In the early spring of 1997, a 900-kVA, utility-tied photovoltaic power station was installed at the US Army Yuma Proving Ground (YPG), in the southwest corner of Arizona. The system will be used to offset peak demand and serve as an emergency power system for YPG's water treatment plant. The power station includes 450-kWp of Siemens M-55 modules, 5600-kWh of C&D motive power batteries, and a 900-kVA power processing and control system from Trace Technologies. Enhanced by the battery load leveling system, the power station has the capacity to reliably provide from 450- up to 825-kVA to YPG's utility grid during the summer peak demand season. The YPG system has three basic operating modes: (1) daytime utility-tied, (2) night-time utility-tied, and (3) stand-alone. The amount of power delivered to the grid is governed by either available power from the PV array or by a power level defined by the user, whichever is greater.