How Effective Are Rooftop Solar Plus Storage Systems In A Blackout? Berkeley Lab Has The Answer

How timely! As millions are without power in Florida following the passage of Hurricane Ian through the state, Berkeley Lab has released a new report that examines how effective rooftop solar plus storage systems can be at keeping the lights on for a 3-day period in every county in the United States.

Most of the findings you can probably figure out for yourself. If the goal is to have a few lights on so you aren’t stumbling around in the dark and enough power to run digital devices, most systems can do that for 3 days or more with ease. But as more power-hungry devices — refrigerators, air conditioners, and electric baseboard heat — are added to the mix, the ability of those systems to cope is compromised. Here’s a summary of that report:

“The study estimates the performance of behind-the-meter solar PV-plus-energy-storage-systems (PVESS) in providing critical-load or whole-building backup across a wide range of geographies, building types, and power interruption conditions. The study also considers a set of 10 historical long-duration power outage events and evaluates how PVESS could have performed in providing backup power during those specific events.

“The analysis is the first in what will be a series of studies by Berkeley Lab, in collaboration with the National Renewable Energy Laboratory, on the use of PVESS for backup power. This initial study, which relies on simulated end-use level building loads, solar generation, and storage dispatch, is intended to provide a baseline set of performance estimates and to illustrate key performance drivers.”

Before we dig into the details of the report, one thing is abundantly clear. Power outages are becoming more frequent as powerful storms and raging forest fires play havoc with traditional energy grids. A good guess would be that solar plus storage will become more of a necessity and less of a luxury as a warming planet confronts us with new challenges. Whether those storage batteries are stand-alone units installed in our homes or the traction batteries in our electric vehicles is a question that cannot be answered accurately yet, but if you said both, you are a highly prescient person.

Solar Plus Storage Scenarios

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Courtesy of Berkeley Lab

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Image courtesy of Berkeley Lab

The Berkeley Lab report focuses on simulated loads, solar generation, and storage dispatch. Backup performance depends first and foremost on PVESS sizing and the set of critical loads selected for backup, it says. If heating and cooling loads are excluded from backup, a small PVESS with just 10 kWh of storage can fully meet basic backup power needs over a 3-day outage in virtually all US counties and in any month of the year,

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Courtesy of Berkeley Lab

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Image courtesy of Berkeley Lab

If loads include heating and cooling, a small PVESS system would meet 86% of critical load on average across all counties and months, while a larger PVESS with 30 kWh of storage would meet 96% of critical load. Backup coverage of heating and cooling loads varies considerably across regions, depending on climate and building stock characteristics. In particular, performance tends to be lowest in regions where electric heating is common such as the southeast and northwest and also in regions with large cooling loads such as the southwest and parts of the southeast.

These results are based on the existing US building stock where heating consists primarily of electric resistance-based heating rather than heat pumps, as well as on typical battery sizing currently in the market. In particular, the report reveals significant performance differences based on heating technology (electric resistance vs. heat pumps vs. fossil heating), building infiltration rates (the leakiness of the building), air-conditioner efficiency, and temperature set-points.

Backup performance for homes with electric heat or high cooling loads is quite sensitive to weather variability. For example, among counties with high penetration of electric heat, between 53% and 96% of critical load is served during winter months, depending on which specific day the outage begins in each month. A similar but less dramatic trend can be observed for homes with high cooling loads. Even greater variability would occur under more extreme weather conditions than explored in our analysis, the report says. [Note: If you aren’t aware that such extreme weather conditions are becoming more common, you haven’t been paying attention.]

Backup performance is fairly insensitive to outage duration beyond one day. In general, backup performance declines as outage duration increases, though the effect is relatively modest, given the ability of PV to recharge the batteries each day. 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% of critical load served for a 1-day outage to 92% for a 10-day outage.

In 7 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. Considerable variability exists among the five hurricane events driven by differences in solar insolation levels. The lowest performing event was Hurricane Florence, where almost no PV generation occurred over the first three days of the ~8-day outage due to cloud cover. For the two winter storms analyzed, all critical load was served in the median case, but a sizable fraction of customers — those with electric heating — saw much lower performance.

The ability to power commercial buildings varies widely, depending on the building type. Schools and big box retail stores with sufficient roof space for solar relative to building power demand fare much better than multistory, energy intensive buildings like hospitals.

The Takeaway

The subtext to this solar plus storage report from Berkeley Lab is that the traditional method for powering our homes and buildings is often unable to provide the electricity we depend on when natural forces interrupt the distribution system. That suggests non-traditional systems such as microgrids that rely on distributed local generation may become more popular in the coming years. Clearly, the system of poles and wires we are accustomed to is the weak link in today’s distribution scheme. There is little discussion about how to address that weakness, but perhaps that is a conversation that needs to happen.

The big question is, how much will this resiliency cost and who should pay for it? A 30 kWh storage battery is not cheap and neither is a rooftop solar system capable of powering a modern single family home. In case you haven’t noticed, electricians aren’t working for free today either. A rough guess would be that the cost of total energy independence for most homeowners would start at $50,000 and go up from there.

The kicker may be V2H bi-directional charging for electric cars. It’s not here yet, and rewiring your house to allow an electric vehicle to charge via a rooftop solar system and power your home when needed can be expensive. You need a bi-directional charger to start with, and modifications to your home’s electrical panel to make it happen. But over time, as more people acquire electric vehicles, such systems may become quite attractive to homeowners and fleet operators who use electric trucks and have solar panels on their roofs.

Never say never. What seems far out there today could be commonplace tomorrow, particularly if EV owners can earn some extra revenue by participating in V2G programs that help stabilize the grid on an as needed basis.

The Berkeley Lab report concentrates on what is reality today, but it also suggests some pathways forward for the future. Major changes are coming to the utility industry. It’s not hard to imagine a time when people will have much more control over the energy supply that has become such a critical part of daily life.


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