Should I build a 12 volt or 24 volt system or even 48v?

12v, 24v, 48 volt, or 120 volt?

Most people begin with a 12 volt system because of it's simplicity, and the ability to charge the battery bank with an automobile alternator.  12 volt battery chargers are cheap and most people already have one.  12 VDC lights are readily available and can be run directly from a battery.  

However, one must consider that the household voltage one obtains from the system is ten time higher. Converting from 12 volts to 120 volts is not efficient.

An ideal system is one in which the DC voltage is equal to the AC voltage. For instance, if you are using a grid-tie system whereby you are feeding your excess power back into the grid, you may want a 600 volt system.  You could coneivably have a 120 volt DC battery bank and convert to 120 VAC household current.  This would be far more efficient than a 12 or 24 volt system, but we also have to consider the price and availability of whatever equipment we choose.

Off-grid systems are most cost effective at 24 volts. Most small to midsize wind turbines produce either 24 or 48 volts. By having either a 24 volt or 48 volt battery bank, you can easily and inexpensively use the same charge controllers for either wind or solar.  24 VDC charge controllers and inverters are very common and inexpensive. 

By using a 24 volt system, the equipment is inexpensive and easily obtainable. One can add a wind generator down the road with no modification of the system.

The higher the voltage, the more efficient the system. Higher voltage also allows for longer distance between the solar panels or wind generator and the battery bank and inverter.

You can start with a 12 volt system and after you become familiar with using solar and batteries and how things work, you will eventually replace your existing batteries.  By using 6 volt deep cycle batteries, you can connect them in series to achieve 12 volts, 24 volts, or any multiple of 6 volts.  The same can be said for 2 volt batteries if you choose to go heavy duty right off the bat.  The piece of hardware that most determines your system voltage is your inverter.  Batteries are flexible, depending on how you connect them and batteries can be charged one at a time if you have to instead of charging the whole battery bank at once, in an emergency for example. 

Inverters are available in 12 and 24 volt almost anywhere.  All truck stops carry 12 volt inverters for truck drivers to watch TV and get their laptop online.  220 VAC inverters and three phase inverters are available, although at a higher cost. 

There is no limit to what you can do with solar, Some things are more practical than others, however.