Duct Heater Replacement

I need to replace duct heater 50kW as part of a refrigeration DX unit model Keeprite (7.5HP Compressor) to heat a space area (5100 ft2) in a commercial building, air flow is 4900cfm by a seven plinth electric heaters 3kW (total 21kW) controlled by SCR 50KW rated SCR. I need to calculate the energy savings. Any body can provide a link to a web site where I can find the calculations

Thanks