Residential solar-plus-storage systems can in some cases meet nearly all of a home’s “critical load” — including heating and cooling — during extended power outages, according to a study from the Lawrence Berkeley National Lab.

But system performance depends on a variety of factors, including the size of the system, where in the U.S. the home is located, and whether the home uses electric-resistance space heating such as baseboard heaters. The study, which was based on models and simulations, described loads from electric resistance heating as “quite large and more difficult to serve.”

The impact of electric resistance heating was one of the surprises to come out of the study, according to Galen Barbose, a research scientist in the Electricity Markets and Policy Department at Lawrence Berkeley National Laboratory and one of the study authors.

“That was far and away the biggest determinant to the results,” Barbose told NetZero Insider.

Berkeley Lab researchers collaborated with scientists from the National Renewable Energy Laboratory on the solar-plus-storage report, which was published last month. Barbose and Berkeley Lab colleague Will Gorman hosted a webinar last week to discuss the study’s findings.

Behind-the-meter solar-plus-storage systems are gaining popularity among residential and commercial building owners, Barbose said during the webinar.

“That trend is being driven by a variety of factors, but certainly one of the major ones has been concerns around grid reliability and resilience and customer interest in using these systems for backup power,” he said.

Yet there has been little research into how well the systems perform as backup power during extended outages, a question the researchers sought to address.

Simulating Outages

The researchers modeled solar and load profiles and then simulated battery storage dispatch during power interruptions. The study looked at outages of a day or longer. These “synthetic” power outages were examined in every county in the U.S. and for every month of the year.

The study primarily analyzed the expected performance of systems where solar provides all of a home’s annual energy consumption, which Barbose said is “pretty typical” for the systems. Systems with 15 kWh or 30 kWh of storage were compared.

The analysis showed the systems could provide enough backup power to meet “limited critical load” in single-family, detached homes. That load includes refrigerators, lighting, well pumps, and power for computers, internet and cell phones.

“Under a limited critical load scenario that excludes heating and cooling, a small [solar and storage system] with just 10 kWh of storage … can fully meet backup needs over a three-day outage in virtually all U.S. counties and any month of the year,” the report stated.

But if the loads are expanded to include heating and cooling, more variation emerges. A system with 15 kWh of storage would meet a projected 85% of critical load including heating and cooling, averaged across all counties and months. A system with 30 kWh of storage would meet 96% of load on average.

With heating and cooling included in load, the backup performance of solar plus storage dips in the winter in the Southeastern U.S. and the Pacific Northwest, regions where electric resistance heating is common, the study found. In the summer, backup performance falls in the Southwest.

In cities such as Chicago and Boston, many homes use gas furnaces for heating, so wintertime heating doesn’t add that much to the electric load, the researchers said. Furnace fans, which often run on electricity, may contribute to the load.

The researchers plan to take a closer look in the future at backup-system performance in homes with electric heat pumps.

Solar Plus Storage Confidence

The overall results may give homeowners more confidence in solar plus storage as a backup power system, especially if they’re primarily interested in maintaining power to a limited load set without heating and cooling, Barbose said in an email after the webinar.

“In cases where customers want to provide backup to heating and cooling loads, the report shows that this may be possible, but requires careful attention to the size of those loads,” Barbose said.

And providing backup for heating and cooling is easier when homes are energy efficient, he said.

Another surprise to come out of the study was that in most cases, the length of the power outage had little impact on how well the solar-plus-storage systems could maintain backup power.

Average load served dropped from 96% on the third day of the outage to 92% on the 10th day, according to the simulations for a 30-kWh storage system that included heating and cooling. That indicates solar energy would largely be able to replenish battery storage that became depleted.

But the longer an outage lasts, the greater the chance of experiencing a cloudy day or increased load, decreasing the percentage of load met, the researchers noted.

In another piece of the study, researchers looked at how well solar plus storage would have fared as a backup system during outages caused by 10 actual weather events.

During a winter storm that hit Oklahoma in October and November 2020, with outages lasting up to 12 days, the study found a median load served of 98% with a 10-kWh battery and 100% with a 30-kWh battery.

The study calculated a median load served of 100% for either a 10-kWh or 30-kWh battery during the October 2019 public safety power shutoff in Northern California. The outage lasted for up to 4.6 days.

But for Hurricane Florence, which caused outages of up to 10 days in North Carolina in 2018, median load served as calculated in the study was 68% with a 10-kWh battery to 76% with a 30-kWh battery.

“Performance can vary considerably over the course of the event,” researchers noted. For example, solar plus storage performance suffered from lack of sunshine during the first days of the outage caused by Hurricane Florence but recovered in later days.