A loose rule of thumb... and we should NOT follow rules of thumb:

It takes around 3 times the watts to START an elec motor as it does to RUN the same motor. Whatever your run watts are (volts x amps)... then 3 times with some wiggle room probably will work.
I agree with Motoguy... a larger unit is a better choice. Remember that generally speaking: A generator will produce close to the same watts on LP gas as gasoline... yet around 20-25% LESS on NG.

Have a LICENSED electrician handle this for you.