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 with the current the device is drawing from the port.
Note there also exists a convention for charging devices. These kinds of ports allow for currents up to 1.5 A (also using 5V). However, the USB port is rated to withstand current up to 5 A--so some manufacturers may go out of spec and offer a higher maximum current.
I used the 'Battery Doctor' free app to determine how much amperage the usb charging port is offering. I use the word offering intentionally, since each device has a maximum amperage amount that it will take in regardless of what is offered.
I found that my 3.0 port on my hp envy laptop, which has a lightning bolt next to it offers 1.5 amps (1500mA), while the 2.0 usb offers only 0.5 amps (500mA).
Although some forums have stated that it is not possible for an app to determine the amount of amps offered to a device, the Battery Doctor app clearly states amps offered accurately and immediately on my ipad (although it may only display up to the maximum allowed by the device - I have not tried this). I have tested the app with a 1.8 amp out wall charger, and a 2.1 amp out power bank, and both are marked as such on the charger. The amperage readings displayed accurately and immediately on the app.