If grid reliability is the real issue, then I suggest you model the grid as an unreliable power supply. The way you do that in HOMER is to represent the grid with a generator as described in article 10463
You can then schedule the generator to be off for a few hours a month. The attached sample file demonstrates.
You can keep your diesel generator in the system, as well as the PV and battery system, and just add a second generator to represent the grid. Modeling grid outages in this way will force the occasional use of the battery and/or the backup generator. If you still find that the system buys more grid power than you would like, you can increase the size of the PV array and/or the battery bank.