How to calculate power required to drive an isolated fan of a TurboFan engine?
https://en.wikipedia.org/wiki/General_Electric_GE90:


  The LM9000 is an aeroderivative gas turbine available in two options; the LM9000 without water augmentation outputting 66 MW (89,000 hp) at a 42.4% efficiency before cogeneration, and the LM9000 with water augmentation outputting 75 MW (101,000 hp) at a 42.7% efficiency before cogeneration.


An aeroderivative gas turbine is basically the same turbine with a driveshaft connected to the low pressure stage instead of the fan. So its shaft output power would be roughly equivalent to the power that goes to the fan in the turbofan version.

The 66MW (the aero version does not use water augmentation) figure corresponds to max continuous power or a bit less. The take-off power is even more.