A 5000 BTU window AC uses around 550 watts when running.
However the 5000 BTU window AC uses around 900 to 1,000 watts or so when starting the compressor.
It takes more power to start the window AC compressor than it does to keep it running.
So if you use a generator then you need double or triple the amount of watts to ensure you have enough wattage to start the AC compressor.
If running a window AC I would recommend you have at least a 3,000 watt or 5,000 watt generator to allow enough wattage for startup and also for other things you may need the generator for.
It's better to have a larger wattage portable generator than too small.
I have a 7,000 watt portable generator that provides plenty of power and I don't have to worry about not having enough wattage.
It uses near the same amount of gasoline as a smaller portable generator does but provides more power.
I can run at least 3 window air conditioners if needed off my 7,000 watt generator so it provides me plenty of power.