I use a lot of power in my office, so much that the four 1500 VA UPS units I have only last me a few min. I needed something bigger so I went on eBay and found two APC Matrix 5000 UPS units for $450 each including shipping. There was only one downside, there were no batteries and new batteries would have cost me several thousand dollars.
The solution? I picked up 8 marine batteries at the auto parts store and wired them up (yes with fuses) into two 48 volt strings connected in parallel.
Note: If you try this, you want to use Marine or better yet Deep-Cycle batteries rather then car starting batteries. Car batteries are designed to give very hight bursts of current and should only be discharged to about 5%. The very thin plates would destroyed over a few hundred discharges rather then the thousands you would get from deep cycle.
P.S. Yes, I built a cover for it!