Hi,
The power spectrum will be in units of Vrms^2 which are power units. There is a very usefull VI that you can use to easily transform the units of your spectrum; the VI is called
Spectrum Unit Conversion.vi. You select the type of spectrum that you have and the units that you want it to be converted to.
Here is a good document on spectral measurements in genereal:
frequency spectrumI hope this helps.
Regards,
Juan Carlos
N.I.