The backup battery choice you didn’t know you had: li-ion, or vanadium flow?

Whether it’s to keep the lights on after a natural disaster or just to avoid peak energy rates, more people than ever are adding battery energy storage to their home solar systems. With some 45% of home solar customers saying yes to home batteries last year, the big question isn’t whether or not to get one – it’s what kind to get.

Two options stand out: lithium ion, and vanadium flow. Here’s the information you need to make the right choice.

SKIP THE STORY: get me prices on both types of batteries.

Lithium-ion or “li-ion” batteries are the batteries you’re probably most familiar with. They’re used in most laptops, smartphones, and EVs – and for good reason!

Advertisement – scroll for more content

Over the last few decades, billions of dollars have been poured into li-ion R&D, and the resulting batteries are energy dense, almost entirely recyclable, and readily available, with literally millions of li-ion batteries in the market today.

That sort of market ubiquity and like-for-like competition among vendors and manufacturers has driven per kWh costs down continually. And those lower costs? They’re driving adoption in big solar markets like California, Texas, and Hawaii, where between 68 and 100% of residential solar quotes have home backup batteries “attached,” and Tesla – thanks to the slick new Powerwall 3 and its integrated solar inverter – is the most quoted brand.

Most quoted home solar batteries

As good as they are, li-ion batteries aren’t perfect. Li-ion batteries degrade over time, losing capacity with use and needing to be replaced after 10-15 years. And, while the risk of a lithium battery fire is increasingly and exceedingly low, it’s also very real – leading to intense fires that are difficult to put out with conventional fire-fighting methods.

Vanadium flow batteries address both of those shortcomings, offering 20-30 years of usable service life without degradation and with little (or, depending on who you believe, zero) chance of the sort of “thermal runaway” that leads to li-ion battery fires.

Flow battery diagram

If you’re not familiar with flow batteries, the Wikipedia page is a great first learning step (I hope – I’m still wrapping my brain around them). Once there, you’ll find that a flow battery works kind of like a fuel cell – charged ions pass through the membrane, causing electrons to flow through an external circuit, generating usable electricity.

The energy capacity (kWh) of a flow battery is determined by the volume of the electrolyte fluids in the tanks (bigger = more), while the power output (kW) depends on the size of the electrochemical cell stacks.

That is, of course, to say that a comparable vanadium flow battery will be significantly bigger and more complex than a li-ion solution. That added complexity, combined with the relative rarity of vanadium flow batteries on the market, mean that the same economies of scale and manufacturer/vendor competition haven’t acted to drive costs down in the same way that they have in the li-ion space.

Spec + price comparison

As you can see, there are pros and cons to both setups. The li-ion batteries are proven tech, but the Gen X-ers and Boomers out there will tell you: 10 years is not a long time, and the thought of replacing your batteries (or, more likely, your inverter) every ten years isn’t a pleasant one.

That said, the vanadium flow batteries I could find seem to “top out” at about 5 kW of continuous output – so if your AC draws more than 5 kW continuously (possible in a large Texas or Arizona home or barnodminium), the system won’t be able to power it.

The final conclusion, then, comes down to what you value as an individual. If you want maximum output to keep the lights on and ride out a power interruption like nothing happened with a decade of reliability baked in, a li-ion setup might work. If you’re primarily interested in keeping the food and medicines in your refrigerator fresh, think replacement is a hassle, and the thought of a one-in-a-million battery fire taking out the 90-point Lancia Aurelia B20 GT that lives in your garage keeps you up at night, the “absolutely will not burn” vanadium flow option might be a no-brainer.

And, don’t forget: while you do need a home backup battery to access solar power during an outage, you do not need a home solar system to take advantage of battery storage.

You don’t need a home solar panel system to reap the benefits of home battery backup. But you’ll get the most out of your system when you pair them together — especially if your utility doesn’t pay you much for the excess electricity your solar panels generate and send to the grid. 

ENERGYSAGE

That’s a lot to digest, but I hope you let us know which way you’re leaning – or what alternative battery chemistry you prefer – in the comments. And, if you’re looking for some options to giving Elon more money to turn Grok into a far-right propaganda machine, check out these great Powerwall alternatives.

If you’re considering going solar, it’s always a good idea to get quotes from a few installers. To make sure you find a trusted, reliable solar installer near you that offers competitive pricing, check out EnergySage, a free service that makes it easy for you to go solar. It has hundreds of pre-vetted solar installers competing for your business, ensuring you get high-quality solutions and save 20-30% compared to going it alone. Plus, it’s free to use, and you won’t get sales calls until you select an installer and share your phone number with them. 

Your personalized solar quotes are easy to compare online and you’ll get access to unbiased Energy Advisors to help you every step of the way. Get started here.