An often asked power-related question is "How do I power my car PC when it's not in the car?" For example, if you are building the system prior to installation, or have removed it from the car for maintenance.
If your system uses an inverter
If you are using an inverter, there's generally no problem. You simply unplug the power supply you are using in the car, bring it indoors with you and plug it into the wall socket.
If you are using a DC-DC power supply
If your system uses a DC-DC power supply, you have to deal with the fact that your computer's power supply is expecting 12 volts as input. You should know better than to plug it into the 110 (US) or 220 (Europe) wall socket in your house, but just in case DON'T PLUG YOUR DC-DC POWER SUPPLY INTO THE WALL SOCKET.
You have a couple of options (thanks to members KByrd and evandude for this info):
Option 1. Unhook the DC-DC power supply from the carpc and use a normal ATX power supply (110V (US) 220V (Europe) in hooked to the wall)
with ATX connection to the carpc motherboard. Manually momentarily short the power-switch pins on the motherboard with a paper clip or something to start the computer, if necessary.
Option 2. Leave the DC-DC power supply connected and provide 12V DC power to it by an AC/DC converter.
Make sure the AC-DC converter provided enough amperage to power your system. A 10 amp system can supply 120 watts (12V X 10A=120W), a 15 amp system can supply 180 watts. In truth, these power supplies often are rated at 13.8 volts, giving 138 watts and 207 watts, respectively.
These AC-DC converters come in various shapes and sizes and run about $100 at Radio Shack.
In terms of affordability, option 1 is probably a bit more affordable than option 2.