That Xantrex looks like a decent unit.
Here's how I design our power systems:
Repeater maximum current draw + Battery recharge current + "N+1" + reasonable overhead for future needs.
Ideally you'd have enough rectified AC power available to run your repeater just off AC. That way you are not constantly dipping into your batteries.
--and--
You need to have enough additional current capacity to recharge your batteries after an extended power outage. Often battery manufacturers will provide a spec for "capacity / 20 hours". This is the amount of current you need to recharge your battery plant from a complete discharge to fully charged in 20 hours. The time to full recharge can be adjusted based off your needs.
--and--
Reasonable accommodations for future growth. Often this can be covered in the excess capacity of the system. Sometimes you'd want to add additional capacity to reduce recharge time on the batteries, cover future equipment or additional site tenants.
--and--
"N+1". All my systems have this N + 1 built in. That way if a rectifier module fails, you can absorb that failure into the system and fix it later. Makes those late night failures something you can put off until the morning.
One thing I've had to start doing at all our sites is to add additional capacity to run an inverter. As more and more radio system are either IP enabled or use IP as a management interface, you need to consider providing power to a router/network switch on site. Most networking equipment won't handle 12 volts DC. Most of the higher end stuff will handle 24 to 48 volts DC. In many of our 12 volt sites, we just install a rack mount pure sine wave 12vdc - 120vac inverter to run the network gear.
I've used these at some of our smaller sites:
12V, 24V or 48V DC | 11-40 amps | Built-In Battery Back-Up | Integrated Power Systems
Just remove the internal battery and hook up your own to the rear power pole connection.
If your site is big enough:
DC Power Systems | 12V, 24V or 48V DC | 150 watts - 14 kilowatts
--or--
12V, 24V, and 48V DC | Rackmount DC Power Systems | Fully Engineered and Integrated by Newmar
If you do go with one of these larger systems, make sure you hook up the remote alarms or get an Ethernet interface. Being able to remotely monitor/control these systems is really handy.