Power factor & Reactive power?

The relationship is determined by the "power factor". This is normally supplied by the generator OEM. It's considering losses of actual power generator and power supplied.
 There must also be kept in mind the grid also has losses so depending on how your power output is measured there may be more correction factors to take into consideration.
 For more in-deep, for example see Wikipedia and search on "power factor. I hope this will help you.
 Reactive power supplied by a generator is dependent on the requirement of the grid to which it is connected. it is absolutely proportional to the terminal voltage of the machine concerned. Otherwise it does not have any relationship with the active power generated of the concerned unit and hence no proportionality can be arrived.
 With the reactive power the generator participates into maintaining grid voltage; With the active power generator participates into maintaining grid frequency. Total generator load is composed from active and reactive power building operational point of the generator which should inside of operational area into generator operational map (diagram). Please look this diagram for you generator. Going out from operational area is alarmed and tripped by generator protection
 The power which can be converted to work is know as active power and the power which can not be converted to work but essentially required for maintaining magnetization of electrical machine is known as reactive power. Hence the total power (MVA) is the combination of active power (MW) & reactive power (MVAR). Power factor (PF ) Indicates how much active power is supplied by the generator power out of total generation(MVA). Suppose power factor is 80% (o.8) it means , out of total generation 80% is only being converted to work and 20% is required for the system for maintaining voltage.
 PF of a machine depending upon the capacity of the excitation system. With lower PF capacity of the excitation system has to be increased accordingly. In India PF of turbo generator is taken as 0.85(Lag).
PF changes according to the grid condition. When grid voltage is high, less reactive power is required and PF may as high as unit or it may go to leading