The stability of a magnetic flux rope is investigated in the presence of an Alfvén wave flux. It is shown that instabilities will arise when the transverse perturbation velocity exceeds the Alfvén velocity for the flux rope. With this restriction the maximum permitted Alfvén energy fluxes have been calculated under a variety of conditions typical of faculae, flares and sunspots. It is shown that the power requirements for faculae are very close to the theoretical limit and that flares may or may not exhibit a flash phase depending on whether the stability criterion is exceeded or not. The Alfvén fluxes which have been suggested for sunspots are, however, generally less than the limit based on this criterion.