Compaq Rack-mountable UPSs provide maximum uptime in the event of a power problem for a variety of rack-based computer systems, providing powerful performance while occupying minimal rack space. The UPS R1500 XR is a high power density solution designed for customers who want to provide power protection in space-constrained rack environments. In addition, the R1500 XR has the capability to be connected to up to two optional Extended Runtime Modules (ERM). Customers can further increase the run-time on their UPS R1500 XR by using this ERM. The Compaq UPS "XR" models denote "eXtended Runtime Capable" UPS models