The Voltage tells you if the battery is at a useable level, but it does not tell you how much energy is stored in the battery.
Basically, a battery is empty if its voltage is below 1 Volt. Rechargeable nickel batteries should not be discharged below that level, because they can be destroyed that way (not chargeable anymore).
The NXT hardware does not include current sensors, so you never know how much current is flowing.
The Battery pack has nominal 9V, if you use 6 alcaline batteries, or 7.2V if you use rechargeable Nickel batteries. A set of DC converters and regulators provide stabilized 5V and 3.3V for the electronic parts. However, the motors are driven with unregulated power directly from the batteries.
But all that does not tell you how much energy is stored in the batteries. The best way to measure the power consumtion would be, to attach an external stabilized power supply of 9V to the NXT, and put an ampere meter in the line. Then you can measure the power consumption quickly (9V x measured current in Ampere = Power consumtion in Watts). But you have to use an expensive true RMS Ampere meter, because the current is not continously.
A much more useful method to measure the power consumption would be: Start something with fully loaded rechargeable batteries and wait until the NXT reports low battery state. The do the same with some else condition.
You will get a accurate RELATIVE comparision of power consumption under different conditions. This is much more useful than any calculated number.