Of course the calculation is different for different situations, but for the typical urban resident with a dependable grid the numbers do not usually come out that way. For me, the Snowpocalypse last February was an outlier; for the 10-15 years prior to that the grid had been down for maybe 3-4 hours total. Again, for me and for most other citizens of Austin, a battery backup system doesn't make much sense economically, and the PV contribution to my bottom line were it to run during outages would be way way down in the noise.