Getting ready to purchase a generator. What size would you recommend for running a heat pump, water heater, well pump, etc?

We are looking at the Generac units. Our neighbor is installing a 17K unit this week. I think Home Depot sized his out for him.

I will be working with a local, highly recommended installer. I prefer the small business folks.

Any suggestions as to what size..

Rusty, my pockets are empty.