Home battery backup has become an increasingly salient upgrade for homeowners over the years as both grid stability and electricity costs have been called into question. Whether tied to an existing solar generation system or set to charge via the grid during off hours, a home battery backup provides not only an opportunity to circumvent more expensive utility pricing, but also the ability to keep a home running during an outage.
Of course, understanding what makes a good solution can be quite the overwhelming lift for someone new to the category. That’s why we’re offering this beginner’s guide on home battery backup for integrators looking to learn more.
You can liken a battery to that of a hot water heater in the home, just replace the water with electricity. The purpose of the battery is to gradually store energy generated either from the grid, a solar array, or a combination of the two. Then, homeowners can pull from that reserve energy later to either keep things running during an outage or to avoid more costly grid energy during peak periods.
Unlike a hot water heater, however, batteries need to convert the energy from the grid or solar array into a more easily stored form. This is where inverters come into play. They’re an essential component to battery backups, and are most commonly used to convert the battery’s DC power into the AC power that most homes run on.
Put simply, the main appeal of a battery backup is to be a better generator during outages.
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“There’s a general confusion that batteries are basically power conditioners, but batteries are only going to function when grid power is either cut off or unavailable,” says Joe Piccirilli of RoseWater Energy.
“You do have that advantage, though, that when the battery’s DC electricity gets passed through the inverter to become AC that you won’t get the same voltage fluctuations associated with the grid.”
These voltage fluctuations, which are common during outages, can spell bad news for sensitive electronics like LEDs, networking systems, and more advanced sound and home theater systems, and while batteries don’t provide power conditioning out of the box, they do provide cleaner energy baseline.
Beyond that, one of the greatest benefits of a battery backup are the transfer times—or: the time it takes for devices to switch over to backup power. Instead of it taking minutes to transfer over like it does with a generator, most batteries nowadays transfer over in milliseconds. Maybe even less.
More advanced systems can also pull from the more cheaply generated battery energy during times of peak pricing on the grid, netting homeowners a less expensive energy bill.
There are, however, many different types of battery backups with many different features. Keeping things simple to start, we’ll talk about the most important aspect of a battery storage system when it comes down to what it can do: its chemistry.
The two big types of batteries you’ll see the most in backups systems are Lithium and Lead-acid batteries, but there’s a few extras you’ll come across as well.
Lithium is perhaps one of the most widely used battery types today. It is the lightest material that can be used for batteries currently, and as such lends itself well for use in electric vehicles.
Lithium is also able to charge and recharge more often while also being able to discharge to a greater extent. This means lithium batteries can lose more of their charge without it drastically impacting performance over the course of their lifecycle.
Lithium does have its drawbacks, however. Number one, it can’t handle itself as well in hot or cold environments. Number two, it is an incredibly dangerous material to have overheat, as lithium fires burn incredibly hot and have unique properties that make them far more difficult to deal with compared to normal fires. However, in most cases, if treated properly, lithium batteries are generally safe.
In addition to lithium, there are multiple lithium compounds that get used in batteries offering varying levels of performance and safety over straight lithium. Lithium Ferris and Lithium Iron Phosphate are two such combinations that offer a little bit more safety. Lithium Titanate batteries, however, are by far the safest and highest performing of the bunch. Consequently, they are also the most expensive.
In contrast to lithium, lead-acid is much harder to catch fire. It’s also much cheaper, and, if you’re interested in sustainable practices, it’s way easier to recycle lead-acid batteries compared to lithium and its counterparts. This means that, in general, you’re going to be able to get more storage capacity out of a lead-acid battery per dollar spent over lithium.
The tradeoff is that lead-acid batteries are far heavier than lithium. Additionally, lead-acid batteries do not respond well to frequent use. Likewise, it can’t be depleted as deeply as a lithium battery, meaning that there is a bit of a difference between its stated capacity and actual usable capacity.
Lithium’s advantage as a battery is its portability and rechargeability. As Piccirilli states, if you’re planning on building a solar system that wants to snap on and off on demand to pull from the battery as needed during peak hours, lithium is the best material to use there.
If rapid on-off charging and usage isn’t a concern, but being able to weather long-term power outages is, however, lead-acid batteries may offer a better value proposition. This is also aided by the fact that because lead-acid batteries are safer, there are less restrictions on where they can be placed in the home.
Because of the danger posed by lithium battery fires, a lot of areas in the U.S. limit where lithium batteries can be placed. Lead-acid batteries do not have these restrictions and therefore offer a more flexible setup when installing.
Baseline, you should know how much energy a battery can store in Kilowatt hours (kWh). This will ultimately determine how long a battery can supply a home with power during an outage, provided you know how much power the home is using on average.
