Best practices are to err on the side of larger than a smaller generator. When generator manufactures label KW they use the higher rating of propane gas whereas natural gas KW rating is less. Example: A Generac generator 22kw running on propane = 19.5kw on natural gas.
Generac generators make the highest wattage 22kw Air-Cooled generator in the industry. This comes with a 200amp whole house automatic transfer switch with load shedding abilities and made for outdoor installations.
Load shedding or smart management modules prevent the generator from stalling on startup due to an excessive load. They wait to engage the load 5min and then re-engage the load once the generator has gotten up to speed. The perfect solution if you have multiple air conditioners or electric water heaters.
Typical California installation of a generator 10ft or less from the electrical and gas meters with a 22kw generator with ATS is about $10k, the further away from the meters will increase the price. This is for the complete installation that includes gas line, electrical, 22kw whole house generator, 200amp automatic transfer switch and sales tax.