That UPS looks very much like the one I have at work under my PC desk..
I forget how many batteries it has inside, I changed them once after
I pulled the unit out of the salvage room..
Did you look at these chargers??
MPPT Charge Controls
The 500W are $200 and the 250W units are around $120 on Ebay these days.
They also make an HV (High Voltage) version of these which are good to 100VDC (or so I've heard)..
Cheers,
Rich