How a home battery works
A home battery acts like a barrier between your home and the power grid. It plugs right into your current electrical system to optimize daily energy use and to provide backup power during a grid failure. It does this by taking in power from the grid or your solar panels and then strategically deploying it on your house.
Home batteries have been around for decades. But like the cell phone before the Smart phone, home batteries used to be bulky, expensive, inconvenient, and limited in functionality. At Swell, we specialize in the new generation of Smart, lithium-ion home batteries.
Smart home batteries use the same advanced lithium ion battery cells that power electric cars like the Tesla. We work with the best battery technology companies in the world, providing compact, automated, Cloud-connected home battery systems that require no maintenance and last over 10 years.
Did you know.
Solar panels are required by law to shut off during a power outage. If they didn’t, they would pose an electrocution hazard to utility workers trying to fix the problem. A home battery makes it so your solar panels can continue running in an outage, providing continuous power to your home and your family.
The Grid: The grid is the largest machine on Earth. Just three grids comprise the entire US power system, interconnecting thousands of power plants and millions of miles of transmission and distribution lines that deliver power to end users, like you.
Main Distribution Panel: Power from the grid enters your home through a single set of wires. The main distribution panel splits power from the grid and sends it along circuits to the rooms and appliances in your house. After installing a home battery, only certain larger loads remain connected directly to the main panel. Anything connected to the main panel (which includes everything in a home without a battery), is susceptible to a power outage.
Home Battery System: A home battery system acts as a buffer between the grid and your home. It charges from solar panels or the grid. In an outage, the battery instantaneously disconnects from the main panel and the rest of the electrical grid, creating a self-sustaining, personal grid that includes solar panels and the important things in the home. When the sun goes down, a home battery can power your home with stored solar energy, all night long.
Solar System. Photovoltaic solar panels convert sunlight into electricity. Because solar panels generate direct current (DC) electricity, an inverter is required to transform solar electricity into alternating current (AC), which is the type of electricity used in your home.
Critical Loads Panel: During installation of a home battery, a certified electrician installs a critical loads panel, which has the same function as the main distribution panel. The home battery powers the critical loads panel, which sends electricity to all of the important rooms and appliances in your home.
How much power is it packing?
Home batteries vary in size significantly, from 6 kWh up to 20 kWh. As an illustration, without an additional charging, a typical 10 kWh home battery could:
- Charge your iPhone 952 times
- Blend 4,000 margaritas
- Microwave 300 Hot s
- Run 30 loads of laundry (without drying)
- Keep your fridge on for 50 hours
- Brew 30 pots of coffee
- Run a space heater for more than 6.5 hours
- Keep an LCD TV on for 50 hours
- Make 360 pieces of toast in a toaster (or 1,000 in a toaster oven)
The research
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. The federal Inflation Reduction Act of 2022 also contains incentives that may affect your decision to add battery backup to an existing solar system: Through 2032, 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 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.
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.
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 450,300. 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.
The Best Portable Solar Battery Charger
We tested 12 solar phone chargers and found that the BigBlue 28W USB Solar Charger is the best option for USB charging in the great outdoors.
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.
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.’”
What kind of solar batteries does Sunrun use?
Brightbox offers two lithium-ion solar battery storage options: Tesla Powerwall and LG Chem. Compared to lead acid batteries, solar batteries using lithium-ion technology are more efficient and have longer lifespans. This makes them one of the best energy storage options when going solar.
Tesla Powerwall can provide whole home backup while LG Chem backs up to four circuits. There are also differences in the software and app functionality.
Request a free quote to get connected with a Sunrun Solar Advisor and determine which back-up solution will best meet your energy needs.
Are there solar storage incentives available?
One of the best solar storage incentives is the Federal Solar Tax Credit. The Federal Solar Tax Credit will deduct 26% of the cost of your solar energy system from your federal taxes. Starting in 2021, the value of the tax credit will step down to 22%. After 2021, the tax credit for residential solar ends.
Another option is to finance Brightbox with a lease. We’ll pass down the savings to you in the form of a lower and more predictable electric bill.
Net metering, which credits any excess power your solar system generates back to your electric bill, is another great incentive for many people that go solar. However, several states have chosen to adopt Time-of-Use (TOU) rates, which complicate the way your net metering credits are calculated. Brightbox can buffer you from peak TOU rates so you may save even more on your electric bill.
Will my Brightbox battery allow me to go off grid?
