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You have a valid question. This forum attracts a wide range of people with greatly varying expectations. If you are going to live in a space equivalent to a small trailer with a floor space no greater than 200 square feet then it is possible to keep the wire run lenghts short enough so the line losses are not too great. As Reboot mentioned long wire runs will produce high losses which in turn requires bigger batteries and or solar panels to compensate with. I have been designing systems both low voltage DC and regular higher voltage AC for several decades. I routinely use a program to calculate how big a wire has to be for a given load current. The other side of the issue has to do with appliances. Doesn’t matter if its a water pump, fan or light. To do work requires power. 65 watts at 12V = 5.4 Amps and the same power at 120V is only 0.54 amps.
Current not voltage creates heat. I’m a Fix-it kind of guy. I have found that DC appliances fail more often than equivalent AC devices, other things being equal. Due to volume of devices being sold you are more likely to find a quality product using 120V AC than a similar product using 12v DC. My observation is that RV type devices typically fail sooner and are generally cheaply made because they are intended for occasional intermittent use.
Most of us drink coffee. Check out how many 12V coffee makers there are; then compare for 120V AC coffee makers. How many 12V coffee makers do you see that last 10 years?
I lived in an RV for five years. When the heater quit I replaced it at a cost close to what a house furnace cost. It failed in 13 months. That is when I was informed I should have bought a ‘parkside’ version with 120V AC fans instead of the 12v DC model. That version had better bearings and last a lot longer.
A lot of people approach OFF-GRID from the pespective of moving from a conventional house. The transition is less stressful if they have familiar things just needing to remember energy conservation.
Almost any home Depot, Hardware store and such have great books for DIY on how to wire houses the safe way. That takes the guesswork out of it.
Finding a GOOD 12V DC book is much more difficult. Some of them are useeless and do not provide data on calculating safe wire sizes. My know it all neighbor burned up his car when his 12V dash wiring modifications caught fire two weeks ago. He figured he knew how to wire.
If you contemplate living in a space bigger than 8′ X 16′ the lenght of wire runs as measured along the wire could be a problem in terms of losses. Overload a wire and you have the potential for overheating and starting a fire. If you do not have the background and knowledge to judge then following a code book is a better guideline.
Cost of copper has recently skyrocketed. Buying larger size wire is being safe but can become very expensive real quick.