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Some computer components, such as the monitor (screen) operate on 120V AC. But the main box (CPU) operates strictly off of 5V and 12V DC (technically +5V,-5V, + 12V, and -12V, but +5V and +12V take most of the power.) The 120V AC is converted to DC by a small switching power supply inside the box. One of the ideas I have played around with in the past is to build a computer power supply that runs off of a 12V car battery that bypasses the internal power supply of the computer. (This is more efficient than converting to 120VAC, then converting back to DC inside the box.) With more low-power technology coming onto the market such as flat screen LCD displays, eventually an entire computer system might be built using only 12VDC as input.

Where on should aim for the assumed voltage, switching power supplies today do have a bit of tolerance on the range of input voltage. The reason is that a switching supply converts high voltage to low voltage in little bursts, variances in the input voltage only change the rate at which these bursts need to occur. Of course you are right in that if the voltage is too high you'll exceed the rating of the parts and something will smoke. But there is still a good range, something like 85V - 150V AC. This is why during a brownout the monitor will go out but sometimes the computer will not. Even running strictly on 12V is not maintenance free. 12V batteries and light bulbs won't last forever either. Eventually the batteries will wear our and the bulbs will burn out. Then what? Perhaps we could grow our own batteries (certain mixtures of foods such as potatoes and lemons can form the basis for batteries), although this won't yield much power.

Offered by Michael S.