more is better
the starting current is what will make your AC unit sing or Die. the ac unit needs a much higher amount of starting current to start, especially after it has been running and built up a good head pressure.
a sing unit with at least 3600-4400 surge watts would handle most 15kw roof airs but a pair of parallel 2000s have the advantage of dual inverters tom dissipate the heat.
Honda and Kipor both have these units and the parallel kits.