700 watts is indeed about the most that should be drawn from a 12 volt vehicle electrical system, This is because the alternator cant supply much more than that, and drawing more than the alternator can supply, will either risk damage, or will discharge the battery even with engine running.
With a large heavy duty battery, several thousand watts can be used briefly, but about 700 should not be exceeded for any time.
(unless the vehicle is fitted with a larger than normal alternator, or double alternators, which is not common)
If the inverter is pluged into the cigar lighter then I would limit the load to about 150 watts, cheap inverters are not that efficient, and an output of 150 watts could be about 15 amps input, which, as others point out, is a common fuse rating.