If you're running at a comfortable volume you can do fine with a 2000-2200w inverter generator.. as long as it's a legit good quality one like a Honda EU2200i.. I have some friends who do park parties with this exact same setup using a Yamaha EF2200i, they run it pretty loud and have been doing it for years with no problem. But if you want to seriously push the sytem to the max all day long then maybe you'd rather a 3000w unit for safety margin.

PS: If you do not have your two subs placed directly next to each other.. do that. It makes an enormous difference in efficiency.