12V voltage drop over long wire
The voltage drop is caused by the current. At 12 V you need a lot of current to transport any amount of power. P = V * I so for 120 Watt at 12 V the current will be 10 A. If the cables have a total resistance of 0.1 ohm (which is quite low for 20m) the cables will drop 10 A * 0.1 ohm = 1 Volt. So you lose 1 V * 10 A = 10 Watt in the cables.

A solution is to increase the voltage. Let's do that again but now use 120 V instead of 12 V:

120 Watt at 120 V gives 1 A, 1 A * 0.1 ohm = 0.1 V drop, 0.1 V drop * 1 A = 0.1 W lost ! So that's 10 / 0.1 = 100 times more efficient !

How to increase the voltage ? You could use step-up (boost converter) to increase the voltage and step-down (buck converter) to efficiently lower the voltage again to 12 V. On Ebay many of these modules are sold. It depends on the amount of current you need at 12 V which type will suit your needs.