Unfortunately, during cloudy days there is not enough current to drive the pump motor.
Present project describes how get pump running also in cloudy days or in shaddow.
Energy from a solar panel (10W peak, 21V) charges a car-audio super-capacitor (1F, 20V), that can drive a pump motor for ca 5 seconds at a time. Circuit is based on a voltage comparator and a logic-level MOS-FET transistor (for example, IRFZ40, BUK555-60A ).
The construction is intended for small greenhouse irrigation. It is self regulating - in sunny days pumps more water to your tomatoes. Could be used for aquaponics.
This is an autonomous circuit and, in principle, could be used to make for example a solar RC boat, grass cutter, or wireless telemetry transmitter.
Please read the attached word file.