How to Wire up Solar Panels

Wiring for solar panels requires thicker cables
Wiring up solar panels and batteries for 12 volt power requires thicker wiring because the currents carried are much larger than the equivalent 240V circuit, in fact electrical cable used to power a 12V device must conduct 20 times the current compared to a similar device that runs on 240V power.

For example a 24 Watt light bulb running on 240 Volt power only has 0.1 amps flowing through the circuit. An equivilent 24 Watt light bulb running on 12 Volt power requires a current of 2 amps.
The obvious reason for needing thicker cable is to prevent overheating the electrical wiring - a serious fire hazard, But wiring also uses up some of the power, This power loss is proportional to the current flow and inversely proportional to the voltage.

The 24 watt /12 volt lighting circuit requires thicker conductors to minimize loss of power due to the cable's electrical resistance known as wiring voltage drop.

Longer cables have more resistance
The electrical resistance of a cable is proportional to length. A 1000 meter length of 6mm cable has an electrical resistance of 4.1 Ohms, if you cut the cable in half then the electrical resistance would halve to 2.05 Ohms.

Thicker cables have less resistance
The electrical resistance of a cable is inversely proportional to thickness (the cross sectional area). Doubling the thickness halves the resistance.
The resistance of a cable with a certain length and thickness never changes but the voltage drop can change depending on the current flowing through the cable, if you double the amps the voltage drop would double.

For example, a 12V battery connected to a 60W /12V lamp requires 5A current flow to operate.

If 6mm twin-core cable 10 meters long with resistance of 0.04 Ohms joins battery to lamp it would lose 5A x 0.04 Ohms = 0.2. volts from battery to lamp and again on the return length from the lamp back to the battery, giving a total cable voltage drop of 0.4 volts.

This reduces the voltage available at the light from 12 volts to (12 - 0.4) 11.6 volts. A 3.3% voltage loss

If we double the amps by using a brighter 120W /12V lamp which uses 10A current to operate then the voltage drop doubles too, 10A x 0.04 = 0.4 volts from battery to lamp and same again from lamp back to battery, a total of 0.8 volts and the voltage available at the light is (12 - 0.8) = 11.2volts. A 6.6% voltage loss.

Increase battery voltage to decrease voltage drop losses
if you double the battery volts to 24V the voltage drop would be much less because the higher voltage requires proportionally less amps to provide the same power, for example a 60W /24V lamp only requires 2.5 amps to operate.

This means the the voltage drop is decreased by half too 2.5 x 0.04 = 0.1 volts from battery to lamp and again from lamp back to battery, total of 0.2 volts and voltage drop is now only 24 - 0.2 = 23.8 volts. less than 1% voltage drop. That will not affect system performance by any noticeable amount.

Use crimp connectors for quick installation
Although the best method to permanently join lengths of cable to components is to solder the unions, the most convenient method of making an electrical connection is with crimp connectors although it takes a bit of practice to do it correctly.