it is possible to hardwire a larger inverter to you battery. But I am assuming this is not an option. If it is an older computer like P4, they were power hogs and are probably pulling a couple hundred watts at idle. Newer core2duos might only pull 100 or so watts at idle. Just because the power supply is rated at 400W does not mean the computer is always using that much. Current is only delivered as needed, so if you dont do anything but start it up and show that it runs that may be okay, IF you have a very small LCD. If you ran games or did anything hard core that put the processor to work you would likely overshoot the 400W mark. I would suggest you try it. I assume these inverters have a fuse. If you pop it, big deal, that is why it is there. Put a surge protector inline to protect the PC. Ohhh that is another idea - if you have a UPS (uninterupted power supply) that would do the job too.