In the same way that the adoption of home solar panels over the past 10 years was accelerated by rapidly dropping prices, cheaper lithium batteries have made solar batteries more attractive.
At the moment, fewer than 5% of residential solar systems have battery storage, though it’s a rapidly growing market. Only 0.2% of home solar systems purchased in 2016 included battery storage, but that rose to 4.4% in 2019 - a huge rate of growth in only three years.
If you’re thinking of adding solar panels to your house, you might be wondering if it’s worth including a solar battery too. There are different reasons why you might want the extra expense, and this article will try to cover the topic in detail.
What is a solar battery?
A solar battery is a rechargable battery large enough to at least partially power your household during a blackout. While older and DIY installations might use lead-acid batteries, the newest generation of batteries use lithium, giving them high output, high capacity, and durability.
Technically, home storage batteries don’t need to be charged from solar panels. They can be set up to recharge from the electric grid, and some homeowners do exactly that. However, these batteries are marketed by solar companies, so the majority of installations are paired with solar panels.
Home solar panels don’t work in a blackout
One of the things that surprises some solar homeowners is that your solar panels will stop producing electricity during a blackout, even if the sun is shining.
This is because grid-connected solar homes generate alternating current that must be synchronized to the public electric grid - 60 hertz in the case of North America. If a blackout happens, there’s no power signal to synchronize with, and the inverters shut down automatically. This is also done to protect utility workers who might be working on downed power lines. It could be lethal if solar panels continued to send electricity into power lines that workers expect to be depowered.
When the inverters shut down, solar panels stop working because there’s nowhere to send the electricity - unless you have a battery system.
How do solar batteries work during a power outage?
During a blackout, a home solar system with a battery can be programmed to run as an island, disconnected from the grid. In this mode, your solar panels send electricity into your home to keep the lights on. If there’s extra solar electricity that your home isn’t using, it’s used to recharge the battery.
Technical side note: battery systems can either be AC-coupled, which means they receive AC power from your inverter, or DC-coupled, which means they receive DC power from your solar panels via a charge controller. Most small off-grid battery systems, such as RV and marine applications, are DC-coupled, while modern home solar battery systems are often AC-coupled. While there are technical pros and cons to both approaches, from the point of view of keeping your lights on, there’s really no difference. Both types of systems will keep your home running.
If your solar panels generate more electricity than can be used by either your home or battery, the inverter will cut off power from the solar panels. Your system won’t send electricity into the grid until grid power is restored.
Transfering loads to your solar battery during a blackout
Even if you have solar panels and a battery, your home will still remain dark in a blackout unless you have a third component: a transfer switch.
With a modern home solar storage installation, this transfer switch works automatically, sensing when a power failure occurs and then switching your home over to an off-grid, solar-plus-battery mode until grid power is restored. This switchover normally takes just a few seconds, during which your lights will briefly be dark.
Home battery manufacturers have different names for this component. For example, the Tesla Backup Gateway sits between a utility meter and the Tesla Powerwall battery and automatically handles the switchover.
Here’s a great video from the Youtube channel Fully Charged that describes the workings of the Backup Gateway:
Nerdy side note: the host in that video is Robert Llewellyn, who played Kryten in the British sci-fi comedy Red Dwarf.
Other battery manufacturers will have different systems, so be sure to ask your installer about this important component.
Which appliances do you want to run on battery power in a blackout?
If you don’t have high-draw appliances such as air conditioning or an electric stove, the simplest configuration is to have the battery power your entire house. However, big appliances can rapidly drain even a large battery. For example, a high-end “prosumer” electric oven can draw in the neighborhood of 10,000 watts. Cooking a turkey with one of these would deplete a Powerwall in short order.
One solution is to buy more batteries, but the more economical approach is to connect only the most critical appliances to your battery. To do this, your contractor can install a subpanel called a critical load panel. The electric circuits in your house that you decide are most important to keep powered in a blackout - such as the one that your refrigerator is on - will be moved to that panel.
You don’t need a critical load panel, but most installations will probably need one. Discuss your power requirements and budget with your solar installer to decide if it’s right for you.
