Current setup is four 100ah lead acid deep cycle batteries.  A Renogy 1000w pure sinewave inverter/40amp "on shore" charger.  RV style.  It started out as a UPS for my starlink and security cameras.  
I now have 600w solar to add to it first chance I get.  

I'm already considering adding more batteries.  Four more just like I have.

My dilemma, for about $425 or so, I can add 4 more lead acid batteries.  Giving me 800a or 400 usable. Which should get me through any cloudy days. Maybe even a little extra.  Or, for about $550, I can get a 200ah li-fepo battery.  Which would not really be an upgrade as far as power but...  

Most of the benefits I see from lithium is size and weight.  Which is no benefit to me.  Any other benefits I'm missing?  I'm stuck in analysis paralysis.   It seems like in a few years, lithium would be more cost effective.  But maybe I'm missing something.  

Longevity, energy per size/weight, energy/dollar. Which seems best to you? In boats and the like, the Lithium Iron batteries are a lot lighter, so there's where their cost is justified. in a home installation, deep cycle lead acid can be just fine, as long as you mange the batteries properly. (This goes for any chemistry) I don't see a huge benefit for the new hotness in the home installation at current prices.

Li-ion technologies have a deeper drawdown so you can't just compare 200 ah lead acid to 200 ah li-ion. The lead acid can only be drawn down to 50% SOC before damage, the Li-ion can be drawn down to 10%. A 111 ah li-ion has the same usable capacity as a 200 ah lead acid.

The lifetime or # of discharge cycles for Li-ion is around 10 times that of lead acid.

When you factor 10 times the life and you only have to buy about half as much battery capacity when using Li-ion the cost per usable capacity over the life cycle is cheaper for the Li-ion.

But you have to be in it for the long haul. And li-ion doesn't do well in the cold so if it's a cold location that's a decision factor.

Came to post this.  Cold location is freezing temps, and you can get batteries with some kind of thermal protection.

Almost zero maintenance as well.  I will be the only person maintaining this equipment and the less I have to do the better,  Life is busy as it is.

I was already considering/ comparing 400a lead acid to 200ah lithium.

They will be in the basement which stays between 40 and 55f year round.  

Space and weight aren't factors of concern.  

Cycles, well, any real draw down should be rare.  This isn't a dedicated power supply, but a backup.  So only time it will draw down much is when the power is out and I'm there using it.

All of this, but a few more points. You can get li-ion batteries with built-in heaters that will keep the batteries above freezing, or at least energize to warm the battery before charging. Li-on is maintenance-free vs lead acid.

And not that it matters with your setup, but the 50% vs 90% Depth of Discharge doesn’t tell the whole story. Lithium output voltage remains nearly constant under load until it’s largely depleted. Lead acid voltage will sag under load almost immediately depending on the load, and it only gets worse as it discharges. As that happens, amperage output has to go up which increases the load vs capacity ratio and you get into a negative loop. So if you stick with the 1/20C rates you’ll get the capacity rating with lead acid, but the higher the load the worse the capacity. If you’re running on solar, lithium is also much more efficient in the charge/discharge round trip so your solar will act like a “bigger” array compared to charging lead acid.

Can you elaborate?   What is the 1/20C?

Also, I hadn't really considered the charge rate/capability difference.  Being in WV not TX, getting everything I can out of my panels will be a chore.

Sorry, I stated that incorrectly. I should have typed the 20hr discharge rate for lead acid, which means depleting the battery (to 50%) over 20hrs. Any higher load than that will give you a reduced capacity.

Fractions of C or a full charge capacity is more of a lithium thing and I used it wrong.

Ok, I understood the concept didn't know the numbers.  I should be very safe in that respect.  My current draw averages around 100watt.  If I had reserves, I might add a little more to double that.  Emergency lights and fan for the fireplace.  Anything more than that, I'll start up the generator.  

LiFePo4 chemistry is superior to Liion, much closer to a direct replacement for lead acid in 12, 24 and 48V configurations.

I'm no Expert, but I can imagine extreme circumstances where the batts (of whatever type) will be drawn-down, absent sufficient re-charging.

In that event, how the batts will survive being "drawn-down", perhaps repeatedly, becomes a question.

OH, having all sorts of weather, having "sufficient" charging capacity seems wise to me.  Having sufficient charging capacity even during multiple days (weeks) of heavy cloud cover is probably where I would build towards.  If it snows, will the solar panels allow snow to accumulate and thus reduce their output?  Does OP have a long "snow brush" or some other decent way to rid the solar panels from snow?

Running any but the quietest genny might not be a wise option in some circumstances.  Genny's tend to burn through fuel at an alarming rate when under load.

Again, no Expert, just posting some thoughts.  OP may well be ahead of my comments, for all I know.

In 2012, a derecho came through.  Took out power to our place for about 20 something days straight.  We ran the gas generator for over a week straight.  None of the local stations could pump gas, the credit card systems were down, everything was a mess.  After that, upsized the generator and converted it to propane.  We have a 300gal propane tank.  Living like we had line power, that would run about 30 days.  If everything goes south locally, I'd just go home.  If that's not possible, I would would be conserving early.  

I'm not looking to solve the world's problems.  Mostly just looking to run the internet and security system in case of a long term outage.  Something to run for a few hours when I'm working from home so I don't have to bail on a meeting to go start the generator.  I do however see it as an opportunity to use as backup power for lights or the fireplace fan if the power is out over night rather than having to go out in the storm to start the generator.  I may buy a cheap suitcase  inverter genny to charge the batteries to avoid running them too low.  I like options.  Along those lines, I would like things to stay as expandable as possible.  Maybe add another few panels and batteries.

Given your use case, you might look into lead carbon batteries. I'm not an expert and just going off of what I've heard else where, but they have similar charge/discharge duties to a LiFePO4 battery, along with being able to be discharged further than a lead acid battery. The "downside" is that there isn't any weight savings, but that's mostly irrelevant to your situation.

If you cycle the thing a lot, lithium iwins out easily.  If you're just keeping small stuff running once a year or so, it's a lot harder to make that argument.  You can buy a whole lot of capacity in Costco 6V, 200AH golf cart batteries for a real decent price....

That's more true of thin-plate lead-acids that are meant for things like starting a car, much less true of lead-acids that are made for deep cycling, which are generally made for ~80% discharges.

Well, it's been sunny the last two days since I set up the panels, but so far I'm impressed with the performance.  Makes me think I should more than double the battery bank.

As of now I'm planning to add another four 100ah deep cycle lead acid batteries.   After that, I'll have a better idea what it's really capable of.  I know what it is on paper, and I have my own prejudices.  But so far I'm happy with the setup.  Good news is, I still believe it's pretty scalable.

ETA, pics or it didn't happen right

Attached File

Not really, sure the batteries not made for deep discharge perform worse under load but it’s true of lead acid in general. Below shows the difference between the 5hr and 20hr discharge rate capacities.

A random chart I found of an AGM battery below. With load-induced voltage sag comes higher amperage so you then have to be aware of amperage limits of your supporting hardware.

You have to figure the cost of a cell balancer for your batteries if you have anything lithium.

I would know, my house burned down charging a lithium battery last summer. It was a charger failure, because it got the cell count wrong at start up and charged to a higher cell voltage than was safe.