What are your thoughts on Nickel-Iron batteries?

While nickel-iron batteries are the most durable batteries around, they are not particularly suited for home energy storage. This is because while the batteries themselves are extremely tough and can last over a century, the electrolyte inside them is not so long lasting. For modern Chinese made nickel-iron batteries, I think it is optimistic to expect the electrolyte to last for 7-10 years before it needs to be changed under normal use, and replacing the electrolyte is a considerable expense.

They also require regular maintenance as they need to frequently be topped up with distilled water and many people considering home energy storage these days aren’t interested in a battery system they have to maintain and prefer something they can “fire and forget”.

They are slow to charge and discharge, which is fine for some applications, but it hurts the economics of home energy storage, as it can potentially make it difficult to fully charge them during the day, and fully discharge them overnight or during peak times.

And they’re not especially cheap compared to their main competition, which is still lead-acid batteries. Most, if not pretty much all, households are going to find lead-acid batteries overall cheaper to use per kilowatt-hour, which is why, with just a few exceptions, they were what was exclusively used for off-grid living until recently.