This all depends on what you think you must have on a back up emergency generator. Kitchen appliances, electronics and lighting, etc. Best way is to determine your current usage is to look at your current utility bill and see what your current daily power consumption usage is in KWH.

whats the best fuel source for a backup whole house generator?

Propane. For a large generator, you'll need to have the propane tank buried in your yard. Local gas/propane companies can come service/fill the tank on a regular schedule. Typically, the generator company can provide those references.

Natural gas is ideal.

22 KW liquid cooled generac being installed at my house as I speak. I'm installing a 1,000 gal propane tank as well.

An NEC load calculation should be performed to determine proper sizing. Many local jurisdictions require this analysis, when puling permits, for installation. KWH does not account for the surge draw of motor loads, like air-conditioners. Thus, utility bills are not generally accurate indicators of appropriate generator size requirements. Contact a certified generator installer, or qualified electrician to size a generator that best suits your needs. This is the key to ensuring that your generator works under all conditions, so you can rely on it, when you need it most.