I don't know of any native power measurement functionality built into either  the Pi or the Arm chip, but there are a couple of options that will get you close to what you desire.

You could use a inline USB Volt/Amp meter like this [one][1], or a [Kill A Watt][2].

Together with a little [Math][3] these should give you what you need. 


  [1]: https://www.banggood.com/LED-Dual-Input-USB-Charger-Detector-Voltage-Current-Meter-Tester-Mobile-Power-Volt-Amp-p-1091367.html?rmmds=search
  [2]: https://www.amazon.com/P3-P4400-Electricity-Usage-Monitor/dp/B00009MDBU/ref=sr_1_1?ie=UTF8&qid=1496190688&sr=8-1&keywords=kill%20a%20watt
  [3]: http://www.wikihow.com/Calculate-Wattage