Our rechargeable solar battery systems are designed to keep you on-grid during normal service times while still providing reliable backup when the sun isn’t shining or during the event of an outage. This is the most reliable and economical way to use your Brightbox system, and the one most strongly recommended by Sunrun’s service experts.
Just like solar-only systems, the size of your rechargeable solar battery system is determined by your unique energy needs and habits. Factors, such as the amount of electricity you use at home and the devices and appliances you want to back up, will play a key role as you select the right battery storage solution for you.
Smarter, more valuable
Without a battery, a grid-tied home solar array will export any unused power directly to the grid in real-time. With a battery, electricity can be charged and discharged in ways that optimize energy bill savings.
Take demand charges, for instance. Some utilities, including providers in Massachusetts, Arizona, and Illinois, charge an additional fee based on the maximum amount of power required over a single hour, or 15-minute period, in a given month. A battery can be scheduled to discharge power during peak use times so that peak energy demand is smoothed, and demand charges are lower.
Utilities also use a billing design called time-of-use, which, similarly to demand charges, are intended to discourage power use during peak demand periods. For example, a utility may charge a lower rate of
Cost factors
A lithium-ion home battery comes with a sizeable price tag, usually around 10,000. Currently, energy storage that is paired with a residential solar system is eligible for the 26% federal investment tax credit.
Certain states and utilities offer additional incentives for including batteries, and in some cases the value adder can make a dramatic difference. The Smart program in Massachusetts, the Self-Generation Incentive Program (SGIP) of California, and incentive programs in Maryland and Long Island provide up-front cost relief for batteries.
Programs like ConnectedSolutions of New England offer further savings to homeowners who opt-in to a “bring-your-own-battery” program, wherein an incentive is paid to homeowner who agree to allow their batteries to export power to the grid during times of extreme peak demand.
In addition to the ticket-price of the battery, there may be hardware costs associated, as well. Tesla estimates supporting hardware for the battery costs 450,100, and installation will range from 800 to 5000,000. Electrical work like a main panel upgrade may drive costs higher, and some installers will require the customer to cover costs like permit fees or retailer/connection charges, said EnergySage.
.15/kWh during off-peak hours, and then increase the rate to
Install now or retrofit later?
Bloomberg’s New Energy Finance reports lithium-ion battery pack have fallen 87% from 2010 to 2019. With global supply chains ramping up lithium production due to increased demand (also driven by electric vehicle adoption) it is possible that further price reductions may occur. But is it worth waiting for a cheaper battery?
The answer depends, but in the past, the cost of installing a battery after-the-fact, or “retrofitting”, is dependent on the inverters on the array, said EnergySage. Most home solar arrays are installed with AC-coupled inverters, which would then need to be re-converted by an additional inverter back to DC for use by the battery. This means a retrofitted battery would come with an additional cost of another inverter, plus installation costs.
A solar system with a battery included at initial install can include just one DC-coupled inverter, which reduces costs, and converts and stores energy more efficiently than AC-coupled solutions.
It should be taken under consideration that some installers do not perform battery retrofits altogether.
Inverter companies are actively working to make retrofits easier. For example, Enphase recently released a microinverter that the company said can be retrofit to any type of home energy storage battery without the need for recoupling
This content is protected by copyright and may not be reused. If you want to cooperate with us and would like to reuse some of our content, please contact: editors@pv-magazine.com.
Ryan Kennedy
Ryan joined pv magazine in 2021, bringing experience from a top residential solar installer, and a U. S.
.32/kWh.
With a battery, the home is able to store energy during off-peak hours, which typically occur during the day when the solar array is producing the most. The battery can then export power to the home during on-peak hours, which are often in the evening as people return from work and run appliances and the array produces less power. Without a battery, homeowners are subject to whatever rate is being charged if the array is not actively producing.
A third benefit is the ability of the battery to provide value in utilities without net metering. Net metering is the process of exporting energy to the power grid in exchange for credits that can be used to offset utility bill costs.
Some utilities offer a one-to-one value, meaning each kWh of power sent out will cover one kWh of utility bill costs. Others have a decreased value for net metering.
For example, power may be billed by the utility at.16/kWh, but the value of exported energy only offers a credit value of.08/kWh. Other utilities offer no net metering credit whatsoever, like in Hawaii. where battery attachment rates are much higher than the rest of the United States at 80%. Again, in the net metering scenario, batteries are able to store and provide power, limiting exposure to the value loss in poor net metering climates.