If I know total power of the signal and spectrum of the signal with amplitudes in dbV, how do I compute bandwidth, which transfers 95% of the power?
You integrate the PSD (power spectral density) of the signal over various widths centered on the center of the channel until you get 95% of the total.
This can be done analytically for some signals, but must be determined empirically for others.
It's an interesting experiment to take the FFT of a signal to get it's PSD, truncate that spectrum to various bandwidths, and then perform the inverse FFT to see how the original waveform gets distorted — and for digital signals, what happens to the eye diagram. It can give you some real insight into what happens with real-world communications channels.
It all depends on the signal. If the signal is flat spectrally but constrained to a certain bandwidth then that's easy. If your signal is more like FSK then there will be a complicated shape to the spectrum and it's harder to compute.
In the simple version (band limited white noise) the power is directly proportional to bandwidth i.e. watts per Hz is constant.