but less dramatic was observed for homes with high cooling loads, the authors added.
In terms of duration, the report found that backup performance is fairly insensitive to outages lasting longer than one day. In general, backup performance declines as outage duration increases, though the effect is relatively modest, given the ability of solar panels to recharge batteries each day, the authors said.
For a PVESS with 30-kWh of storage and critical loads that include heating and cooling, backup performance drops from a population-weighted average of 100 percent of critical load served for a one-day outage to 92 percent for a 10-day outage, the report found.
In seven of the 10 historical outage events analyzed, the majority of homes would have been able to maintain critical loads with heating and cooling, using a PVESS with 30 kWh of storage, the report said. However, the authors noted that there was considerable variability among the five hurricane events analyzed, which was driven by differences in solar insolation levels.
The lowest performing event was Hurricane Florence, where almost no solar generation occurred over the first three days of the roughly eight-day outage due to cloud cover. For the two winter storms analyzed, all critical load was served in the median case, but a sizeable fraction of customers—those with electric heating—saw much lower performance, the report said.
The major constraint to backup performance for commercial buildings were roof area constraints on solar system sizing, the report said. “Providing full-building backup for a multi-day outage would require significantly larger systems than what is typically observed in the market today, for systems installed primarily for other purposes,” the authors said.
LBNL said the report is the first in a series it plans to do in collaboration with the National Renewable Energy Laboratory on the use of PVESS for backup power. The report’s authors plan to host a webinar summarizing key findings of the new report on Oct. 6.