HP ProCurve 2520 Planning And Implementation Manual page 139

To achieve the 35 ports at 15.4 watts the PoE devices must be divided up and
connected to two different modules. Remember, as soon as a module is
installed into the switch, 22 watts is reserved for its use.
In order to use those watts, devices must be connected to that module or PoE
power must be disabled to all ports on that module.
If PoE power is disabled to all ports on a module the 22 watts that was reserved
for that module is returned to the pool of available watts and can be used by
another module's ports.
In this example the load must be balanced or split between two or three
modules in order to effectively use all 546 watts. The number of devices and
wattage must be split between the modules. This would also help limit the
effects of a single module failure. If one module fails, only the devices on that
module would lose power.
In this example, there are three modules in the chassis and therefore 22 watts
is reserved for each module. In order to use the 22 watts, PDs must be
connected to each module. Or all ports on one module could have the PoE
power disabled.
There is no redundant power with this configuration. If a power supply fails,
the remaining power supply can keep the switch running, but cannot supply
all the PoE power needed by the modules.
Source of Power Watts Available # of Ports Powered and Average watts/
Two Internal 546 (without
Power Supplies redundancy)
(J8712A)
Planning and Implementation for the 5400zl/8200zl Switches
Port
35 @ average 15.4 W each
72 @ average 7.5 W each
136 @ average 4.0 W each
Configuration Examples
Redundant # of Ports Powered and
Average watts/Port
None
10-19