I've used Freq drives, however I'm no expert.
Freq drives have many different applications and advantages.  I'm not sure that reducing the energy bill is one of them.
Controlling start up torque is just one of many issues that they can address.  Motor speed (how quickly) they come up to speed as well as how they (slow down) and stop are the more common applications.
What are these motors doing?  their applications?
Like I said, I doubt that they will cut back on the power bill however its possible that "payback" may come in other forums.
Improved production ect...ect..

ax7..   Mark