Question USB 2.0 Port Theory Question

[VegasBobby]

The specification for a USB 2 port is 5V at 500mA or 2.5 Watts of power. One of my USB 2 ports on my desktop only puts out 4.75V. To provide 2.5 Watts of power, my port would need to put out 526mA of current. Is Watts or power the actual limiting factor for a USB port or is it separately Volts and Amps. That is, could I have any combination of Volts and Amps and not risk frying anything so long as I didn't exceed 2.5 Watts of power? This makes sense since a Watt is the amount of energy, Joules, that flows through a device per second. If the energy is the same, does it really matter what the individual Volt and Amps are? Maybe I'm missing something. Been a very long time since I studied electricity and magnetism and I really didn't understand it then.

 

[RealBeast]

While the USB standard is 500mA, there are fast charge USB 2.0 ports that can provide a great deal more power. It is safe to use them for lower power devices though as the standard includes a battery charging specification of 1.5A that can safely be used with all standard USB 2 devices, they just won't draw as much current if they require less.

With a port delivering voltage at only 4.75V it is very likely that it just delivers power below specification, not the spec power at higher current.

That port would likely only be a problem in that it will charge devices slower or will not provide enough power for USB port powered devices to run well -- like having issues with portable port powered drives.