Grid-tied inverters are designed to connect to your home to supplement mains (Eskom) power. When there is solar energy to use, the system will deliver it to your property. When there is insufficient energy to deliver, the system will switch back to grid power.
Grid-tied inverters work with mains to provide energy wherever possible. They have the processing intelligence to know when to deliver power and when to not deliver it and will synchronise power delivery with grid power.
An on-grid inverter converts solar power DC which is constantly varying and feed it into the mains power supply. It synchronises its output voltage and frequency to the mains power supply it is connected to. As the power of the solar increases, so does the output but it can do this at leisure. There is no forced power demand or instant requirement, it simply puts power though as and when it can.