The first is where you are starting and where you see the future of your system. If, for example, you have an existing 120VAC system (either grid connected or just things you run off a generator), 120VAC is more likely the right answer. On the other hand, if you are just starting, 12VDC is probably the best choice. But, even then, it is not necessarily the best choice for everything.

With the advent of bright and efficient LED lights, now mass produced for the 12 volt market, 12 volt lighting usually makes sense. Even if you are just starting with a generator, adding a small deep-cycle battery to run lighting which can be charged off the generator (most generators have a 12 volt battery charging output) makes a lot of sense. The lights will be available all the time. Even just having your lighting when it is type to start or refuel the generator is a big plus.

As for higher power devices, how much time they are used can help you make the right decision. A good example is an electric coffee maker. Our 120VAC unit draws about 1500 watts but is only used about 10 minutes a day. A quick multiple tells you that the total consumption is about 250 watt hours (1500 watts x 1/6th hour). It's pretty low on the total energy budget. Further, most 12 volt coffee makers draw a lot less power but the end result is that they just take longer to produce the same amount of coffee. Thus, little if any energy savings but less convenience and higher cost.

You can use this same type of analysis for other devices. On my list of AC devices you find a table saw, radial arm saw, circular saw, recriprocating saw, hand drill, drill press, rice cooker, blender and more. There would be little if any efficiency gain for using other than a 120VAC unit and they are just not used that much.

On the other hand, I have a desktop computer system which is on 24/7. It is a low-power motherboard but the system drew about 80 watts when running off a typical 120VAC PC power supply. It is now running off a special power supply which can run on from 6 to 32 volts DC. The same system draws about 30 watts. While saving 50 watts sounds small, we are talking about a savings of 1200 watt hours each day or almost five times the total consumption of the coffee maker.

Another item where a DC appliance is typically a big win is the refrigerator. AC refrigerators are pretty much designed to be inefficient. First, the upright design means big losses when you open the door. Self-defrosting freezers are another big loss and, finally, think insulation. Thus, a 12 or 24 volt chest-type refrefrigerator can be a huge win.

If you are just starting with off-grid, going beyond a battery for lights may make sense. For example, laptop computers, TVs, stereos, DVD players and such designed to operate in a car or RV, offer good choices. Size your battery such that it will cover your needs without being dischared more than 50% and you can wait a while before investing on other charging options.

Note that all this doesn't have to paint you into the 12 volt corner. Most efficient DC refrigerators are designed to run on 12VDC or 24VDC and generally will automatically configure themselves. You just need to connect them. For low-current devices such as lights, there are inexpensive DC-DC converts which can step your battery voltage down efficiently.

The one place where you need to plan ahead is the inverter. While there are cheap intermittent-duty 12V inverters, the industrial strangth units suitable for off-grid living are available in 12, 24 and 48 volt units. They are almost always inverter-chargers. This means when there is AC power available, they will charge the batteries. That AC power could be from the grid or from your generator. The important point here is that before you buy a commercial-grade inverter, you need to pick your battery voltage.

Bottom line is that you really need to evaluate your long-term plans before you put anything in place. While there is no one right answer, figuring out where you are headed will determine the right answer for you.