Everyone’s looking for a way to keep the lights on when the power goes out. With increasingly intense weather knocking the power grid offline for days at a time in some regions, traditional fossil-fuel–based backup systems—namely portable or permanent generators—seem increasingly unreliable. That’s why residential solar power combined with battery storage (once an esoteric niche industry) is rapidly becoming a mainstream disaster-preparedness choice, according to more than a dozen installers, manufacturers, and industry experts we interviewed.
For homeowners, multi-kilowatt batteries that charge from rooftop solar panels promise resilience in the event of a natural disaster—a reliable, rechargeable, instantaneous source of electricity to keep important devices and appliances running until the grid comes back online. For utilities, such installations promise a more stable and lower-carbon electrical grid in the near future. Here’s how you can set it up for your home. (Just brace yourself for sticker shock.)
Who should get this
Backup power in an outage is crucial for anyone looking to maintain basic comfort and communication abilities. Scale it up to a larger system, and you can go beyond the basics, backing up more appliances and tools for more time until the grid power returns. These solutions are too customized for us to recommend specific batteries, to suggest how many kilowatt-hours of storage you need to run your home when the grid is down, or to outline how much solar production you need to keep your battery charged. Keep in mind, too, that other variables—including your specific energy needs, budget, and location (just about every state and utility has its own incentive programs, rebates, and tax credits)—all factor into your purchase decisions.
Our aim is to help you think through three things: the questions you need to ask yourself about the whats and whys of installing solar battery backup in your home, the questions you should ask potential installers when you meet with them, and the question of whether a battery-storage system primarily represents an investment in your own home’s resiliency or in the future grid as a whole. “That’s just like the first hour and a half of my conversations: telling people what they need to think about,” said Rebekah Carpenter, founder of Fingerlakes Renewables Solar Energy in upstate New York.
I can see why. I needed to put in hours of research just to wrap my head around all the ins and outs, reviewing installation examples and playing the role of a prospective buyer. And I empathize with any person making this investment. You’ll be facing a raft of major decisions—from your choice of contractor to the design and manufacturers of your system to financing. And all of it will be wrapped in layers of technical jargon. Blake Richetta, CEO of battery maker Sonnen, said one major challenge he faces is simply to translate this information for his customers, or, as he put it, to “make it palatable for regular folks.” There truly is no simple way to address the question of whether, how, and why you should adopt solar battery storage.
Why you should trust us
Before I began this guide, my only experience with solar power was getting zapped by sun-powered cattle fences on a ranch in the high desert. So to give myself a crash course in solar battery storage, I spoke with more than a dozen sources, including the founders or executives of six battery manufacturers; five highly experienced installers, from Massachusetts, New York, Georgia, and Illinois; and the founder of EnergySage, a respected “unbiased solar matchmaker” that offers free and detailed advice to homeowners on all things solar-related. (EnergySage vets installers, who can then pay a fee to be included on the company’s list of approved contractors.) In an effort to provide a breadth of views as well as depth of knowledge, I sought out installers in areas of the country not always seen as solar-friendly, as well as those of diverse backgrounds, including one who focuses on providing solar power to impoverished rural communities. Late in the process, just for fun, I joined a call between an installer and my brother and sister-in-law (prospective solar and battery buyers in Texas), to hear what kinds of questions a pro asked them (and vice versa) about planning a new installation.
What does solar with battery backup mean, exactly?
Solar panels with backup battery storage are nothing new: People have been using banks of lead-acid batteries to store solar power for decades. But those systems are bulky, require regular maintenance, rely on toxic and corrosive materials, and often must be housed in a separate, weatherproof structure. Generally, they’re limited to rural, off-grid applications. This guide focuses on so-called grid-tied solar systems, in which solar panels supply power to both yourself and the grid. So we’re talking instead about the modern, compact, high-capacity lithium-ion batteries that first appeared in the 2010s.
For many people, the first such system they heard of was Tesla’s Powerwall, announced in 2015. As of 2022, according to EnergySage founder Vikram Aggarwal, at least 26 companies are offering lithium-ion storage systems in the US, though just seven manufacturers account for almost all installations. From highest to lowest share, those manufacturers are Enphase, Tesla, LG, Panasonic, SunPower, NeoVolta, and Generac. You’re likely to encounter several of these names as you begin your research. But to ensure that you’re giving yourself the widest array of choices, it’s important to speak with multiple contractors, since most of them work with only two or three battery makers. (The differences between the batteries largely come down to chemistry, the type of input power they take, their storage capacity, and their load capacity, as described in the following paragraphs.)
