Sunlight, not heat causes molecules on the panels to excite, they then generate DC electricity current which flows into an inverter which converts this to pure sinewave AC current which is used by your home.
A portion of the DC current is fed into the battery bank, charging the batteries.
The inverter first seeks out power from the panels, when it sees no current from there, it seeks out battery power. When batteries are exhausted it will then allow Eskom power in.
The System is modular and can be started small and built up over time by adding additional panels and batteries.
It is always a good idea to reduce your load first with a solar geyser, gas cooking and LED lighting before embarking on the photovoltaic system, as your load determines how big the system should be, smaller the load, smaller the PV system you would require, reducing capital layout.