Understanding PoE power needs

I have an old 3com 4500 PWR 50 port switch that I want to replace with an HP. I think an HP 2530 48G PoE (J9772A) should work, since the old switch is only PoE and the new switch does PoE+. But then I see models like the HP 2920 which come in a 370W and 740W model. This starts to seem like speccing a UPS. I had always assumed that if a PoE switch had 48 ports, you could do 48 PoE devices. Do you have to keep the overall power consumption under a certain level, or should a 48-port PoE+ switch handle 48 PoE+ devices?