Battery Bank Design & Sizing

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FLAs can offer large-capacity battery banks at lower upfront cost; the flip side is that regular watering is required. These batteries from HuP Solar-One are available in large amp-hour capacities and can minimize the number of parallel strings.
VRLA batteries require far less maintenance than FLA batteries. Since they do not gas when charging within specifications, they require less venting. They do require a more controlled charging regimen, however. These 2 V absorbed glass mat (AGM) cells from OutBack Power are combined in a single string for 1,716 Ah at 48 V (82.4 kWh).
NiFe batteries are designed to be deeply cycled and can last decades. Due to their substantial gassing, these banks will need adequate venting and regular watering. Single-point watering systems can simplify and speed up the watering process.
LFP batteries are a very stable lithium-ion battery option. Like all Li-ion batteries, they can offer long cycle life and are maintenance-free. High energy density combined with deeper DOD means a smaller battery bank footprint and less space required for Li-ion energy storage.
A common approach in off-grid systems is to design for only a few days of autonomy and include a backup generator.
Energy meters take the guesswork out of determining the daily watt-hour consumption of 120 VAC appliances.
The battery bank size for backup, GT systems depends on the critical load profile and the number of outage days anticipated. This AGM backup bank requires no watering and only minimal venting.
Some battery banks are designed to provide energy only for critical loads, such as a chest freezer.
The sonnenBatterie eco series energy-storage systems combine Sony LFP batteries with OutBack Radian inverters in a freestanding enclosure. They are available in capacities ranging from 4 to 16 kWh (in 2 kWh increments).
The sonnenBatterie eco series energy-storage systems combine Sony LFP batteries with OutBack Radian inverters in a freestanding enclosure. They are available in capacities ranging from 4 to 16 kWh (in 2 kWh increments).
sonnenBatterie eco programming options include the ability for users to change their settings from primarily a self-consumption system to a fully backup system, or somewhere in between.
Current transformers (CTs) are placed around a home’s main service conductors to track household energy consumption.

Batteries are used in renewable energy (RE) systems for many reasons. For example, if your home is off-grid and powered by a PV array, you’ll need to store the solar energy in batteries for use at night or during cloudy weather. If your home is on the grid, a battery bank can provide electricity during a utility outage. In areas with utilities unfriendly to net billing, some systems are designed for self-consumption—with batteries to store RE energy as it’s created and all of the RE-generated electricity used on-site.

Which Batteries?

In these situations, you’ll need to figure out which batteries are best for your situation and how many of them you will need. There are many battery types to choose from and battery bank designs depend upon factors such as application, budget, and maintenance preferences. For background information on the common battery types used in RE systems, see “Battery Chemistry“ in HP179. The battery types discussed here are being mass-produced—there is a formalized testing process in place with material safety handling data sheets, etc.—and are currently available for the U.S. residential storage market.

While each battery chemistry could hypothetically be used for any system, certain battery types are more commonly deployed in specific types of systems. And while this may change as prices for different battery chemistries fluctuate, the following is currently how we are seeing batteries being deployed in homes.

Flooded Lead-Acid

Flooded lead-acid (FLA) batteries are most common in off-grid systems. These battery banks tend to be larger than for on-grid. Since there is no electric utility to depend on, they have to support all of the household loads. The lower purchase price of FLA batteries compared to other options is attractive when you need a lot of storage. FLAs require regular watering and maintenance. However, folks living off-grid generally are more self-reliant due to living farther way from in-town services. They are usually very hands-on, often dealing with pumping their own water, maintaining road access, etc. Thus, maintaining a battery bank becomes another part of the overall property upkeep (see “Methods” in this issue for information on maintenance and watering systems).

Valve Regulated Lead-Acid

Valve regulated lead-acid (VRLA, also known as “sealed” lead-acid) batteries are often used for battery backup in grid-tied systems. These systems spend most of their time in “float” service (fully charged), waiting for a utility outage—they are cycled less than in off-grid and self-consumption systems, which cycle batteries daily. Compared to other chemistries, sealed batteries have a low to moderate cycle life. They also have a lower self-discharge rate (how fast they lose energy while sitting unused), which is helpful since they spend so much time in standby mode.

Sealed batteries are more expensive than their flooded counterparts. However, since battery backup systems usually need to provide energy only for the home’s “critical” loads (such as refrigeration, medical equipment, and communications) during an outage, the battery bank can be smaller, which can make up for their higher cost. Since they do not need to be watered, they can make sense for those on-grid folks who may not be willing to perform the upkeep required of FLA batteries. Additionally, some off-grid systems will also employ sealed batteries to avoid the required maintenance of FLAs (and thus reduce the risk of ruining those batteries if they are not watered).

Comments (9)

Greg Smith_0_0's picture

NEC 708 has very specific litany for "critical loads" and most storage or ESS systems that are used in the context of most conversation are actually backed up, protected or essential loads. I know that I am fighting a losing battle with this term, kind of like how people refer to the things on the roof feeding solar inverters as "panels" instead of modules (inverters don't heat water, people), but I had a partner who got into trouble using the term critical loads panel. His customer was in the medical industry and was very disappointed when the left support system did not immediately shift over when the grid went down (during testing, thank goodness).

Maybe I'm just nitpicking...

Michael Welch's picture
Hmm, maybe -- maybe not. Does 708 actually mention the phrase "critical loads?" (I don't have access to the wording from here). I know it is about "critical operations power systems," but should we stop using a term that we standardized on years before the Code just because it is sharing one word of our terminology? Can we easily enough distinguish between the two?

That said, I'm not against changing our terminology if appropriate, but I'm also not (yet) convinced we should. I do like your alternatives, especially "essential loads."

Thanks for bringing it up, Greg
Greg Smith_0_0's picture

Like I said, I might just be nit picking, but there are a lot of places in the Code where the word "critical" refers to life support systems- 517.30 being another one, I guess it isn't overtly stated and even the word "essential" drifts in and out of these articles. However, I was never a fan of, "That's the way we've always done it" as a reason for keeping something around, although if the Code wasn't written by lawyers then perhaps we wouldn't have so much trouble with it!

Justine Sanchez's picture
Good discussion. Greg, you bring up a good point that I will consider in the future when discussing this topic. On the flip side, usually whenever we start the discussion on critical loads we follow with a list of examples, to make sure readers know what we mean here. However, there are definitely other terms we could use that would eliminate any confusion altogether. Thanks for bringing up this issue. Best, Justine Sanchez PV Technical Editor - Home Power Magazine
Marc Fontana's picture

In your "Web Extras" I didn't see a link to a tool for sizing the battery bank for Off-grid systems. Wasn't there a spreadsheet available? Perhaps I found it elsewhere.

Michael Welch's picture
Hi Marc. There is a graphic representation of the loads spreadsheet at the top of the article, in "Inside this Article." And the article's sidebars and graphs are up there too.
Keith Winston's picture

I believe you use "self-consumption" where you meant "self-contained" in several places.

Ian Woofenden's picture
Hi Keith,
"Self consumption" in this context refers to systems that are using their production directly, instead of having or using the option to send surplus energy back to the utility." It's a rather new term, coined in the era of utilities pushing back and making it harder to maximize the usefulness of renewable energy systems.
Regards, Ian Woofenden, Home Power senior editor
Keith Winston's picture

Ah, sorry, I now see that. Interesting point actually. I was skimming too fast.

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