I've been trying to do this calculation myself. One factor I hadn't initially put in was the fact that I didn't need to think about a backup generator for our (east coast) storms.
You might—I do. Your inverter won't power the house when the grid supply is down. Why? Well, because if the grid supply's down, it's probably because there's a line down somewhere. Therefore there's a lineman trying to pick that line up somewhere. He's going to be handling both ends of a broken, downed line. The generator-side line is tagged out, that's safe. The house-side line? Well, that's up to you—and so your equipment will deactivate to avoid creating a safety hazard for grid maintenance staff.
A generator probably beats a lot of battery systems (which along with the right inverter and a transfer switch is another way to use the solar for backup power).
Well one issue with grid-tied systems is that when the grid is offline they provide no power (they explicitly don't want to back feed into the power lines and kill some lineman who things power is off). So one of the certification issues is to make sure that your inverters recognize that state and shut down.
But a side benefit of putting in the system was we did a lot of electrical panel work, and as as result put 8 circuits on to a transfer switch (the incremental cost was the transfer switch and the electrictian's time wasn't materially affected.) So when the power does go out we can fire up a generator and switch over to gen power easily.
