I haven't worked through this calculator for OP's setup (he didn't mention motherboard model, I don't think), but honestly I have to believe your 611W figure is way too high.
I'm basing this on my APC UPS status which tells me how much power is being used at the moment, as part of its calculation as to how much time could be supported by the battery should there be an outage.
I have a Supermicro C2SBX board with Intel E6850 3.0Ghz dual-core CPU, 4GB DDR3 memory, two 120mm case fans and large CPU cooler/fan. I have 2x7200rpm SATA drives, 1x10Krpm SATA drive, and 1x10K SCSI drive. Also have Adaptec 39320 PCI-x SCSI adapter, SATA BluRay/CD/DVD burner, HP DAT160 tape drive, ATI TV Wonder 650 PCI tuner card, Ceton 4-tuner PCIe card, USB 3.0 PCIe card, combo floppy disk/SD card reader, and ATI HD4850 PCIe video card. Also have two 24" Eizo LCD monitors. All of this is plugged into the battery-backup sockets of the UPS. Other peripherals (e.g. speakers, scanner, printer) go into the non-battery sockets.
And the UPS claims I'm using 360 watts (on the battery backup sockets), which is far below my Nesteq ECS6001 600W PSU's capability.
OP's only got two hard drives, and pretty much not much else. How could it add up to 611 watts?? I'm honestly curious (and dubious).