Flow capacitor unit is an amperage storing device (connected in series) which starts the pump faster, boosts its water pressure & helps to keep water flow steady in case of momentarily sunlight shortage, or when the sunlight is too weak.
This unit is designed to work with the following pumps & panel combos:

2 Watt Solar Panel With Submersible Pump Combo Kit

Solar Panel 6 Watts With Submersible Pond Pump Combo

Solar Powered Pond 10/20 Watt Water Pump With Panels Combo

Solar Powered Submersible 30/40/50 Watt Water Pump With Panels Combo