According to http://www.bitsavers.org/pdf/dec/pdp11/rt11/v2c_Jan76/RT11v2C_SysRefJan1976.pdf RT-11 V2C will run in 8KW.
How usable that is, it doesn't say!
Warm feelings for this version of RT-11, as this was the first DEC operating system I used in my working life.
The documentation and...