The thickness of the cable is calculated according to the power that the installation will work and it is important to know what voltage the panel is and what power. Requests are reduced if thicker cables are placed or the distance traveled is shorter. Unless you're talking about high voltage, 0.75mm you don't even have to start. In 220VAC electrical installations it is used as a return of the lighting lamps that if they are 100W (unusual in domestic installations) handle less than 1 / 2A