The performance of the inverter is also a crucial factor in determining battery performance. Because the inverter is what converts the battery’s DC power into usable AC power, the rate at which it can convert that power (measured in kilowatts (kW)) is the ultimate limiting factor in how much of a home’s power the battery can supply at once.
Transfer times give insight into how quickly the battery will be able to start providing power once the home has been cut off from the grid. As an example, a typical standby uninterruptible power supply (UPS) will have an average transfer time of eight milliseconds.
A battery’s depth of discharge (DoD) is the last piece in understanding the full functional capacity of a battery. What DoD means is how much energy a battery can lose before it starts to shorten the lifespan and impact future performance of the battery. If a 10 KWh battery has a DoD of 80%, then that means the battery should never dip below 2 kWh of stored capacity remaining and that the battery has a usable capacity of 8 kWh.
Speaking from his own experience installing these types of systems in homes, Orrin Charm of Charm & Imagination states that having a capacity of 26 kWh is good when dealing with the average-sized home. For larger homes, however, the likes of which integrators might be dealing with, a capacity of 52 kWh or above may be ideal. Supplementing this assessment, the U.S. Energy Information Administration states that in 2022, the average U.S. residential customer used roughly 899 kWh of electricity per month. Dividing by 30 days, that gives a rough estimate of an average home consuming just shy of 30 kWh of energy each day.
While it’s not necessary to have a solar array set-up to charge your battery backup, Charm has several reasons as to why you would want to pair a battery backup with solar generation over relying solely on the grid or a generator.
When talking about generators, these options are generally less cost-effective in the long-run and can still result in that gap in service if a battery needs to be recharged in the middle of an outage.
Additionally, Charm notes the scarcity of fuel that often comes with a loss of power.
“When you think of events that usually cause an outage, like say a hurricane or a blizzard, those events also usually make it a lot harder for people to get their hands on fuel if they’re relying on using a gas or oil generator for power.”
And if you’re solely relying on grid power to charge the battery, once the grid goes out, whatever power has been stored in the battery up to that point is the only power you’re getting until the grid comes back online.
Unfortunately, there’s no easy answer to how much a battery costs. Factoring in battery chemistry, installation and additional features, battery costs can vary significantly from manufacturer to manufacturer.
Using some rough examples, a 13 kWh capacity Powerwall 2 (which uses a lithium battery and provides intelligent load balancing) costs around $15,000 with installation costs included. A 30 kWh lead-acid battery that provides nothing more than power storage, meanwhile, might run around $12,000 with installation costs. The U.S. Department of Energy, meanwhile, sets the average cost at anywhere between $12,000 to $22,000.
At the end of the day, the cost of a battery is going to depend heavily on the desired functions of said battery.
Most lithium batteries come with a 10-year warranty on them, which, in Charm’s experience, is a fairly accurate estimate for how long these batteries last. Usually at the end of those 10 years, provided that the batteries have been treated properly, they’ll be operating at roughly 70% of their original performance.
In speaking with Piccirilli, RoseWater, which uses lead-acid batteries in its solution, generally offers a seven-year life expectancy on all its products. An added perk to RoseWater products, however, is that they’ll send out alerts when the charge dips below 60%, making it a lot easier for homeowners to avoid overdrawing their battery.
Ultimately, the lifespan of the battery will only hold true if homeowners stick to the listed DoD.
Up until this point, we’ve spoken mainly on the base functions a battery provides. When looking at proprietary offerings, however, these systems will sometimes mix and match batteries with varying utilities and functionalities. Some might offer intelligent load balancing while others could provide added functionalities with EV charging.
In speaking with Charm and Piccirilli both experts offered their own opinions of addons that can either be viewed as necessary, or incredibly salient, as part of a battery backup system.
For Charm, an energy monitoring system like one provided by Savant or Emporia, provide extra insight into how much power is being used throughout the home. This makes it easier to understand the capacity requirements of a battery as well as the flow rate required to adequately supply core systems in the home.
Piccirilli meanwhile, highlights load shedding as a valuable function of any battery backup system.
“If an inverter has a 5 kW flow rate, and the house has a 15 kW draw, you need to find a way to manage the energy loads, otherwise breakers are going to get tripped, and then the house will be without power regardless.”
Within the professional channel, integrators need look no further than RoseWater Energy and Savant. Both manufacturers offer uniquely different products when it comes to the scope and functionality of each, so they are both worth checking out in terms of understanding what a specific type of system can provide in the home.
Outside the custom channel there’s also Tesla with its Powerwall as well as Franklin Power who serves as direct competition to Tesla in terms of the functionality and performance of its battery offerings.
Both Generac and Enphase are other names in the category that have longstanding legacies built up behind them. Enphase in particular is a product that gets widely used throughout the channel currently for its dependability on projects.
Then there’s the more unique offerings, like Schneider Electric’s which bundles inverters batteries, smart load systems and even lighting controls into a single offering.
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