Well the PSU is rated at a steady-state power of 325W with a maximum peak power of 420W.
Now this is a maximum, obviously load is based on how hard you're working the box and you don't mention what country/voltage you're in but if you're using a 120v input then that's 3.5 Amps and if it's a 220-240v input then that's 1.75-1.9 Amps.