Efficiency of the inverter should be better than 75%, while the ATX is a switcher which is about 80% efficent, so you are only wasting about 80W, overall efficiency is about 60%. How hard are you going to work to save maybe an additional 20% of 200W?It had a problems:unused a lot of watts
current of +5 and +3.3 is very high and if I use downregulated,the sum of current to be use is go high(because current in the Input and Output of regulator is same),but when use an ATX power supply,current is low.Your car supplies 12 to 14V DC.
Your PC wants +12, -12, +5, +3.3 VDC.
You do not need to modify the ATX power supply. You do not need neccesarely to go into AC inverters. You need to downregulate with beefy DC-DC converters.
It will not be cheap. Probably more $ than an used laptop plugged direct to the cigarette lighter.
ATX power supplies schematics are somewhere on the net.
if you notic:for example: pc use +5V\20A and +3.3V\15A then sum of watts is about 150watt,if I use regulator to reduse +12 to +5 and +3.3 the used current from +12 is 35A and the used watts is 420 watt,in this metod the unused wattsis 420-150=270 watt and thet so much for car battry.Current for +5 and +3.3 is whatever it has to be for your PC to run.
And that same exact current is the one supplied by the ATX supply if you use it.
Why do you say the ATX supply current is low ?
Yo do not waste Amperes; you waste Watts !
That would be assuming that you are using a linear regulator as your supply (output current equals input current). There is no way you're going to use linears for the 5V and 3.3V rails. You would have to use buck regulators. Assuming an efficiency of, say, 85% for each regulator, the power from the 12V rail would be about 176W, or about 15A.if you notic:for example: pc use +5V\20A and +3.3V\15A then sum of watts is about 150watt,if I use regulator to reduse +12 to +5 and +3.3 the used current from +12 is 35A and the used watts is 420 watt,in this metod the unused wattsis 420-150=270 watt and thet so much for car battry.