The thing I value most about my lithium iron-phosphate bank is that it seems to charge a lot more efficiently. I have a home-base setup that never moves anywhere, so it always gets the exact same amount of sun, and the load I've put on it has remained very nearly constant since 2018. I replaced my 9600 watt-hours of lead acid with 6000 watt-hours of LiFePO4 mounted on a wheeled cart that I can move back and forth into my trailer last December, creating a nearly perfect apples-to-apples comparison. Since that time, I've noticed that I routinely switch over to "float" about an hour to an hour and a half earlier in the day than I used to. (In winter this is later in the day than in summer, of course, but it's consistently earlier.) My understanding is that in both charging and discharging there's a lot less internal wastage with LiFePO4 than with lead acid, and this lines up nicely with what I'm actually seeing. So in essence I'm getting additional useful power without adding more panels or upgrading the rest of the system.
Here are some other "hidden" cost savings...
1) Because LiFePO4's are lighter and you only need half as many I was able to mount my entire system including the controller and inverter on a cart that I can easily roll back and forth between my home base and trailer. Only the (roof-mounted) panels need to be duplicated. So, I only need one controller, one inverter, and one set of batteries for both home and travel use. This saves me a _ton_. While in theory I could do the same with a large lead-acid battery bank and dis-assemble it every time I want to move it, in practice I'm in crappy enough physical shape that this just isn't going to happen.
2) I like to run an air-conditioner off my battery bank, so I need a large one. The lead-acid battery bank I previously used for this in another trailer weighed approximately 540 pounds, not counting (twice as many) cables, acid-catching trays, etc. The new one weighs about 125 pounds not counting the cables (and needs no trays). This not only saves on gas and wear and tear on my tow vehicle, but in essence adds significantly to my useful cargo capacity in terms of both weight and volume. In essence I get a slightly "bigger trailer" and "bigger tow vehicle" for free. And who knows? Not ever having to move those big lead bricks ever again may also save me from a back injury that could cost me not only thousands of dollars but also most of my freedom. I've come close to seriously hurting myself moving them several times, I suspect.
3) How much is time and aggravation worth? While I'm aware that some folks use AGM's that require no maintenance, the really cheap lead acids-- like I used to have and that most people cite in their cost comparisons-- require regular watering and monitoring. Because mine were installed under a bed in my trailer, I had to dismount and move my largest item of furniture every time they needed looking-into, as well as maintain a supply of distilled water. Not anymore! Plus, with unsealed lead acid there are annoying-- sometimes even destructive or toxic-- fumes to deal with as well.
4) Just as most cost-comparisons cite the very cheapest lead-acid batteries, they also tend to compare against Battle-Born LiFePO4's. These are fine, well-made batteries-- perhaps the very best! But... They're far from the cheapest. I bought my five LiFePO4's-- 6000 watt-hours-- directly from Renogy during a combined Black Friday/closeout sale. These are their old-style-- no longer cataloged-- 100 amp-hour cells. They were on sale plus a large percentage off, so I forget the exact price. Shipping and all, however, they were within spitting distance of $575 apiece, little more than half the Battleborn price. So far they work just fine and, while I don't expect the same life out of them that I would from a Battleborn, Renogy is a well-known, respectable company that stands behind their products. Indeed, I'll point out here that to purchase an equivalent lead-acid battery bank -- maintenance free, and 200 amp-hours for every 100 amp-hours of LiFePO4 so that you don't damage the lead-acids by going under 50%--- Well, the cost difference becomes a lot smaller than some would have you believe. It could be made smaller still by assembling your own batteries from prismatic cells, which was my original intention. However, at the price I paid it was worth it to me to have Renogy's warranty standing behind them.
If money was my one and only consideration-- if I had to stretch every dollar to the absolute limit-- I'd be all about lead acid. But given at least a few extra nickels to rub together... I'd wait for Black Friday if I could, and go for LiFePO4 on sale. The advantages are just huge, and the longer I own them the better I realize they are.
