This is a single replacement module for the FSP Twins PRO 500W ATX PS2 1+1 redundant power supply (TwinsPro500W). The FSP Twins PRO power supply is a market leader in redundant ATX power supply solutions. Under normal operation, the twin modules share the load evenly divided. In the event of a module failing, however, the remaining module will safely and efficiently power your system until the downed unit can be repaired or replaced. This allows for continuous uninterrupted operation ideal for servers, home systems, and more.