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 keeping them powered for longer.
Battery backup systems 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. (And which brand you wind up getting is often tied to your choice of installer, besides.)
Other variables — including your budget and state and utility-based incentive programs — factor into your purchase decisions. The federal Inflation Reduction Act of also contains incentives that may affect your decision to add battery backup to an existing solar system: Through , you’ll be able to claim 30% of the project cost as a tax credit when you file with the IRS.
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 empathize with anyone considering making this investment. You’ll be facing a raft of major decisions, from your choice of contractor to the design 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 and “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.
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 the modern, compact, high-capacity lithium-ion batteries that first appeared in the s.
For many people, the first such system they heard of was the Tesla Powerwall, announced in . As of , according to EnergySage founder Vikram Aggarwal, at least 26 companies were offering lithium-ion storage systems in the US, though just seven manufacturers accounted for almost all installations, and Enphase was the market leader. Since then, the Powerwall has become dominant, but Enphase, LG, Panasonic, SunPower, NeoVolta, Generac, and others still have a presence.
You’re likely to encounter several of these names as you begin your research.You’ll also run into newcomers to the market, including some names that may be familiar: EcoFlow, Anker, and Jackery. These companies have their roots in “van life,” and are known for making small, portable campsite batteries that recharge from equally portable solar panels. They have now adapted the portable solar battery idea for home use.
Finally, be aware that it’s possible to have battery backup without any solar at all. By installing what’s called a hybrid inverter, you can charge a battery off the grid. This is most commonly done in places with time-of-use electricity rates, where the cost of grid power rises during peak demand periods (like midday in the summer, when air conditioners are in use). By charging a battery during cheaper low-demand periods and using it to run their home during high demand hours, homeowners save money on their electricity bills. But you could also charge a battery off the grid in anticipation of a predictable natural disaster, like a hurricane or blizzard.
To ensure that you’re giving yourself the widest array of choices, it’s important to speak with multiple installers, since most of them work with only one or two batterymakers.)
Fundamentally 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.
Some 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 can involve 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. (Low-voltage DC solar panels are also available — ask potential installers about them.)
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 . That means no high-voltage current enters your home. integrated microinverters in the battery itself then reconvert the electricity to DC for charging, and convert it back to AC when the battery is sending power to your home. The downside is that AC-coupled batteries are slightly 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 is those hybrid inverters 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.
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.
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. (The punchline is that almost nobody here has outdoor space to begin with.) But even if I could install a battery, researching and writing this guide made me question whether I would.
For starters, installing battery storage is inherently expensive. EnergySage’s data shows that in , , the median cost per kilowatt-hour of battery storage was about $1,000. “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. (When we first published this guide in , the average kilowatt hour cost $1,300.)
But 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.
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And the smaller but still substantial whole-home systems from EcoFlow, Anker, Jackery, and others are an interesting middle ground. They pair moderate-capacity, moderately priced portable batteries with a smart transfer switch (and sometimes a smart panel) connected to your home’s circuit box. That lets you use the batteries as backup for important circuits in your home’s wiring (say, the ones for the fridge and a few lights and outlets). Charging the batteries with portable solar panels gives you a low-hassle way of maintaining emergency power in your home, even if the grid stays down for an extended period. And because the systems are modular, you can add more batteries if you decide you need to run additional circuits. The cost is far below that of installing rooftop solar and a high-capacity battery.
We first encountered this sort of system in , when we spoke with Goal Zero. The company had had success selling solar generators to campers and RVers, and had begun to offer home integration kits that used those generators to power houses. It relied on a manual transfer switch to isolate the home from the grid during a blackout, and though Goal Zero has since expanded its battery lineup, it still does not offer a smart transfer switch.Still, we were impressed by the idea then, particularly by the option to stack multiple, relatively inexpensive batteries to customize your storage capacity to your needs. The fact that many other companies have adopted (and adapted) the idea since then is evidence of its utility
All of these options have their downsides and limitations, and for large systems price is a big factor.
It’s important to weigh your expected future hardships in an emergency against the cost of gaining increased resilience. I spoke with Joe Lipari, vice president for projects at Brooklyn SolarWorks, and he mentioned the great Northeast blackout of . 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 outage — was it a crisis to fortify against or a minimal risk to tolerate? “People bring that up to us,” he replied. “Paying an extra $20,000 to get a battery storage system? Probably not necessary.”
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 , according to US Energy Information Administration figures, the typical US home consumed 29.6 kilowatt-hours per day. Even the largest single solar battery we were able to find in , a Generac PWRCell, has a capacity of 21.6 kWh. “I don’t have to tell you that this cannot run your whole house for a day,” said EnergySage’s Aggarwal. 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 , 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 had run multiple tests on their 3 kWh battery, and they typically were able to go for a day and a half with “fridge running, multiple recharges, and master bedroom and bathroom with lighting.” They also did tests with their solar panels hooked to the battery. Even bearing in mind that Vasefi has an interest in selling this tech, 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. Via an app, they let you 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).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 reality 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.”
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 and Jen Gushue.
Lead-acid batteries are the industry standard for electrical energy storage. The cost-effectiveness of lead-acid batteries is its most prominent advantage. As it is less expensive than to pay for a power mains grid extension, they are often used in rural and remote areas. Lead-acid batteries have a long-term, consistent output because they are deep-cycle batteries. There are two types of these batteries: sealed and flooded. When installing an off-grid solar system, lead-acid batteries are the first option as they are reliable and easy to repair. They are still used in emergency power backup systems across the US.
The latest lithium technology comes with a reduced risk of fire. Although they are costly and temperature-sensitive, they are still the most popular backup battery in the market. Li-Ion batteries need little to no maintenance and their energy density is higher. This indicates that if we compare lithium-ion vs lead acid batteries of the same physical size, Li-Ion has more storage capacity. Also, they have deeper depths of discharge and longer lifespans due to their extended life cycles. At an 80% depth of discharge, the lithium-ion battery may provide 4,000–6,000 cycles and still last up to 15 years. The automobile sector is now using lithium-ion batteries. As electric vehicle makers compete for this battery, demand is at an all-time high.
Since flow batteries (also known as redox flow batteries) are primarily made for commercial applications, they are less often used in home systems. They work best when they endure several cycles of charging and discharging each day.
Compared to lead-acid batteries, nickel-cadmium batteries have twice the energy density. Because of its high cost, high durability, and ability to withstand high temperatures, nickel-cadmium batteries are a desirable option for use in business and industry. Cadmium is usually not suitable for use in homes because it is poisonous.
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