Minimum battery sizing for PV system with prime generators

[pucksr]

If you want to save money then IMO build a generator system that is fueled from different sources and forget about using batteries. You will save money in the long run.

Well, not really.
We are talking about islands in the middle of the pacific or mountains in Middle Earth. If I design a dual-fuel system, that just means I have to pay someone to barge two different fuels out to a remote location. It doesn't really help with anything.

The idea behind using PV-prime and generator backup is that it reduces EG operation hours, which reduces maintenance hours. Most of these sites are highly inaccessible. One big trip every 5 years to replace batteries isn't bad. One trip every 3 months to deliver diesel is a big deal.

[bcroe]

One consideration is your weather. Clear skies are nice, but frequent clouds will bring
your generator back on much more often. Bruce Roe

[SunEagle]

Something you have to also figure in the calculation for the battery size is that if you discharge more than the mfg says you should you will shorten the life of the battery system.

That 5 days is really based on not needing to discharge more than 20% a day but still allows you a little space if you have to discharge more a few times over the life of the battery.

If you want to save money then IMO build a generator system that is fueled from different sources and forget about using batteries. You will save money in the long run.

[pucksr]

So, this may be a little abnormal for the typical discussions here.
I am an EE trying to design some off-grid systems(or rather augment some off-grid systems) in the 50-150kva range.

We currently operate as generator prime systems with 24/7 generator operation(or very near it). I am trying to make an argument for converting to PV prime with standby generator operation. We have had success with this type of design at smaller facilities, but not at larger facilities.
I know that IEEE 1562 typically recommends a minimum sizing of 5 days for batteries for off-grid applications. I know I can shrink that number if I have a standby EG. I also know that if I shrink that number too much, the "standby EG" will be required to operate on a daily basis.

Example: If I only have 20 hours of battery, my EG will not be a standby generator.
If I have 36 hours of battery, I can be fairly certain I will have a standby generator.

I am trying to figure out, from a design perspective, where IEEE or similar puts that number.

Any thoughts?