Fundamentally, though, all of the batteries work the same way: They store power from rooftop solar panels as chemical energy during the day, and then they release it as needed (most commonly at night, when the solar panels are idle, as well as during power outages) to keep your home’s appliances and fixtures running. And all batteries charge only via DC (direct current) power, the same sort that solar panels produce.
But beyond that, there are many differences. “Batteries are not made the same,” Aggarwal said. “They have different chemistries. They have different wattages. They have different amperes. And how much amperage can be extracted from a battery at a given time, i.e., how many appliances can I run concurrently? There is no one-size-fits-all.”
The amount of power that a battery can store, measured in kilowatt-hours, will of course be a key factor in your calculations. If your area rarely experiences long blackouts, a smaller and less expensive battery may suit your needs. If your area’s blackouts last a long time, a larger battery may be required. And if you have critical equipment in your home that absolutely cannot be allowed to lose power, your needs may be higher yet. These are all things to think about before you contact potential installers—and those professionals should listen to your needs and ask questions that help you refine your thinking.
You have to consider a few other things, as well.
The first is whether you’ll be installing a new solar system at the same time that you install battery storage, or whether you’ll be retrofitting a battery to an existing system.
If everything will be new, you’ll have the widest range of options in both your choice of battery and your choice of solar panels. The majority of new installations use DC-coupled batteries. That means the DC electricity produced by your panels feeds into your home and directly charges the battery. The current then goes through a device called an inverter, which converts the DC (direct current) electricity to AC (alternating current) electricity—the type of power that homes use. This system offers the most efficient way to charge the batteries. But it involves running high-voltage DC into your home, which requires specialized electrical work. And several of the people I spoke with expressed reservations over the safety of high-voltage DC.
So you can instead opt for what are called AC-coupled batteries, and install a solar array that uses microinverters behind each panel to convert their output into AC on your roof (which means no high-voltage current enters your home). To charge a battery, integrated microinverters in the battery itself then reconvert the electricity to DC, which gets converted back to AC when the battery is sending power to your home. AC-coupled batteries are less efficient than DC-coupled batteries, because with every conversion some electrical energy is lost as heat. Have a frank discussion with your installer about the pros, cons, and relative safety of each approach.
If you already have a solar array and want to install a battery, the big news is simply that you can now do so. “I’ve been doing this for 20-something years, and being able to go in and look at a system and retrofit it is amazing,” said Rebekah Carpenter of Fingerlakes Renewables. “I remember when there was absolutely no option to retrofit a system. You just weren’t going to be able to use solar at all if the grid went down.”
The solution lies in hybrid inverters, which offer two key abilities. First, they take input as either AC or DC, and then they use software to figure out where it’s needed and make any conversions necessary. “It’s an either-or-and,” said Carpenter. “It’s using it to charge batteries [DC], it’s using it for the home or grid [AC], or if it’s got enough power coming in, it’s using it for both at the same time.” She added that what she terms “agnostic” hybrid inverters are of particular value for retrofitting battery systems, since they can work with batteries of several different brands; some battery makers restrict their hybrid inverters to working only with their own batteries. Carpenter mentioned Sunny Island as one maker of agnostic inverters. Sol-Ark is another example.
If you already have a solar array and want to install a battery, the big news is simply that you can now do so.
Second, hybrid inverters can generate what’s called grid signal. Solar arrays need to sense that the grid is online in order to work. If they lose that signal—which means there’s a grid outage—they stop working until the power returns; this means you are without power until that time too. (It’s a matter of safety, explained Sven Amirian of Invaleon: “The utility requires that you don’t feed back energy when there are [people] working on the lines.”) By generating grid signal, hybrid inverters let your existing solar system keep running in an outage, powering your home and charging the battery by day and using the battery to power your home at night.
