The Power Request
Made within the USB CONFIG DESCRIPTORs. You can have more than one with different power requests, but host devices may not support dealing with multiple configurations.
General Power Availability
A device that wants to be able to function on all systems and hubs should request 100mA max.
Multiple Power Configurations
One solution to a high power requirement is to have multiple configurations, each configuration having its own power budget to let the host decide how much power to give. The problem with that is that Windows does not bother with other configurations (may be out of date now with Win7?) and you may need to to writing a host driver for Windows to handle these issues.