By modelling the typical orbital deviation and the deposition profile of tangentially injected neutral beams, the heating efficiency is derived in heliotron/torsatrons. It is found that the changes of the configuration due to finite beta effects alter the NBI heating efficiency greatly and that the magnetic well or hill condition, which is important for the MHD stabilities, is also an important factor in determining the heating efficiency in a weak magnetic field. By combining the energy confinement scaling law with the heating efficiency, the reachable beta value is evaluated and it is found that there is an optimum value of the magnetic field strength for obtaining a high plasma beta from the point of view of the NBI heating efficiency