In addition to storage capacity, measured in kilowatt-hours, batteries have load capacities, measured in kilowatts. The term continuous capacity refers to how much power the battery can send out under normal conditions, and it indicates a limit on how many circuits you can run at once. The term peak capacity refers to how much power the battery can put out for a few seconds when a large appliance, such as an air conditioner, kicks on and creates a sudden, brief need for more juice; such an event requires a robust peak capacity. Consult your contractor to find a battery that will meet your needs.
Lithium-ion battery chemistry is complex, but there are two main types used for solar. The more common ones are NMC, or nickel-magnesium-cobalt, batteries. Less common (and a more recent development) are LFP, or lithium-iron-phosphate, batteries. (The odd initialism comes from an alternative name, lithium ferrophosphate.) NMC batteries are the more power-dense of the two, as they are physically smaller for a given storage capacity. But they are more sensitive to the heat generated during charging and discharging (they have a lower flash point, or ignition temperature, and thus in theory are more susceptible to what’s called thermal runaway fire propagation). They also may have lower lifetime charge-discharge cycles. And the use of cobalt, in particular, is of some concern, since its production has been tied to illegal and exploitative mining practices. LFP batteries, being less energy-dense, need to be somewhat larger for a given capacity, but they are less sensitive to heat generation and may have higher charge-discharge cycles. Ultimately, you’ll wind up with whichever type of battery best fits into the design you settle on with your contractor. As always, however, be proactive and ask questions.
And that brings up a final point: Speak with multiple solar installers before you pick one. “Consumers should always, always comparison shop,” said EnergySage’s Aggarwal. Most installers work with just a few battery and panel manufacturers, which means you won’t get a full picture of what’s possible from any one of them. Keith Marett, president of clean energy services at Generac—a manufacturer of fossil-fuel backup systems that’s rapidly expanding into renewable backup—said that “the big thing for homeowners, really, is figuring out what they want their lifestyle to be during an outage, and building a system to support that.” Adding battery storage is a major investment and, to a big degree, locks you into a particular system, so don’t rush your decision.
What will this cost—and do you really need it?
I live in New York City, where indoor solar battery storage is not allowed because of the fire code, and outdoor battery storage means navigating a Kremlinesque bureaucracy (PDF). (The joke being that almost nobody here has outdoor space to begin with.) Nor could I install a battery even if it were allowed—I live in a co-op apartment, not a freestanding home, so I don’t have my own roof for the solar panels. But even if I could install a battery, researching and writing this guide made me question whether I would. It’s worthwhile to ask yourself some fundamental questions before you pull the trigger.
For starters, installing battery storage is inherently expensive. EnergySage’s data shows that in the last quarter of 2021, the median cost per kilowatt-hour of battery storage was almost $1,300. Of course, that means that half of the batteries on the company’s list cost less than that per kilowatt-hour (and half cost more). But even the lowest-cost battery maker on EnergySage’s list, HomeGrid, charges over $6,000 for a 9.6 kWh system. Batteries from the “big seven” (again, that’s Enphase, Tesla, LG, Panasonic, SunPower, NeoVolta, and Generac) cost from nearly one and a half times as much to over twice as much. “Currently it is for the well-to-do,” said EnergySage’s Aggarwal with a sigh. He added, however, that the cost of battery storage has long been on a downward trend, and he expects the trend to continue.
Do you really need to spend a ton of money to meet your needs in a power outage? There are less-expensive options than high-kilowatt solar storage, including portable gasoline generators, lithium-ion portable power stations, and small solar battery chargers aimed at keeping devices running.
Those portable methods—even the rechargeable ones that are safe to use indoors—aren’t as convenient as plugging things into a wall outlet. Yet there are even ways to get household circuits working in an outage without a traditional rooftop-solar system. Goal Zero, which has had success selling solar generators to campers and RVers, also offers a home integration kit that uses those generators to power houses. In a blackout, you manually disconnect your home from the grid (a physical transfer switch is included in the installation work). You then run your home’s circuits on an external Goal Zero battery and recharge it with Goal Zero’s portable solar panels. In some ways, this Goal Zero kit splits the difference between the fully installed solar-plus-battery system and a more-basic solar battery charger. The use of a manual disconnection switch adds an extra step versus the automatic transfer switches used in grid-tied solar systems. The price? “We start at about $4,000 installed in your home for our 3-kilowatt-hour battery,” said company CEO Bill Harmon.
