Losses in DC generator
The 1250 W no load motor losses are those losses that occur when the machine is turning. The largest of these is 'windage', the viscosity and turbulence of the air surrounding the rotor. There are also bearing friction and core eddy current losses.

These will all be the same when the machine is turning, if the speed and the flux are the same, whether it's running as a motor or generator. The 1250 W loss therefore has to be added to the copper losses.