The hard drives are pretty moot when it comes to discussing power requirements, especially if one or more are SSDs that consume a trivial amount of wattage (<1W idle, <5W under load). An optical drive uses maybe 5W idle but never is going to break 20W even in use. A modern system with 4 HDD, 2 Optical Drives, and SLI would have power usage requirements something like this: CPU+MB - ~150W Graphics card 1 - ~150W Graphics card 2 - ~150W 4 HDD - ~50W 2 Optical Drives - ~25W Other (RAM etc.) - ~20W However it's not realistic you'd have everything under max load simultaneously. I'd jump for a 650-750W PSU for something like that. But note it's not the HDD or optical drives or what-not that have much bearing on this. It's really a matter of how many graphics cards you're looking to run. Cut one of the graphics cards out and you'd be fine with a ~500W PSU.