A standard Computer PSU rectifies the incoming mains (120 or 240 volt) and the "chops" it in a switchmode type power conversion.
Therefore as it starts with a high DC voltage (120 or 240 V AC rectified to DC) there is no easy conversion to make it use 12 V instead. Without a complete rebuild and redesign of the PSU.
By far the cheapest and easiest way is to use an off-the-shelf 12 V inverter that converts 12V to 120 or 240 V AC, and then you dont need to bother rebuilding the power supply from scratch.
Has the advantage of easily sourced power supply units for the PC. You would likely get away with a small 200 Watt unit.
Therefore as it starts with a high DC voltage (120 or 240 V AC rectified to DC) there is no easy conversion to make it use 12 V instead. Without a complete rebuild and redesign of the PSU.
By far the cheapest and easiest way is to use an off-the-shelf 12 V inverter that converts 12V to 120 or 240 V AC, and then you dont need to bother rebuilding the power supply from scratch.
Has the advantage of easily sourced power supply units for the PC. You would likely get away with a small 200 Watt unit.