Solar Battery Buying Guide: Everything You Need to Know
If you've got solar panels already, adding batteries can increase your savings and independence. Here's what you need to know.
Solar panels are a great way to capture free, clean energy from the sun for yourself or to feed it into the grid for more savings on your utility bill, but it's attaching a battery bank that gives you real energy independence.
"Something that people don't consider is that if they're on natural gas and the power goes out in the winter they think their heater is going to work, but most have an electric fan, so if the power goes out you still need something," said Mike Murphy, owner of Utah-based PrepSOS, which sells solar batteries, generators and other emergency preparedness equipment.
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Batteries are the key component that ensures your home can always stay lit and warm and that vital pieces of electronics such as medical equipment continue functioning. Depending where you live, they might be able to save you some extra money by returning energy to the grid at times when it's more valuable.
However, all of these benefits come at a cost, and wading through specifications to find the right match for your needs can be a bewildering task. Here's a concise primer on what you need to know before you go shopping for independence in a box.
How solar batteries work
The simplest way to think of batteries is to imagine that the electricity in your house flows through wires in much the same way as water through plumbing. Batteries fill the role of a storage tank, making electricity readily available when it's needed, just as indoor pressure tanks and water heaters do with water. When batteries are tapped for energy, their reserve of stored electricity is depleted, but in a properly wired system they can be automatically replenished by "catching" any excess electricity flowing through the system from sources like solar panels or the grid.
Depending on your goals for installing batteries, your system might look a bit different.
Connect just to solar panels: Batteries connected only to solar panels will fill when the sun shines and discharge when you use electricity and the sun is down or behind clouds. It's one option if you are off-grid and away from electrical utilities.
Connect to solar panels and to the grid: If you have a solar inverter that can temporarily disconnect you from the grid, you have what's known as a hybrid solar system. In such a system, you can charge your battery with your solar panels or the grid and use the energy stored there in your home or send it back to the grid and save some money via rate arbitrage (if you have time of use rates). A hybrid system can also keep your house powered during a power outage.
Connect just to the grid: While we wouldn't call them solar batteries, you can install batteries without solar panels at all. They would charge from the grid and would be useful for back up power or for enrolling in a virtual power plant.
Pros and cons of solar batteries
The pros and cons of buying a battery largely boil down to savings (and backup power) versus cost.
The extra solar electricity you store in your solar batteries can be used in place of electricity you'd normally have to buy from your utility or sold back to the grid when it's most valuable. This can save you some money and relieve some pressure on the grid when there's peak demand. (Whether this is a viable money-saving option for you depends on your utility's net metering rules.)
Some companies are starting to allow people to enroll their batteries in virtual power plants, a fleet of batteries, smart thermostats and other household appliances that work together to decrease demand on the grid. Where available, virtual power plants might come with additional perks for the battery owner.
In addition, you'll be able to use your battery bank in the event that the grid goes down due to a failure, natural disaster or even a solar flare. A backup generator can also help keep the power on in an emergency (and charge your batteries), but it requires burning fossil fuels, usually either gas or propane.
Batteries do add considerable expense to your home energy system, but federal tax credits and other incentive programs usually can be applied to the cost of storage.
Can reduce energy bills
Savings vary depending on local utility and net metering rules
Getting cheaper, and eligible for tax credits and incentives
Stored power for when the grid goes down
Large battery banks require significant storage space
Storing and releasing energy during peak usage hours can reduce stress on the grid
Not all utilities encourage or reward this practice
Cleaner source of back-up power than a generator
A generator may be more useful in a major emergency
Latest battery chemistries (like Li-PO4) are exceedingly safe and simple to maintain
Some batteries pose health and fire risks and require management and maintenance
Increased energy independence
Different types of solar batteries
There are several kinds of batteries used in battery backup systems, including lithium-ion and lead-acid batteries. Here's a quick overview.
There are multiple lithium chemistries on the market, including nickel-manganese-cobalt, lithium polymer and lithium iron phosphate. The latest lithium technology comes with less danger of fire than older headlines might lead you to believe. They're capable of a deeper discharge than lead acid batteries (you can use up to 90% of a charge per cycle without inflicting much damage) and are much easier to maintain with a longer lifespan. They're also significantly more expensive and sensitive to temperature. Increasingly, though, they are becoming standard in residential solar applications.
The basics of this technology are essentially unchanged for over a century. They remain inexpensive and widely available. For solar systems, it's popular to use somewhat more expensive sealed batteries that require less maintenance and eliminate the risk of dealing with a potential acid spill and hydrogen off-gassing. For a while, sealed lead acid seemed to be the future of solar batteries.
However, all lead acid batteries require more careful monitoring of charge levels compared with lithium-ion and can't compete in terms of efficiency, energy and lifespan but are a good and plentiful budget alternative.
Flow batteries (or redox flow batteries) are less common in home systems since they're mainly designed for commercial use. However, the technology appears promising, and it could become more widely used in residential battery backup systems in the near future.
