Well, efficiency doesn’t come into it because the 120V 2.75A already includes efficiency.
120 * 2.75 = 330VA
Do a 350W inverter should do the trick, even if the charger has power factor correction.
However, the charger may have poor power factor - maybe 0.8 - in which case an inverter rated for 250 W at 0.8 power factor may do the trick.
Finally, you have to consider whether you need a true sine wave inverter (recommended) or can use a cheaper ‘modified sine wave’ inverter (which will be cheaper but may make some electronics very angry).