In a blackout, your solar battery will send power to the critical load panel. Circuits that are not on that panel will remain dark.
Lithium-ion batteries dominate the home storage market
In the early days of solar electricity when it was more of a hippie-in-a-cabin sort of project, lead-acid was the go-to battery technology - and it still is for small off-grid applications such as recreational vehicles and boats.
Lead-acid batteries are the type used for golf carts, mobility scooters, and for the starter battery in gasoline cars. Lead-acid has the advantage of being a cheap and mature techology, but with significant disadvantages, especially weight and durability.
These days, the dominant technology for large-scale battery storage is lithium-ion. This is the same type of battery found in your laptop, smartphone, and newer electric vehicles such as Telsas.
Li-ion batteries normally cost more upfront than lead-acid, but they are less toxic, have higher energy density (ie. they can store more electricity), are physically more durable, can be deeply depleted without damage, and can withstand a greater number of charge-discharge cycles. In other words, they’re superior in almost every way, which is why they’ve taken over in markets such as electric vehicles and even utility-scale energy storage.
All of the batteries mentioned later in this article are li-ion.
Your battery might not be able to fire up your appliances (continuous versus startup power)
A lot of things in your house, such as LED lights and even televisions, don’t use much electricity. Appliances that will quickly drain even a big solar battery are those with resistance coils for heating (such as space heaters or ovens) or motors: air conditioners, central heating, refrigerators, and clothes washers and dryers.
But there’s an important wrinkle when it comes to electric motors: starting current.
Starting current - also known as inrush current - is a brief surge of electricity needed by an electric motor when it’s switched on. This means that if your central A/C draws 5,000 watts when it’s running steadily, it will briefly need a lot more when starting up. As a rough rule of thumb, the starting current might be double the steady-state current, meaning that your A/C might briefly need 10,000 watts at startup.
If you have a storage battery with a 5,000 watt output rating, it wouldn’t have enough power to start up that air conditioner.
Exactly how much electricity is needed to start a motorized appliance? Look on the label for locked rotor amps (LRA). Here’s an explanation of LRA and a way to calculate it if you don’t have the LRA rating.
What to do about about starting current
If you want to be able to operate a large appliance like a pool pump or central air conditioner but your battery doesn’t have the output capacity to satisfy the starting current, don’t despair.
A device called a soft starter moderates the brief surge of electricity when a motor starts up. By spreading out the electric surge over a longer time, a soft starter can make it possible to operate an appliance that otherwise couldn’t be powered by a battery. Talk to your solar installer or electrician about getting one of these installed.
It’s worth nothing that the Enphase Ensemble battery has a soft starter built into it, eliminating the need to install a separate device.
Price arbitrage: using a battery to avoid time-of-use charges
@DawnTJ90 @FriendsOScience Solar powered cars might be worthwhile except for the fossil fuels for manufacture & lack of component recycling resulting in lots of toxic bits of solar panels in landfill, leaching into groundwater. Solve those & less— CL Moller (@SteamPoweredDM) Jul 16, 2021
One of reasons why you might want to have a storage battery is price arbitrage. This is a fancy way of saying that you charge your battery when electricity is cheap to avoid using grid electricity when its expensive.
Many solar homeowners have a time-of-use (TOU) plan that causes the price of electricity to be low during off-peak hours (such as during the night) and expensive at peak hours. This is especially true in California, where it’s now mandatory for new home solar installations to have a TOU plan.
Peak hours (or even super-peak hours) occur when electricity demand is highest. This is usually the early evening on weekdays, typically from about 4 pm to 9 pm. This also happens to be the time of day when solar power generation starts to drop off, which means that if you have solar panels without a battery, you’ll probably be cooking your dinner at least partly with expensive peak-hour grid electricity.
How does a battery help in this situation? On a clear day, solar power generation is greatest for a few hours on either side of noon. This period usually isn’t peak hours for utilities, and because the average homeowner is also often away at work or school during this time, their solar panels may be generating excess power that is sold into the grid.
