This PSU Wattage and Rails Calculator can calculate both wattage and individual voltage rail amperage (3.3V, 5V, 12V and 5VSB) required to run a PC system. Data provided by calculation is based on accurate manufacturer supplied information (TDP or otherwise) and measured values from various professional sources, like following:
Lost Circuits, Silent PC Review, Storage Review, Tech Report, Xbit Labs, iXBT Labs.