Switching a capacitor bank is not ordinary contactor duty: the inrush can reach many times the rated current and destroys standard contacts. UA capacitor contactors include damping resistors and early-make contacts that limit that inrush, which is why they are specified in power factor correction panels. We also supply GA DC contactors for direct-current switching duty.
Capacitor switching produces very high transient inrush current. A capacitor contactor adds pre-charging resistors on early-make auxiliary contacts to limit that surge. Using a standard contactor typically leads to welded contacts and a failed step within months.
By the capacitor step rating in kVAr at your supply voltage, with allowance for harmonic loading and detuning reactors if fitted.