Have a talk with your band's sound guy. They may not want to play off generators, and you'll need more than one. Solid state amps and home generators do not get along.
What type of music do they play?
What is the size of your audience?
I'd play it safe and rent one that comes on a semi trailer.
Hopefully Gogar will share some of his knowledge.
Edit: To elaborate
Crown has an amp sizing tool on their website.
https://www.crownaudio.com/en-US/tools/calculators#amp_power_required
Equations used to calculate the data:
dBW = Lreq - Lsens + 20 * Log (D2/Dref) + HR
W = 10 to the power of (dBW / 10)
Where:
Lreq = required SPL at listener
Lsens = loudspeaker sensitivity (1W/1M)
D2 = loudspeaker-to-listener distance
Dref = reference distance
HR = desired amplifier headroom
dBW = ratio of power referenced to 1 watt
W = power required
Once you know the size of the pa amp, you can look up how much power it needs.
Add in the guitar and bass amps, lights, smoke machine, apparatus to spin the drum kit, etc, to get the total power required, and then you'll know how big of a generator you'll need.
Hearing damage occurs at sustained exposure to 90-95db, so I'd plan on at least 100db at your desired distance. But that's just me, your taste may vary.