The old physics formulas will help here
Power is your girlfried Ivy Watt
or P=I/V Watts (I = amps and v = volts)
so if you have 90odd watts (plus some loss on the coverter) divide that by the battery voltage will give indicative amp draw from the battery, maybe around 7~8amps. Cant remember what the battery capacity is but you want to leave a safe margin for starting... and batteries never improve with age or use.
All figures are wet finger and should be checked verified by someone who gives one . that said if it was my batt trying to charge/run a laptop i'd save that for hotel rooms or when the engine is running.
ps any cable u use to wire up the 12v socket will need to be good enough for the expected current draw ...