How to measure voltage in a PS/2 keyboard cable?
I don't really care about the battery though, what I am interested in is what is the minimum watts needed to use the keyboard. What other way there is to decrease power consuption?


First thing you need to think about that traditional keyboards were not designed for low-power applications, thus if you think you are critical for power probably you need to use some special input device especially designed for these purposes.

Look here. Power consumption will depend on what keyboard is currently performing. If it is idle there will be small current, if you press 10 buttons on it and have all LEDs turned on, it will be another current and thus another, bigger power consumption.

In my opinion, the only way to properly decrease power consumption is to turn keyboard device off when it is not expected to be used. When operator or application will need to use it, you should refer to maximal device's current per its datasheet.

I just took PS/2 keyboard I have, and on its rear side there's label saying that it requires 5 V DC. No information about current though.

There might be information on the web about maximal PS/2 ratings, I found this one which says max current is 275 mA.

Also please keep in mind that you are currently thinking or talking about your particular case with particular keyboard. Imagine you are designer of consumer product, which can be used virtually with any keyboard, thus you can not know which exact current attached device takes. And thus you should refer to minimal and maximal definitions within the standard, planning your device operating cycle accordingly.