This is a good question and most people are probably heavily influenced by what they have in stock, just as you say. There is not an firm rule, but anything over about 150 amps of current is a good candidate to move up to the next available voltage. This would imply a break-out of about 1800w as the limit for 12v, 3,600w for 24v and so on. Most inverter manufacturers follow close to that line, although Magnum has some high current units like the MS2712E (225 amps) which is mostly aimed at the RV/caravan markets where the coach battery bank is 12v.
The main thing is to get the cable sizing and breakers/fuses correct – check the wire sizing chart and overcurrent protection charts on our site.
In a perfect world, we would work at very high voltages – this would reduce power loss due to heat, allow smaller cable sizes, and put less stress on the weak parts of the battery cabling (like terminal connections). But the solar industry was built around automotive battery standards, so we are all working in sets of 12v at the moment. There are more and more inverters and charge controllers available at higher voltages now, but because of our batteries, those are still only practical for fairly large systems. As the battery industry changes, we might see different voltages for storage, which would allow changes in all of the other components.
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