kVAR of PF Correction Capacitor = kW X [(tan(cos-1(OLD PF))) - (tan(cos-1(NEW PF)))]Kilowatts, "kW"
I recommend measuring kW using a power meter. You will have better results choosing a power factor correction capacitor for your motor, especially if it is not fully loaded. But, if you don't have a meter, you can estimate kW with this formula.
kW = (HP X 0.746) / Eff.