Hello.
I plan to build an off-grid solar power system with about 4kW output (using 20 x 200Wt solar panels).
I happen to have 15 rack-mounted Minuteman battery pack units. Each unit has 6 x 12V8.5Ah batteries. I am totaly naive about the requirements for the batteries that are used in solar systems...

If I do the simple math, for example, 3.5kW solar system calls for 12 x 6V225Ah batteries (total 6V2700Ah) and my capacity is 90 x 12V8.5Ah = 12V765Ah My guess is that UPS batteries have a similar purpose to solar batteries - they are trickle charged all the time and power the load under normal conditions. During the power outage, they provide necessary output for the connected load...

My question is Will I still be able to use these battery packs for solar enerdy storage in day time and to power my house appliances over night?

Thank you.

My guess is that UPS batteries have a similar purpose to solar batteries - they are trickle charged all the time and power the load under normal conditions. During the power outage, they provide necessary output for the connected load...

My guess is that UPS batteries are not designed for a daily charge/discharge cycle. They are designed to stay topped off and for the occasional discharge, but I haven't checked out what Minuteman uses in their particular rackmounts.

I think the question is "how long will they last?" and less "will they work".

Thank you for your feedback.
I wonder, if someone with practical experience or theoretical knowledge can answer "how long will they last" question?

"how long will they last"

It depends on the type of cell, the charge rate and the discharge rate. Backup battery banks for solar systems use deep cycle batteries with capacities in the hundreds of Amp Hours. You probably have sealed lead acid batteries there. They can last as many as 300 cycles or more and as little as 50 cycles depending on the charge and discharge rate. With what you are fixing to do, I'd guess it would be more like the 50 number....or less.

Are these batteries new or have then been in service in another application for a few years already? Have they been maintained properly?

These are new (never been used) battery packs. They retail for several hundred dollars each, but I got them for free. I would like to do something creative and solar power project seemed to be a good application

So, if I can't use this stack of 90 batteries to power the house needs (because of the low capacity), would it be possible to use a couple of 200 Wt solar panels to charge the batteries, if I use them for their intended purpose - to back up the power of my computers/servers? Will I need an inverter to convert the voltage from the solar panels to 110V in order to charge the UPS batteries?

Yes, it is (possible) to use them that way, but why not just use them in a rackmount UPS as intended? Even if you had a solar engineer come in and work up an appropriate system to charge the batteries during the day and discharge them at night to power a single server or server group (so you could advertise "This system powered 100% by the sun!"), your little experiment would be done in a year or less due to the daily cycling on the batteries.

If I had that windfall, I would take them down to the local battery store and see if I could trade them for something more appropriate to my solar aspirations.

There are certain specifications you should use when evaluating your solar battery options, such as how long the solar battery will last or how much power it can provide.