Cassini had a radar subsystem

 - To determine whether oceans exist on Titan, and, if so, to determine their distribution.
 - To investigate the geologic features and topography of the solid surface of Titan.

The primary goal of the RADAR was to pierce Titan's thick veil of smog and aerosols to map its surface.

The paper found by PearsonArtPhoto has a list of the instruments:

    Cassini Radar (RADAR)           			   108.4 W
    Radio-Science (RSS)				                82.3
    Imaging Science (Subsystem (ISS))			    59.3
    Composite Infrared Spectrometer (CIRS)		    43.3
    Ion and Neutral Mass Spectrometer (INMS)		26.6
    Visible and Infrared Mapper Spectrometer (VIMS)	24.6
    Magnetospheric Imaging Instrument (MIMI)		23.4
    Cosmic Dust Analyser (CDA)				        19.3
    Cassini Plasma Spectrometer (CAPS)			    19.2
    Radio and Plasma Wave Science (RPWS)		    17.5
    Ultraviolet Imaging Spectrometer (UVIS)			14.6
    Dual Technique Magnetometer (MAG)			    12.4
    Sum						                       450.9 W

There were other systems of the space craft needing power, the transmitter and receiver for communication, the control computer

There were 885 watts (633 watts at end of mission) from the RTGs, but simultaneous operation of all 12 instruments was not possible. So one of the three RTG was needed due to the very long mission time of nearly 20 years.

See https://pds-imaging.jpl.nasa.gov/documentation/RADARUsersGuide2ndEdV2.pdf

https://pds-imaging.jpl.nasa.gov/data/cassini/cassini_orbiter/CORADR_0284/DOCUMENT/BODPSIS.PDF