This PSU calculator can calculate both the wattage and indiviual voltage rail amps(3.3V, 5V, 12V, and 5VSB) required to run your system. Data is based off any accurate manafacturer supplied data (not TDP!) and measured values from the following sources: Digit-Life, LostCircuits, SilentPCReview, StorageReview, TechReport, XbitLabs