[jrockway]

Your UPS doesn't save money by discharging its battery back to the grid when it's not in use.

Your UPS handles an order of magnitude (or more) less power than a whole-home solar installation.

None of these problems are intractable of course, but you are oversimplifying the problem a little.

[lostapathy]

I'd be willing to accept not discharging my solar system's battery to the grid when not in use, I can use it at night.

One can get a UPS affordably that will power 1500 watts of continuous power. Being able to supply just that much, or twice that much, from solar panels in a grid-down scenario would be tremendously useful, even if it's not enough power to fire up my welder.