How much mAh am I using?

Question

I have a 12V battery with a circuit that leads to a USB port where I charge my phone. I know the USB delivers approx. 5V and 0,4 A (= 2W) when I charge my phone.
The question: If I charge my phone for one hour, you could say I am using 1h * 0,4A = 0,4Ah of the battery load. 

However, since the battery does not deliver the 5V there is probably a transformer in the circuit. For the same power as the usb delivers, the battery would in theory deliver 12V at 0,17A (= 2W). If you would calculate it from that point of view I only used 1h*0,17A = 0,17 Ah of the total battery load.

So how much did I actually use, 0,4Ah or 0,17Ah?

Answer

Its 0.17 Ah, as that is the current leaving the battery terminals.
As you have calculated, the Watts being drawn are the same, so the enegy (as a watt is a joule/sec) deliverd by the battery and at the USB can only be identical.

How much mAh am I using?

Answers (1)