Now let's move right.
First question: do you have plenty of money? If, go for LiFePO. Why? You can charge and discharge often, using stated capacity.
If not having money that much, you need to think about lead acid like myself.
They are heavy, but sometimes cheap if used and you can do some kind of recycling by having used ones.
Go for agm type or gel.
Capacity: bigger is better. There are cloudy days and plenty nights.
(General schematics https://mstdn.social/@hamjb/103182013100915068)
What voltage for battery?
If your primary intention is having 230V AC, go for higher voltage. 24, 36, 48V etc. Less less during step up to 230V
Becomes problematic during time. It is cells in series internally, they age, breaks faster.
Secondly, if having old batteries, you can't place in series. To different in general. (Without plenty of additional electronics, with a lot of things possible).
If thinking about used batteries: check carefully that not broken.
It is fine if having half capacity left only, but they should hold it!
How to check:
Let it rest a day.
Voltage should be in range from datasheet.
Discharge, use lamp etc. to cutoff voltage. Count time. Calculate Amphours from it, you know what your lamp has.
Repeat process 2..3 times.
Capacity should be about same each measurement or increase, but not drop.
During research on (used) batteries available your area, you get idea, if plenty of 12V or 24V etc.
This leads to decision, what voltage for solar system itself.
My place it has been 12V.
If 24V at yours, fine.
@hamjb did you build your own battery pack?
used lead acid agm, placed together.
like schematics with C1...C3.
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