This question is a very open-ended one which depends on many factors – load, incoming supply voltage, utility costs, and several others – and so cannot be simply answered. A good place to start customizing the inquiry for your use would be at ABB’s web site; they have tools for rough calculations for energy savings using VFD’s.
Energy Saving Tools
ABB has developed calculation tools for estimating the energy savings that become available when applying electric speed control to certain flow machines. Using PumpSave and FanSave tools, AC drive control in pumps and fans can be compared against traditional flow control methods in terms of energy usage. The tools also provide financial figures for assessing the profitability of purchasing an ABB drive. PumpSave and FanSave run in Microsoft Excel and utilize VBA macros.
FanSave for comparing AC drive control against traditional flow control methods in fans. Calculate the savings you can achieve by replacing outlet damper, inlet vane or pitch control methods with electronic speed control from an ABB drive. FanSave also provides financial and environmental figures concerning the control method retrofit project and recommends a suitable ABB drive type.