As the title says, what's the power output of a USB port?
Is it a standard value, or it may change depending on manufacturer/model, and so on?
If that value is not standard, how can one determine it?
As stated in Wikipedia
As power is equal to current times voltage, all you have to do is multiply 5V per the current your device is drawing from the port. This is true, however, for the regular USB standard. as it turns out, there also exists a convention for charging devices.
These kinds of ports allow for currents up to 5 A. However, the USB port is rated to withstand current up to 1.5 A.