I wouldnt waste money on batteries. extra expense, and too much toxic heavy metal waste. just use the two way meter and fall back to the grid at night. Im practical, not a dooms day survivalist. the grid wont go away over night, or even in 50 years. but solar can first replace the day time peak demand on the grid. when that is done, then we can work on the night time base load with storage. power demand is lowest at 3-6 AM each night and rises to a peak around 3-6 PM each day . like a sine wave above the base load. perfect for solar. sun comes up, power goes up. just like the california grid.
http://www.caiso.com/
the baseload is the low point of about 20,000 mega watts. summer day time peaks are often over 45,000 megawatts. so sun comes up, solar can make power. but sun comes up, and we turn the lights on over the tank. the sump pump is the baseload running round the clock.