Can you count signal energy equivalent in frequency domain? I read about method of analyzing audio signals where FFT result is bandpass-filtered and the energy of this band is counted. 
Can you count the momentary energy in frequency domain?
Does this mean summing magnitude value from all frequency bins (after filtering)?

(I don't care whether this will yield the value similar to the one counted in time domain but just if this can be of some benefit - reflect the energy present within this band.)
Parseval's theorem applied to the signal itself:

$$\sum_{n=0}^{N-1}|x[n]|^2 = \frac{1}{N}\sum_{k=0}^{N-1}|X[k]|^2$$

So, if you sum the magnitude squared of the frequency coefficients of the DFT, you get the energy of your time-domain vector.

If you want power (i.e. energy per sample), divide both sides by $N$.