Today's cars use a 12 volt system hence a 12 volt battery. With the engine off, the power for such things as the radio or the cigarette lighter comes only from the battery which should be around 12 volts. When the engine is running power is supplied by the alternator but is higher than 12 volts which then keeps the battery charged and in standby ready for use should the alternator stop producing power or the engine is not running and the driver turns the key to the start position. Power from the alternator should be no higher than about 14.5 volts.Your car's cigarette lighter is connected to the electrical system and therefore gets either the lower 12 volts when the car is off or 14.5 volts when the car is running. Be aware that some cars turn off all accessories when the key is in the off position so you may lose power to the cigarette lighter when the key is in this position.Amperage is another matter... Amps is determined by the load put on the electrical system, i.e. what you plug into the cigarette lighter. To protect the electric circuit, they placed a fuse in that system. Assuming a 20 amp fuse, you would have to limit what you plug into the cigarette lighter so that it would never draw more than 20 amps or the fuse would blow and cut off all power to the cigarette lighter.With most laptops having an AC adapter to convert household AC into DC for the computer, you would have to buy an inverter (available darn near everywhere) that will produce a fairly low amperage 120 volts AC so you can merely plug your computer in. Most laptops never even come close to 20 amps and I would bet that 300 milliamps would be more the norm or 0.3amps never even coming close to blowing that fuse. However, car power is known to be "dirty" meaning it is full of noise spikes coming from the rotating alternator and mediocre regulator that is designed for the engine and radio... not a computer. You should get an inverter that filters out most if not all this noise so your computer doesn't have problems.Good luck!