The idea for household energy storage didn't originate with Tesla, but like with many things, Musk is translating it into action.
Some earlier comments were about increasing generation. One way to do that is with reduced demand. You don't need to supply as much if the demand goes down. Energy efficient appliances and lights are one example. That's why there are incentives to buy efficient appliances. If people use less, the power generators need less capital to build.
Smart meters came into play so that the power company can reduce load during peak demand. If people are willing to allow the electric company to shut down their homes during certain hours, they can get cheaper rates overall. There are incentives also for people to charge their EVs at night when demand is lower and power companies still maintain spinning generators that need load.
EV cars were a concern initially, not just because they would need more electricity, but because the loads would be constantly moving. This lead to planners to not just think about EVs as loads but also as lots of micro generators. All these plugged in cars could be accessed for power at peak hours, which are during the day when people are working. If the people are working during the day, their cars are parked and their batteries are available. Why not tap into them for extra juice?
This is related to the household battery system. Used car batteries that have lost capacity for use in an EV could still be used in a household system. But not just used batteries, but batteries made by companies that make batteries for cars then have another outlet for sales. People have described Tesla not as a car producer but as a battery producer. The ability for homes to store electricity also means all those wind generators and solar can be stored.
Smart meters are making this work together as an integrated system. It's all pretty interesting and gives reason to hope for a better future. Trouble is there's a limited amount of time. It's great to see what Tesla has accomplished. There aren't enough EVs in homes yet to make some of the bigger plans a reality. But it's exciting for me to learn about what people are thinking about. There seems to be a great capacity for innovation that could achieve even more with a greater will.