Converting Power Spectral Density to Power
Convert PSD to dBm/Hz  * Hz BW = dBm ( average sub-intervals if not flat)

Or convert dBm @ BW to 100kHz RBW 

e.g. 

20MHz BW @ 10dBm time domain (RMS power meter??) displayed as PSD    @ 100kHz RBW  normalized by = 10 log ( 100K/20M ) = -23dB    

thus OFDM signal should average 10dBm - 23dB = -33 -13dBm over a 20MHz BW