All of these options have their downsides and limitations. A solar device charger will allow you to keep in touch with loved ones and give you access to news alerts in an emergency, but it won’t keep the fridge running. Fossil fuels can run out, leaving you stranded, and of course a fossil-fuel generator is not environmentally friendly. “But, that being said, if you’re only going to run it twice a year, two or three days a year, maybe you can live with the impact for now,” Aggarwal said. Several battery makers have incorporated the ability to use fossil-fuel generators to charge their batteries in the event of an extended blackout. Sonnen chairman and CEO Blake Richetta said if your goal is maximum resilience after a disaster, “You really should have a gas generator—a backup for the backup.”
In short, it’s worth weighing your expected future hardships in an emergency against the cost of gaining resilience. I spoke with Joe Lipari, vice president for projects at Brooklyn SolarWorks (which, as the name suggests, operates in New York City, where, again, batteries aren’t yet an option), and he mentioned the great Northeast blackout of 2003. It was an unpleasant couple of days before the power came back on. But I’ve lived here for nearly 20 years, and it’s the only time I’ve ever lost power. Purely from an emergency-preparation perspective, I asked Lipari what I should take away from the 2003 outage—that is, was it a crisis to fortify against or a minimal risk to absorb? “People bring that up to us,” he replied. “Paying an extra $20,000 to get a battery storage system? Probably not necessary.”
How long can you run your home on solar battery backup?
We asked a lot of experts how long these systems can last in an outage, generally speaking. The short and conservative answer: less than 24 hours on a single battery. But claims vary so widely that the thorough answer to this question is less conclusive.
In 2020, according to US Energy Information Administration figures, the typical US home consumed 29.3 kilowatt-hours per day. A typical solar backup battery can store somewhere around 10 kilowatt-hours. “I don’t have to tell you that this cannot run your whole house for a day,” said EnergySage’s Aggarwal. Batteries are generally stackable, which means you can string multiple batteries together to increase your storage. But, of course, doing so is not cheap. For many people, stacking is not practical—or even financially possible.
But “how long can I run my home” is really the wrong way to think about solar storage in the context of a blackout. For one thing, you can expect your solar panels to both deliver power to your home and recharge your battery during the day—in sunny weather—thus continuously regenerating your backup power source. That adds a form of resilience that fossil-fuel generators lack, because once their gas or propane runs out, they’re useless until you can get more fuel. And that may be impossible in an emergency.
More to the point, during an outage, how much energy you conserve is at least as important as how much energy you can store. In order to make your battery last as long as possible, you’ll need to cut way back on your usage. Having lived through Hurricane Andrew in Miami, in 1992, I turned the challenges of that experience—no power for days, rotting groceries—into a line of inquiry. I asked all of the installers and battery makers I spoke to the same question: Assuming I want to keep the fridge running (for food safety), keep a couple of devices charged (for communication and information), and keep some lights on (for nighttime safety), how long can I expect a battery to last without recharging?
Keyvan Vasefi, head of product, operations, and manufacturing at Goal Zero, said he and his wife have run multiple tests on their 3 kWh battery, and they typically can go for a day and a half with “fridge running, multiple phone recharges, and master bedroom and bathroom with lighting.” They have also done tests with their solar panels hooked to the battery. Even bearing in mind that Vasefi has an interest in selling this tech, I can say that he does make a compelling case for it: “We try to pretend it’s the end of the world and see what happens, and we can effectively get an indefinite run time” on those limited circuits, he said. “Batteries back to a hundred percent every day at 6:00 p.m. And we feel really good about that.”
A 10 kWh battery can typically run a fridge, some lights, and several device chargers for two to three days, said Sven Amirian, vice president of Invaleon, a Massachusetts-based installer. That timeframe was echoed by Aric Saunders, senior vice president of battery-maker Electriq.
When you get a battery installed, your contractor may ask you to choose a limited “emergency subset” of your home’s circuits, which they’ll then route through a subpanel. During an outage, the battery will feed only these circuits. (As an example, my dad has a propane backup generator at his home in Virginia, and it’s hooked up to one of his three air-conditioning units, the fridge, the kitchen outlets, an on-demand water heater, and some lights. The house doesn’t have TV, laundry, and other conveniences until the grid comes back. But having a partially cooled home and cold drinks has meant the difference between comfort and misery during the frequent summer blackouts.)
