Adding Battery power to existing Grid + Generator setup

I'm currently connected to the grid and have a whole home standby generator.  We have frequent and sometimes extended outages so would like to add battery storage so the generator is not running constantly during a power outage. Located in the SF Bay Area.  My buddy in the area has an off grid gen + solar setup with mostly EG4 gear... 

My setup is as follows:

-125A grid main service disconnect (exterior with PG&E meter)

-200A Kohler "dumb" ATS.  Exterior, adjacent to the main service disconnect. ATS out feeds whole home main interior subpanel.  There is no load switching, etc. 

-14Kw Kohler standby unit (Max 70A output), propane powered. Can pretty much run the whole smallish home.  Hot water & range are propane, biggest loads are 220v mini-split and dryer

-No solar and don't plan on installing any.

-Batteries could be interior/protected from weather or exterior. Maybe start with a minimum stack and then increase later.

Thought is to replace the Kohler ATS with an EG4 inverter and add a battery stack.  So if grid power fails, batteries kick in first, then generator kicks on automatically when batteries become low & powers house + charge batteries.  When batteries are recharged, gen auto off, run off batteries, rinse and repeat as needed for duration of outage.

Any recommendations for this scenario?  I've been looking through the All-in-one inverters and seems like it's doable with them but need some technical guidance with the above electrical setup & loads/capacities in mind.