Very few people have an excess of power from their solar grid - unless they aren't at home (which, come to think of it, is an excellent use case for a home batt

Very few people have an excess of power from their solar grid - unless they aren't at home (which, come to think of it, is an excellent use case for a home battery). For most people, the home battery is going to be useful from day one by entering a TOU contract with their local utility, and then charging at night, and using the power during the day. Solar power, on top of that, will be a bonus. For the summer, California E-6 Residential rates are $.32/kWh during peak times, $0.13/kWh during off peak. If your house is using, say, 4 kW, for 5 hours during peak - then that 20 kWh will cost save you  20 * ($0.32-$0.13) =  $3.80/day in power costs, approx $115/month. Do that for 4 months/year ,that's $460/year. With a 5 year life, that battery would need to be less than $2300 to come out even. http://cleantechnica.com/2014/09/05/teslas-gigafactory-may-hit-100-per-kilowatt-hour-holy-grail-ev-batteries-report-predicts/ http://cleantechnica.com/2014/09/05/teslas-gigafactory-may-h... suggests that $100/kWh is in the ballpark - so this might make sense right out the door.