Hi,
For a DC supply you don't calculate with RMS at all. Just average.
P = V × I, the above is generally true when V or I is fix: power supply, voltage drop on a diode.
For linear loads (ohmic, or complex) where V and I relate on each other one needs to calculate with RMS.
Example:
To do the power loss calculation on an SCR you need both. Because when they are ON, they cause a constant voltage drop combined with an ohmic voltage drop (the one relates on current the other not).
The total power dissipation is the sum of Pc and Pl where Pc = V_const x I_average and Pl = I_RMS ^2 × R
Klaus