Nickel-cadmium batteries have a high energy density with double the energy of a lead-acid battery. Nickel-cadmium batteries are very durable, expensive and work well in extreme temperatures making them a good choice for large-scale commercial and industrial projects. Cadmium is toxic, however, and generally not appropriate for residential use.
Buying a backup battery system
In general, a solar battery bank can cost between $10,000 to $25,000 for 10 to 25 kilowatt hours of power. (The US Department of Energy says solar batteries can cost anywhere from $12,000 to $22,000.)
That said, it ultimately comes down to your budget and energy needs. A small off-grid solar system with enough battery capacity for the basics (no air conditioning or electric heaters allowed) using a pair of high-capacity flooded lead acid batteries can be had for $500 total. Upgrading to lithium ion cost $1,300 for a system with comparable capacity.
Adding batteries is a significant expense for any system, but the good news is that nearly a third will come back to you in the form of the 30% federal renewable energy tax credit. Other incentives may also be available from state and local governments, utilities, and even credit unions.
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It's best to use a certified solar installer or electrician to install your solar batteries and connect them to your solar array, your home, the grid and an EV charger, if you desire. You should expect to spend a few thousand dollars for the labor involved, and there may be additional components like inverters, charge controllers and EV charge stations that can also add hundreds or thousands of dollars in expense.
If you have confidence in your DIY skills and experience with electricity, it is possible to install batteries yourself. Just be sure to check with local regulations, building codes and equipment warranties.
If you go the DIY route, make sure all batteries are the same age (ideally new). Don't mix new batteries with older ones to help make sure they take a charge evenly.
Maintaining a battery backup system
For all batteries, follow any manufacturers' instructions regarding monitoring the depth of discharge. Generally for lead acid batteries, this means trying to keep them over half charged as much as possible. Many lithium ion batteries can safely be taken down to only 10% charge. Flooded lead acid batteries also need to be topped off with distilled water a few times a year.
Lifespan and warranties
A few years after installation, you may notice that your battery backup system doesn't hold a charge as well as it used to. That's because, like other types of batteries, battery backups lose storage capacity over time.
To account for this, battery backups include a warranty that expresses how efficient the battery should be by the end of the warranty period. Many of the top solar batteries offer 10 years and 70%, meaning that by the end of the 10-year warranty, the battery should still operate at 70% of its original performance. Lead acid battery warranties typically last for two to five years.
Inverters and batteries
Inverters play an important role in how the battery stores and converts solar energy. While solar panels generate electricity in direct current, the electric grid and homes generally use alternating current. An inverter can convert AC to DC or vice versa, and most solar batteries include an inverter to store the energy in DC form, as well as an inverter to convert it back into AC to be used in the grid or the home.
Because inversion of current isn't perfectly efficient, battery producers are always experimenting with how to invert less often and increase the efficiency of the battery. As a result, some batteries will not have inverters for both input and output included in the system. Talk to your solar installer about the battery system you're considering so that you can make sure you have all the external inverters you need.
Beware cheap inverters, which are everywhere. To power modern household appliances, a pure sine wave inverter is essential, otherwise you may end up frying some of the more delicate electronic circuits in your home.
Why do I need a battery with solar panels?
Solar power is available for only part of the day. Adding a solar power storage battery system ensures you always have power when the sun isn't out or during a power outage.
Can I install my own solar battery?
A professional solar electricity system installer can help you select a battery that works well with your goals, whether you want to be entirely off-grid, have an emergency store for outages or minimize your costs from the electric company during peak hours.
DIY battery installation instructions abound online, but beware. Working with electrical equipment is dangerous and doing so without the proper training may also void your warranties and put you out of compliance with codes and other regulations.
Do solar batteries qualify for the federal tax credit?
How many solar batteries do I need to power my house?
It depends on how you intend to use them. Likely, fewer batteries are required if you simply hope to maximize net metering savings. To keep your home powered during an extended blackout, you will need to calculate your total power needs
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Eric MackContributing Editor
Eric Mack has been a CNET contributor since 2011. Eric and his family live 100% energy and water independent on his off-grid compound in the New Mexico desert. Eric uses his passion for writing about energy, renewables, science and climate to bring educational content to life on topics around the solar panel and deregulated energy industries. Eric helps consumers by demystifying solar, battery, renewable energy, energy choice concepts, and also reviews solar installers. Previously, Eric covered space, science, climate change and all things futuristic. His encrypted email for tips is email@example.com.
ExpertiseSolar, solar storage, space, science, climate change, deregulated energy, DIY solar panels, DIY off-grid life projects. CNET's "Living off the Grid" series. https://www.cnet.com/feature/home/energy-and-utilities/living-off-the-grid/Credentials
Finalist for the Nesta Tipping Point prize and a degree in broadcast journalism from the University of Missouri-Columbia.
Taylor Freitas is a freelance writer and has contributed to publications including LA Weekly, Safety.com, and Hospitality Technology. She holds a B.A. in Print and Digital Journalism from the University of Southern California.