If all of your loads were going to go through an inverter and come out at 120 volts, having a 24 volt or even a 48 volt battery bank would be a no brainer. Like some of the other posters here, I thought if I ever had the chance to change to a higher voltage battery bank, I would do it. The higher the voltage, the smaller wires you can use. There is a little efficiency gain if your inverter is going from 48 volts DC to 120 volts AC. The biggest advantage is that your battery bank has more series connections and fewer parallel connections. I will try to add more about parallel connections in a future post.
I had my chance to jump from 12 volts last month. I had to replace my inverter/charger and therefore had free reign to buy whatever I wanted. Going to a higher voltage bank all sounded good until I started considering all of the things I had to do to have a higher voltage battery bank. All of my lighting (interior and navigation) is 12 volts. I have a 12 volt unit that uses UV light to kill bacteria and viruses in my drinking water. I have both entertainment stereo systems and a marine radio that are 12 volts. Even my TV and VCR are 12 volts. I added up all my 12 volt loads and figured I would need to plan for 50 amps to run everything that operated on 12 volts. In theory, you can tap ¼ of a 48 volt bank or ½ of a 24 volt bank to supply 12 volts. But in practice, that can cause battery bank imbalances. Battery bank imbalances are a big deal. They can dramatically shorten battery life.
I considered separate battery banks. But for two banks, you need two chargers, two monitoring systems, and a lot of wiring. It gets complicated.
I considered using a DC to DC converter to drop 24 or 48 volts from a battery bank down to 12 volts to use. But for 50 amps, a converter was running about $600. The manufacturer rated it with a lower operating temperature than the 110 degrees I often see on my lake. By the time I found a DC to DC converter that would reliably convert 24 or 48 volts to 12 volts at higher temperatures, I was looking at a $1,000 converter. I even considered using an MPPT solar charge controller to drop voltage from 48 to 12. But they aren’t cheap and not really tested or made for what I had in mind.
In the end, I decided to make the choice to stay at 12 volts. It didn’t have so much to do with money as it did with simplicity and reliability. I never know when my life might depend on things like nav lights, water purification, and a marine radio. I want as few things as possible between those devices and their power source. Fewer “things” means fewer things to break. I go to my boat to play, not to work on things. So given the choice, I have a 12 volt battery bank.