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APC recommends calculating the total average power consumed and give yourself around 60% headroom to handle future additions to your network closet. I usually shoot for the small 350-500 watt units for an individual PC and monitor, laser printer, etc. But for servers I don't use any less than 1000-1500 watts because the Xeon cpu's are pretty power hungry - plus you have to plan for storage growth and adding cards or hard drives as necessary as your system grows.