Finding a motor that is 100% efficient is not possible. This means that 100% of the electricity supplied to the motor is converted into useful mechanical energy. Highly efficient motors are able to operate at 80-90% efficiency, but this is only under nominal conditions. An inefficient motor can even operate at 40% efficiency or less. Efficiency losses in motors can be attributed to heat, friction (bearings at both ends of the shaft are ideal), wind resistance (any motors have integrated cooling fans), coiling of the wires and control method. Some manufacturers are kind enough to include the graphs of efficiency versus voltage and rpm though others prefer not to make that information public.
This is a companion discussion topic for the original entry at https://community.robotshop.com/index.php/blog/show/robot-operating-time-amp-efficiency