A local battery store sells used batteries for cheap. They sold us a one year old 195 amp hour, 12VDC deep cycle lead acid battery for $80. It weighs 123lbs and they claimed this battery could have a $40 scrap value, which would put us $40 out-of-pocket after using up this battery completely.
In the grid-tied system, we have to pay $18/month minimum service charge. That works out to $216/yr, even if more electricity is produced than consumed. At $80 per battery, we could buy 5.4 batteries every two years with that money instead. Often, people think that off-grid solar is a bad idea because the batteries go bad every 3 years or so. However, the on-going cost of being grid-tied makes investigating off-grid worthwhile. It may not be as uneconomical as initially thought.
Bring Down the Cost of Electronics
Solar panels are pretty cheap today. Over half of the up-front cost for grid-tied solar electricity is due to the power conversion electronics required to turn the 30VDC from the solar panels into 240VAC. Off-grid solar electricity has even more difficulty -- you need a charge controller to keep from overcharging the batteries AND you need an inverter to convert DC to AC. The most fancy charge controllers also provide maximum power point DC-DC power conversion to match the solar panel's output voltage to the battery's voltage. Unfortunately, these units cost a lot.