If you have a battery, the homeowner can instead save that excess electricity. When early evening comes around and homeowners return home, they start to use electricity - maybe you have a smart thermostat that turns the air conditioning on when you come home. And because the sun is lower in the sky, solar panels are generating less electricity just as peak rates kick in.
Fortunately, if your battery was recharging earlier in the day, it can now discharge that electricity to power your home. If you have enough capacity, the battery could allow you to keep your home powered without using grid electricity until after peak rates end.
If you have a big enough solar and battery system, you could avoid ever using grid electricity at all. But even with a smaller battery with only enough capacity to get you through peak hours, the savings from price arbitrage might make possible for the battery to eventually pay for itself.
I wrote an article all about solar homes and time-of-use rates that goes into this topic in a lot more detail.
Smart home battery storage
With the advent of lithium, battery technology has gotten better, but there’s more than just an improvement in chemistry. The 12 volt battery in your car is just a dumb block of lead and sulphuric acid, but modern lithium home storage batteries are smart. In most cases, you can monitor and control your battery from your smartphone, and program them to charge and discharge according to specific conditions.
Every manufacturer has their own software, but it’s common to have a choice of three automatic modes:Self-consumption. In this mode, the battery will store as much of your excess solar electricity as possible, then use that stored energy at night to avoid using the grid as long as possible - up to the battery’s capacity. With the Tesla Powerwall software, you can even set a reserve level of electricity in case of a blackout. Or, if you don’t care about backup power, you can configure the Powerwall to let your home use all of the battery capacity and have it charge when the sun starts shining again.Time-of-use (ie. arbitrage). If you have a time-of-use plan, this mode will configure the battery to charge and discharge according to utility peak hours, saving you money and also helping to provide the maximum environmental benefit from your solar panels. This is because peak-hour electricity also happens to be the dirtiest. So-called “peaker” power plants are powered by fossil fuels, and are often older equipment that is kept on standby. You can check out the non-profit WattTime to learn more about grid emissions intensity.Backup-only. You would choose this if you have frequent power outages or have critical equipment - such as medical devices - that absolutely must be kept running. In this mode, the battery will only supply electricity to your home during a blackout. This will maximize the running time you get from your battery, but you won’t get the financial or environmental benefits that the other modes provide.
If you’re thinking about getting a home battery, the quality of the software will have a big impact on the product experience. One way to research this is to visit the Google Play or Apple’s App Store and check out their reviews.
Tesla, in particular, is simply better at writing software than the other companies. Their app, which is also the same app Telsa drivers use to manage their vehicles, gets a 4.3/5 on the Play store.
Sonnen’s software, used to manage the Sonnen Eco battery, does much worse: it gets only a 2.9 and also has many one-star reviews.
Are solar batteries recyclable?
Lithium-based solar batteries are very recyclable. This is especially desirable if the battery contains elements such as cobalt or rare earth elements.
Telsa intends to implement recycling at its Nevada Gigafactory, which would close the loop on its product lifecycle. Sonnen also mentions the recyclability of its Eco battery, although the company is less specific about how the process will be implemented.
Are solar batteries safe?
On rare occasions, lithium-based batteries can catch on fire. Because lithium batteries store so much energy, these fires can be extremely energetic and difficult to extinguish.
Sounds scary? If you’re a homeowner, you shouldn’t worry about this. Solar batteries like the Powerwall have intelligent software and hardware that prevents them from being overcharged and overheated. Also, these fires typically occur when the battery is physically damaged, such as being crushed or penetrated. Home solar batteries have hard cases and are mounted in an out-of-the-way location, keeping them nice and safe.
Popular home storage batteries
There are now several batteries on the market, from the relatively well-known Telsa Powerwall to lesser-known products such as the Sonnen Eco. LG Chem RESU
Special note about about the Sunrun Brightbox: it’s a rebranded version of the LG RESU. I’m not sure if the software is the same, but the LG software gets a pretty low rating, so this is something to keep in mind if you’re thinking of getting the Brightbox.
