You can add up all of the wattage of everything that want run, and them make sure that your generator can put out that juice.
You want to make sure that your generator has enough wattage to handle the peaks that your whole system will be putting out.
I have had few experience with underpowered generators and it's awful. First, tube amps will start to sound really bad - breaking up - making splat sounds... Then digital gear, like keyboards will act up - powering on and off and other weird stuff. If the generator has a breaker on it, then the breaker will trip also. Power amps for a PA can eat up a whole lot of wattage, especially for bass.
Whatever the math comes out to - add more to it. Better to have more wattage headroom than you will need. It's miserable if you don't have enough.
What kind of PA do you have and what is its wattage? What about monitors - are they powered or do you have power amps? Also, what else are you going to be running through it - lights, recording gear...?