How we decide the Voltage of Transmission Line? Explain in detail the role of Distance, Line Voltage, Line Load etc.?

The empirical formula for selection of transmission voltage relating transmission distance, transmitted power is given by-

V =5.5√ ((L/1.6) + (P×/Cosø × N ×))

Where,

V = Transmission line voltage in KV

L = Length of line in Kms.

P = Power to be transmitted 

N = No. of circuits.

So by knowing transmitted power, transmission distance, power factor and no. of circuits transmission valtage can be obtained.

I agree to masruddin and he gave sample formula to understand

Here what is the unit of power W,KW MW or GW???