You can also manually shut off individual breakers in your panel to limit the battery to feeding only those you consider critical. And all solar storage batteries come with apps that show you which circuits are being used, helping you find and eliminate power draws that you may have overlooked. “In real time, you can change your habits and maybe stretch out an extra day,” said Amirian. Note, though, that customer reviews of the apps are the same kind of mixed bag that we find for every smart-appliance app we test: Some people love them, while others are frustrated by glitchy performance and buggy updates.
Finally, battery makers are beginning to offer smart panels. Through these you can use your app to toggle individual circuits on and off remotely and thus customize which circuits are in use at various times (say, disabling the bedroom lights and outlets during the day and turning them back on at night). And the battery’s software will also take steps to optimize your power usage, closing down circuits that aren’t needed. But Amirian cautioned that installing a smart panel is not simple or cheap. “There’s a lot of customer education that has to happen, the pros and cons, costs and benefits, of ‘I want to be able to control every circuit’ versus ‘That’s going to be $10,000 of electrical work for a two-day blackout.’”
The bottom line is that even with limited solar recharging, you’ll be able to increase the time you can maintain power off-grid—but only if you demand less of your battery. This calculation was neatly described by Jonnell Carol Minefee, co-founder of Solar Tyme USA, a Georgia-based solar installer that focuses on rural, minority, and impoverished communities: “I understand we’re Americans, we love our whatever-whatever, but we have to learn how to exist without all our luxuries some of the time.”
How solar and battery backup could make the biggest impact
Although solar battery storage will keep important appliances and devices running in an outage, the manufacturers and some installers I spoke with all said they consider that to be a useful but secondary function. Primarily, they view such systems as a way for homeowners to limit their utility bills by practicing something called “peak shaving.” At times of peak demand (late afternoon to early evening), when some utilities raise their rates, battery owners switch over to battery power or send power back onto the grid; this earns them rebates or credits from the local utility.
But an even more important use for batteries is on the horizon. Utilities are beginning to upgrade their grid infrastructure to be able to use privately owned batteries as virtual power plants, or VPPs. (A few are already operating, and such systems are expected to become widespread over the next decade.) Right now, there’s so much rooftop solar and so many solar farms that they stress the grid during the middle of the day. All of the power they produce has to go somewhere, so it flows onto the grid, forcing the utilities to power down some of their big fossil-fuel plants, to keep electricity supply and demand in balance. It sounds great—cutting CO2 emissions is kinda the point of solar, right? But that sundown spike in demand arrives right as solar panels stop producing electricity. (The daily cycle of excess midday solar production and evening excess demand produces what’s known as the “duck curve,” a term you may run across in your own research into battery storage.) To meet the surge in demand, utilities are often forced to fire up “peaker plants,” which are less efficient than the main fossil-fuel plants but quicker to get up to speed. The result, on some days, is that the utilities’ CO2 emissions actually exceed what they would have been were there no solar panels at all.
Virtual power plants will help solve this problem. Excess solar power will charge up homeowners’ batteries during the day, and then the utilities will draw on it during the evening spike, instead of firing up the peaker plants. (Battery owners will enter legal agreements with the utilities, granting them the right to do this and likely earning a fee for letting their batteries be used.)
I’ll give Sonnen’s Blake Richetta the final word, since there’s no way I could better convey what a revolution VPPs represent:
“The swarm control of batteries, to respond, to breathe in and out to a grid operator’s dispatch, to provide generation that replaces a peaker plant’s dirty generation, to make the grid run more efficiently, to decongest the grid and create deferrals on the cost of grid infrastructure, to stabilize the grid and to provide, to be totally frank with you, a much cheaper solution to the grid on frequency response and voltage regulation, literally to take solar from being a nuisance to being an asset that adds value, and, to capstone it, even to be able to swarm-charge from the grid, so if there are tons of wind farms in Texas producing gigantic amounts of power at 3 o’clock in the morning, to swarm-charge 50,000 batteries and soak that up—this is what we’re really for. This is the use of the battery.”
This article was edited by Harry Sawyers.
Post time: Jul-07-2022
