With 15 HP electricity power how much of a capacitor bank can be used?
Answer This question makes no sense. Please clarify and check your terminology.
With 15 HP of power it would be worth having a capacitor bank to correct the power factor if:
(i) your load contains a large percentage of appliances with a poor power factor e.g. induction motors
(ii) your power company is loading the charge for your kWh usage because of your poor power factor.
If those two items don't apply to you, it isn't worth it.