Jon,
All S2500 P models have a 580W power supply as you noted. 400W is allocated for PoE function while 180W is allocated to system function. In truth, a 48 port model running all ports simultaneously will only get to around ~120W from the system allocation.
So you can look at this several ways, worst case scenario of all PoE consumed, plus max forwarding gives you 520W which you then divide by 85% which is the worst case power conversion efficiency for the PSU given max load, you are then looking at a total draw of 611.76W from the PSU inlet. Using the "show poe controller" output, you can modify the PoE value above to get something closer to your environment.
Best regards,
Madani