Here’s a quick rundown of the four most popular batteries in the US market:
Enphase EnchargeUsable capacity13.5 kWhOutput power 7 kW peak, 5 kW continuousWarranty10 yearsApp rating4.0 out of 5
LG Chem RESU 7H / 10HUsable capacity6.6 kWh / 9.3 kWhOutput power 5 kW peak, 3.5 kW continuous / 7 kW peak, 5 kW continuousWarranty10 yearsApp rating2.6 out of 5
Sonnen EcoUsable capacity5 to 20 kWh (in 2.5 kWh steps)Output power3 to 8 kWWarranty10 yearsApp rating2.9 out of 5
Telsa PowerwallUsable capacity13.5 kWhOutput power 7 kW peak, 5 kW continuousWarranty10 yearsApp rating4.3 out of 5
Prices aren’t listed because installation costs will vary, so ask your installer for a quote.
Bottom line: Should you get a solar battery?
Despite the steady drop in prices for lithium-ion batteries, the cost still hasn’t quite reached the point where it makes financial sense for many solar homeowners to install a battery. A single Powerwall will cost approximately $7,000 to $8,000 to install, but many homes will need two (or even more).
Meanwhile, you can walk into your local big box home center and buy a natural gas-fired home generator with 20,000 watts of output for around $5,000.
Still, there are several reasons why a solar homeowner might decide to add battery storage in spite of the price premium:
Incentives can push the cost down.
The 26% federal tax credit can be applied to home battery storage, but only if the battery is charged exclusively from your solar panels. There are also state-level incentives in California, Massachusetts, and New York. Read my article on battery rebates to learn more.
You don’t want the inconvenience of a home generator.
While gas generators are cheaper upfront, they require maintenance and are noisy when running. Just like car engines, home generators need oil changes, and periodic spark plug and air filter replacements. Li-ion batteries, on the other hand, are dead silent and don’t require any maintenance at all. They also come with 10 year warranties, while gas generators often come with only a 5 year warranty.
You have a time-of-use plan.
As described in the price arbitrage section above, it is possible for a battery to save you a little money every day by letting you avoid peak utility rates. Will it be enough for you to pay back the investment? There’s no general answer I can give, because it depends on your personal electricity usage patterns. If you tend to use a lot of peak-hour electricity and also generate enough excess solar power during the day, it could be a wise investment. To figure this out, work with your solar installer or break out a spreadsheet to do the calculation.
You really want to avoid fossil fuels.
Helping the environment is a big reason - if not the main reason - that many homeowners choose to go solar. If that’s the case for you, the financial payback might be less of a concern than knowing that a battery can help you reduce your carbon footprint. Even if you don’t have a time-of-use plan with your utility, you can make the assumption that late afternoon and early evening is when power plants are working hardest, and fossil fuels are often called into duty to generate more power. By using a battery to reduce your load on the grid during this time, you can lower your carbon footprint.
You’re really into tech.
Are you a nerd? I’m a nerd. Having a giant battery in your house that you can control from your phone, and being able to tweak the energy flow to and from the battery exactly according to your needs is pretty cool. If you love tech, having a solar + battery system will give you lots of graphs and other nerdy things to obsess over.
Lower cost alternative: portable solar generators
If your main goal is emergency backup power and your power requirements are modest - a single refrigerator and perhaps some electronics - a lower cost option is a portable solar generator.
These are lithium batteries that are small enough to be portable, but come in sizes large enough to keep a refrigerator powered for a day or so. They’re called solar generators because you can plug a solar panel directly into them (such as when camping) but they can also be charged from a wall outlet.
You would keep one of these plugged in at all times, ready to go in the case of a blackout. When the power goes out, you would move the battery next to the appliance you want to power, and plug it in.
While this isn’t automatic and as seamless as a Powerwall or similar storage battery, a portable battery is a lower cost option that might be good enough for you. Read our article on solar generators to learn more.
Rebates and next steps
I wrote a separate article about state and utility rebates for batteries. Be sure to check it out, and then use the link below to be connected with local contractors who can help you determine if a home solar battery is right for you.