Why do you need to assume 100% generation???

It's all about the all-in cost of the system vs the reduction in utility bill.

If your system runs 20% of your usage, it still has whatever ROI it has.

Building to cover 100% usage isn't a typical goal if you're planning to keep the utility anyway. Most net metering doesn't pay you if you make more than you use, so if you go over, you spent too much on your system.