Easy to change system output to 124 volts from 120 volts with Outback FM80?

Good point and a brain fart here. Yes  - the inverter does this. No wonder I found nothing in my manual perusals. Gadzooks!

This is just ridiculously easy to do. Currently see no downside to increasing system voltage. Especially when one has very long runs or very large loads. 125 volts is the max setting for the Outback MATE.

In theory a motor receiving 120 volts will use less power than the same motor receiving 110 volts. My uncle was part of a group that decided the "quality of the power supply" to various regions. He definitely felt that 120 volts was a higher quality supply than 110 volts. Of little surprise in a way, his house received 122 volts. Or was it 124 volts? A likely coincidence - hard to say.

The only time this issue comes into play is powering the well pump - where it only received 108 volts with the pump running. Now it should receive 113. Pretty long run to the well since it has to be 100' from the septic leach field.

But we had a poster claim that no harm was ever done when his equipment regularly ran on 90 volts. Opinions seem to vary. Don't they always?

"The idea that "more voltage is better" can be counter-productive to the power factor as well." This is why the inverter maxes out at 125 volts. So my fridge and freezers now receive 122-123 volts.

There is speculation that well pump motors are designed to accommodate lower voltage since they always drop the voltage with their powerful motors. I still feel better at 113 than 108 volts.

It might prove to be a small amount of "free energy". Worth a try. Nothing to lose. I'm pretty certain my electronics are fine with 122-123 volts.

But I do not recommend going over 125 volt inverter output. 

And - so far so good. I'm not going to scientifically verify a 2% gain in electrical efficiency. But I'll take it.

Your obsession about my system voltage range of 120 vs 125 is reminiscent of my obsessing about someones choice of breakfast cereals. Then tossing in my special expertise on breakfast cereals - which I make my living at. A very good living involving many government organizations!

Got a nice explanation that went like this: 
"If your loads are mostly resistive in nature, with a power factor close to 1.0 then it would make sense that raising the voltage increased the power used. Let's say you have 5 100 watt incandescent bulbs, rated at 120 volts. This works out to 500 watts / 120 volts = 4.1667 amps. 120 volts / 4.1667 amps = 28.8 ohms. Now we up the voltage to 125 volts. That same 28.2 ohm load now pulls 125 volts / 28.8 ohms = 4.3403 amps. 125 volts x 4.3403 amps = 542.53 watts. The power goes up at a square of the voltage in a resistive circuit. 120 to 125 volts is only a 4.1667% increase, but the wattage going from 500 to 542.53 is an 8.5% increase in power. If your inverter is 90% efficient, it was pulling 555 watts before, and now it needs 603 watts. This will certainly show up on the battery useage. On a 48 volt battery, this is 11.5 amps to 12.6 amps, just by raising the voltage 5 volts.

Not all loads are purely resistive. Some motors and such could be even worse and make more heat and use more power, or might get more efficient with a little more volts, it is hard to say for sure. Reactive and capacitive loads are not easilly predictable with changing supply voltage. But most will probably still take a little more power at a higher voltage. Modern switching regulated power supplies will still pull about the same power by pulling less current at the higher voltage, but an old style linear regulator will have to drop the voltage more and turn more power into heat to keep the same output voltage. Take an old 5 volt logic supply. They normally have about 7 volts before the regulator. If it is running at 10 amps, that is 50 watts to the load and 20 watts going out as heat from the regulator. 7 - 5 = 2 volts, x 10 amps = 20 watts. Raising the voltage going in by just the 4.1667% makes this look a lot worse. We now have 7.3 volts before the linear regulator. It still drops the output to 5 volts for 50 watts at the load, but now it is dropping 23 watts of heat instead of 20 watts. 7.3 - 5 = 2.3 volts x 10 amps = 23 watts. This is 15% more wasted power than before."

Think I'll try 118 volts next - never satisfied.