What Should I Look for When Buying a Solar Battery?

Trying to decide on the best solar battery for your situation is a bit more difficult than you may have expected.

The main reason for this is because different solar batteries have been developed for different situations.

For example, an off-grid solar array has different battery storage needs than an on-grid one. A large solar array is different from a small one. An existing array is different from one not yet installed. A boat is different from a large house. Besides all that, there are also just different brands and models for different budgets. There are also different types of solar battery chemistry.

So, to help you narrow down your choices for what kind of battery is best for your solar energy system, here are eight core features that will help you understand what you want and need.

1. Solar battery safety

Which solar battery technology is the safest one?

The safest one in existence today is lithium iron phosphate battery technology, or LFP. These are highly stable and have no chance of overheating. They also run very cool which means they can be placed inside your home, if desired, and won’t require extra ventilation or cooling.

The next most popular solar battery chemistry is lithium nickel manganese cobalt oxide, or NMC. This type of battery is also widely used, and poses almost no risk of overheating. However, they do run hotter and require extra ventilation. As such, you generally don’t want these types of batteries in your home.

Older solar batteries used lead instead of lithium, but this technology has mostly fallen out of favor because it is less safe and prone to leakage as the batteries age.

Among lithium ion solar battery technology options, Coastal Solar considers LFP to be the safest choice.

2. Battery capacity – how much energy it can store

It’s a common question — which solar batteries last the longest?

Battery capacity is one of three factors that help answer that question. The others are power and depth of discharge, which we’ll talk about next.

Capacity refers to how much energy you can store in the solar battery. Capacity can be as low as 1kWh and as high as 20kWh. You can also buy modular battery systems where you can link multiple batteries together to get your desired capacity.

For example, sonnen offers 2.5kWh capacity lithium iron phosphate batteries that you can link together to create more capacity, as in 5, 7.5, 10, 12.5, etc.

More capacity means you can store more energy, and that in theory, the batteries will last longer during the night or in a power outage if your system is set up to run in that situation. But that’s not the only factor in how long a battery will last.

3. Battery power – how much energy it can discharge

So, how long will a 10kW battery last?

Capacity stores the energy, but power determines how much of that energy can be used at once.

Think of it this way: Can you drain a swimming pool through a straw? Yes, but it would take a really long time.

A low solar power battery power output works in a similar way. You might have a lot of battery storage capacity, but with a low power rating, you can only use so much of that energy at a time. Cell phones work like this, because people want the batteries to last a long time (capacity), but not much power is required to actually run the device. So cell phone batteries have high capacity but low power.

But powering your home is a very different story.

A large capacity solar battery system in a residential garage in Georgia.

With more power, you can run more appliances and more systems in your home at the same time. With high power combined with high capacity, you can run more appliances for longer. Make sense?

A 10kW battery will last as long as it has remaining capacity, and depending on how much power it’s using per hour. Use more power, and it will run out of capacity sooner.

4. Depth of discharge (DoD) — how much stored energy is usable

This one is easy to overlook after understanding solar battery power and capacity. It’s easy to think that all you have to do is go look for the highest capacity and highest power ratings for the longest lasting solar batteries.

Not quite.

A battery cannot typically use all the energy it has stored. Running the storage all the way down to zero makes the battery work much harder to build its energy storage back up. So, batteries are equipped with a ‘depth of discharge’, which determines how much energy it uses before it is considered ‘empty,’ and must refill its supply.

For example, the Eguana Evolve has a DoD rating of 90%, which is very good. That means, if a battery were to store 10kWh of solar energy, it would be able to actually use 9kWh of that.

This is one reason solar battery prices can vary so much. You might find an ultra-low cost battery that seems to have a high capacity and a high power rating. But if its depth of discharge is only 60% upon installation, you’re never going to use a large portion of that capacity. So it’s not really a good deal. Most of the top solar batteries in 2022 have very high depth of discharge.

