What is the power output of a USB port?

As stated in Wikipedia

The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire to power connected USB devices.

A unit load is defined as 100 mA in USB 2.0, and 150 mA in USB 3.0. A device may draw a maximum of 5 unit loads (500 mA) from a port in USB 2.0; 6 (900 mA) in USB 3.0.

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.

There are USB power adapters on the market explicitly stating "10W adapter". As USB is 5V the 10W result in 2A = 2000 mA. The net effect is that devices connected to this adapter charges its battery 4 times faster than with a "normal" 500 mA USB port.

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.