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If I isolate the fan from a TurboFan jet engine (specifically GE90) and power it electrically with a motor,
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.
1) Based on flow rate and nozzle exit velocity you can calculate the minimal power for acceleration of the air. This assumes 100% efficiency of the fan and nozzle. Actual power will be more because they are not 100% efficient.
2) Estimate the power output of the low pressure turbine, which roughly equals to the power input to the fan. This could be done by connecting the LP turbine to a dyno and operate the assemble as a turbo shaft engine, or roughly estimate it by fuel consumption rate, fuel energy density, and thermal efficiency.
3) Model the takeoff process as a constant-power process, calculate the accumulated kinetic energy of the airplane and divide it by time and number of engines, which is a even more ideal case than 1). This ignores the most inefficiencies and should give the lowest number.
Remember, power isn't a concept tied to electricity. So the power to drive the fan has noting to do whether a electric motor is driving it or the LP turbine is driving it. The numbers are the same (if you live in a region where mechanical power and electric power are measured in the same units).
Edit:
Thinking about it again, I think this question got the logic wrong.
Before the design work of a turbo fan engine, one of the most basic spec is how much power do you want to drive it with (other questions include bypass ratio and thrust).
This is not something you estimate after you have an actual engine. It's something you have to know before the engine is designed (and simulated when it's designed, and tested after it's designed).
The quickest way to know the power required to drive GE90's fan is to ask GE engineers.
