The 90VDC was not/ is not an Edison standard.
It was selected as a DESIGN NORM for small DC loads driven by rectified AC current of as low as 110 VAC.
The difference between 110 VAC RMS and 90 VDC was 'chasmic' because the rectification technology then existing was a POWER HOG.
Indeed, MOST DC motors were power gluttons -- for every manner of reasons:
Variable speed -- and resistive controls being at the top of the list.
New solid state technologies have 'rewritten the book' -- so much so that many classic DC motor loads have been entirely switched over to 'smart' AC schemes.
BART subways (San Francisco) has entirely remotored from DC to AC. IIRC, the rails still send DC -- in switched blocks -- they only heat up when the subway approaches -- then smart inverters generate the three synthetic phases at variable speed.
ALL modern diesel electric power systems have also gone over to smart AC.
In EVERY engineering text I've read it is explicitly spelled out that RMS IS the mathematical correction to SINUSOIDAL AC power equivalence to steady state, DC, power flow.
As to economics, the Poco BILLS you at the RMS calculation for Watt-Hours consumed. That's a HINT right there.
