I believe a lighter is 20 amps so you can run a max of 16 amps on it. Take the watts and divide them by the volts and that gives you the amperage of the device. For example 200watts divided by 12volts equals 16.6 amps. That is pushing the circuit. Check out this site.
http://www.zena.net/htdocs/alternators/alt_inf2.shtml