Higher depth of discharge means the battery lasts longer each day.

5. Wait — what about cycles? Defining battery longevity another way

Cycles closely relate to depth of discharge. A cycle happens each time a battery uses up its energy and then recharges. For solar batteries, this generally happens each day, because during the night it uses the excess energy stored during the day.

You can think of depth of discharge as how much battery capacity will be used during each cycle.

And you can think of the number of cycles as how many years your battery will perform at an acceptable level. Typical solar batteries are expected to perform for 3,000 to 5,000 cycles.

This again is a price differentiator. One of sonnen’s most expensive solar batteries is priced that way because it’s good for a whopping 10,000 cycles. Assuming one cycle per day, that’s over 27 years. So you pay more up front, but it’s built to last longer than most other batteries. We consider sonnen’s signature model to be one of the best lithium batteries available for solar power.

Back to the question of how long a solar battery lasts, you see now that there are two ways to answer this:

  1. How long a battery lasts each day — depth of discharge, capacity, power
  2. How long a battery lasts over its lifetime — cycles

6. Solar battery warranty

Over time, all batteries lose depth of discharge. So, a typical solar battery warranty lasts ten years. The battery will still work after that time, but the warranty is meant to guarantee a minimum depth of discharge through at least that period of time.

The Powerplus Energy battery, which is ideal for off-grid solar arrays, is warrantied to deliver an 80% depth of discharge after ten years, which is very high after that much time. 60-70% is a more typical number.

7. Round trip efficiency – solar energy that gets lost

Solar batteries require power to operate themselves, in addition to the power they supply to your home. Round trip efficiency refers to how much energy is used to power up the battery and to discharge that energy to power your appliances.

This is another price differentiator. A cheap solar battery might make big promises about capacity and power, but if its round trip efficiency is some pathetic number like 60%, you’d be losing almost half your energy just to make the battery work.

Ideally, you’d like to see an efficiency above 80%.

8. Compatibility with Your Situation

So, that’s enough of the technical stuff for solar batteries. There’s more, but those are the most important considerations.

Beyond those, you need a solar battery that’s compatible with your existing setup. For instance, some batteries cannot be installed with existing solar arrays. For those, you need to install the battery at the same time as the solar panels.

Other batteries are equipped for different sizes of solar arrays. Some are built for off-grid systems. Some are built for outdoor situations like boats and RVs.

A happy customer stands next to a sonnen solar power battery installation in his home near Savannah, Georgia.

This is why you didn’t see much discussion of price on this list of what to look for when buying a solar battery.

Price doesn’t matter if the battery isn’t compatible with your situation or doesn’t suit your needs. Powering a home takes a lot more capacity than powering a street sign.

How to Choose the Best Solar Battery for You

What you need to do is consider what you want out of your solar batteries. Then, figure out which of the above factors will deliver that for you. Then look for batteries that have those features and levels of performance. And then, you’ll have a list of qualified choices and you can start considering price.

For instance, do you want to be able to power your entire home with solar batteries, such as in the case of a power outage?

Not all batteries are built for this purpose. And some say solar battery storage cannot promise to do this. Meaning – if the power goes out, you would need to turn off certain systems, like your hot water heater or air conditioning, and only use your battery storage to power the lights, freezer, and other key systems.

But on the other hand, this too depends on what kind of house you live in. It’s more feasible to fully power a 1000 square foot home than a 4000 square foot home with solar batteries. But still – for how many days? The longer the power outage lasts, the harder this will be to sustain.

That’s why most people buy solar batteries not for power outages, but to simply make it through the night on battery power and maximize their savings.

But all of this is up to you.

What do you want out of your solar batteries? Start there, use this article to decide the type of battery you want, and start shopping.

Coastal Solar offers batteries from sonnen, Eguana,  and other reputable brands to our customers. We believe in their high safety factors since both use LFP technology, and both score very well on the other factors listed in this article.

Keep learning: See our full guide to solar and storage battery systems.

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