Work must be done against the electromagnetic torque in order to generate the voltage (and current) supplied to the electrical load connected to the generator. Therefore energy conversion occurs from mechanical energy to electrical energy. 

The mechanical power input is T.ωThis is converted into electrical power e.I_a (Some may be lost due to friction and windage losses in a practical machine)

The electrical power out = V_out.I_o

V_out is less than the induced emf 'e' due to volt drop across R_a and the brushes