Let me start by saying I love the Idea of Tesla and the electric car.
Well, I've done a lot of research into his newest adventure, and that's personal batteries for load shifting.
Load shifting is a trillion dollar business, and I don't come to those numbers lightly. It makes healthcare seem cheap. People happily give thousands to the power company to keep homes warm in the winter and cool in the summer, not to mention the gobs and gobs of power industry and Amazon server farms hum away for all this digital noise.
Few people understand electricity anyway, it might as well be magic to most.
In a nutshell his plan is to sell people batteries that charge when the grid has too much cheap power and discharge when the power is expensive, putting the difference in the users pocket as savings.
We already do this with hot water heaters that heat the water at night and just keep it warm until you need it during the day.
But Musk plans to do this with electricity, and here he will face several problems.
1. Most batteries have only so many charge/discharge cycles. Like the battery in your car or phone, they generally die after so many charging cycles. A led acid car battery dies after 5 years, or 2,000 charges, about the same as a lithium battery.
So, let's take an average car battery as an example. Its about 1,000 watts and takes half a day to charge and half a day to discharge (without hurting itself). That's about 12-18 cents worth of electricity per day. That's at most $65 a year that you can save if you charged it for free. Now, a car battery costs $100 and you need a new one every 3-5 years. So, that's $20 a year at best. 65-20 =$45 a year you could save IF YOU WERE CHARGING THE BATTERY FOR FREE. You are not. Let's say its being charged for half the price, then to charge it costs $33 a year, minus your battery, and your down to a modest $13 a year savings for a lot of trouble and danger dealing with batteries.
I have personally seen a car battery explode, it went off like a bomb, bent the hood and everything, and Musk is calling for something 10 or 20 times the size of a car battery.
2. It isn't as efficient as everyone thinks.
In general, it takes 100 watts of charging to store 80 watts of power. And to turn 100 watts of DC battery into TV watching AC you're generally losing just as much. You lose 10-20 percent charging a battery and another 10-20 percent going from battery back to AC.
These are just laws of physics. You can never get more out than you put in.
But, if someone could store massive amounts of power for next to nothing, there is a fortune to be made doing it.
For me, it's the million ton flywheel, which was city scale power for the price of water and a tunnel.
But maybe Musk knows something nobody else does. I suspect he has found a way to make batteries work where nobody else could.
I suspect he has found... a federal tax subsidy :)