A: In general batteries have two uses.
The first is to take advantage of time-of-day (TOD) tariffs. A TOD tariff is where your electricity company charges different rates for electricity at different times of the day. You can use this TOD tariff to buy electricity at a cheaper rate and sell it back at a higher rate. We do not have a TOD tariff where we live so I have no direct experience of using batteries for this purpose.
The second use is as an alternative to a diesel or propane emergency back-up generator for when the power lines go down in a storm. In December 2020 I bought batteries to replace my back up generators at three houses.
The Tesla Powerwall is considerably cheaper per kilowatt-hour of electricity stored than other batteries from companies like Sonnen, LG Chem, Generac and Simpliphi. However, at about $7,000 it is considerably more expensive than a diesel generator which costs $2-3,000. However, I have a propane back-up generator and it costs me $500 a year to get it serviced so that it actually works when we need it. I have learned this lesson the hard way – our previous back-up generator was not serviced, and it failed to come on when we needed it – during a power outage. Over 10 years this maintenance cost is $5,000. So, the full cost of a back-up generator over its lifetime is about the same as a Powerwall.
Also, in Massachusetts, the new SMART subsidy program for solar panels includes an extra subsidy (an “adder” they call it) for a battery. This is currently about 4c per kWh generated by the solar panels. If you have a 10kW array generating about 10,000 kWh a year then this is worth $400 a year for 10 years which brings the cost of the Powerwall down to about $3,000 which makes it similar to the upfront cost of a diesel back-up generator. Only now you have no maintenance cost. This is why I intend to install a battery to replace my defunct back-up generator in the near future. The regulations on qualifying for this adder are very complicated!