If you want power when the grid goes down, you will need a battery system to store power. Your power will not work (even in full sunlight) with a grid-tied system when the grid is out. You need a lot of batteries to run your house for 12 hours, so plan on spending a bundle on those (and periodically replacing them).
Also keep in mind that selling power back to the grid is typically a terrible idea, since in every state I know of, the utility will only buy power from you at wholesale rates (much, much less than you pay). Unless you assume really crazy inflation in power prices, it's a terrible investment from that standpoint. Most grid-tied systems are carefully designed to produce 95-99% of the necessary power over the course of a year so that you end up net-neutral.
There are good reasons to do solar PV (live far from power grid, power only needed intermittently/during daylight hours, freedom from grid dependency, warm fuzzy feeling, etc) but they are generally not financial. The low hanging fruit is using the sun for HEAT, because that's what it already produces for you. Passive solar design, solar hot water, etc, are great ways to do this. For electricity, the easiest way to save power/cut costs is to use less by using efficient devices/lighting. PV is pretty far down the list for return on investment still, though it's always improving.
-W