Most wouldn't respond with "The Skin" when asked "which is the largest organ in their body?"
Equally, most would not respond correctly when asked which is the largest electrical system in your home? The answer, 99 out of a 100 times, is the wiring. The wiring fault protection systems would be next, but not even close. From the breaker box to the outlets and lights in every room is a patchwork of wiring that connects it all. If you think of the distance each outlet sits away from the box, and the number of walls the wires have to snake through, it get's pretty impressive.
To wire a residential home requires it be done to code, and therefore we can get some standards. For every 1200 square feet you would need about this much wire to thread it per the electricians:
* Notice that the amount of wire involved is longer than double the square footage.
With this much wire involved, there must be a reason, and that reason is efficiency. Wire is not a perfect conductor of electricity. When you add more wire, you make the system more efficient. By separating circuits, adding more breakers, and adding more wire, electricians can install safely, the power necessary to run your day. Without this much wire, things get hot and there are fires. This is the reasoning behind the NEC (North American Electrical Code). DO FOLLOW IT FOR YOUR OWN SAFETY AND THAT OF THOSE AROUND YOU!!
If you are building something outside of the standards of code, makes sure you do the math:
A: If you are generating power, your power is worth getting to where you want to use it. Only a fool makes power and drops it in the transmission lines.
B: If you plan to upgrade later, do the wires NOW! The need for bigger wires is obvious and imminent, as the ability to stack wires like legos isn't quite there. On the other hand, adding another solar panel later is fairly straightforward.
C: No matter the application, do the math 3x for fault tolerance. Then upsize the wires for added precaution. Without the luxury of product liability to fall back on, over engineer your own projects for safety's sake.
D. Remember, the wires are a major contributor to the efficiency of your system.