There is no question about the need of efficient, affordable and reliable batteries. Is his Powerwall a breakthrough? I don't know, time will tell. At least it is a push.
The following is an extract from Bloomberg:
The new Tesla Powerwall home batteries come in two sizes - 7 and 10 kWh - but the differences extend beyond capacity to the chemistry of the batteries. The 7kWh version is made for daily use, while its larger counterpart is only intended to be used as occasional backup when the electricity goes out. The bigger Tesla battery isn't designed to go through more than about 50 charging cycles a year...
Here’s where things get interesting. SolarCity, with Musk as its chairman, has decided not to install the 7kWh Powerwall that’s optimized for daily use. Bass said that battery "doesn't really make financial sense"...
But if its sole purpose is to provide backup power to a home, the juice it offers is but a sip. The model puts out just 2 kilowatts of continuous power, which could be pretty much maxed out by a single vacuum cleaner, hair drier, microwave oven or a clothes iron. The battery isn’t powerful enough to operate a pair of space heaters; an entire home facing a winter power outage would need much more..
But I would also be interested to know what kind of battery it is: lithium-ion or some other, Musk didn't mention it in his presentation. What is weight, and what are the dimensions? Do they require rare elements and their manufacturing is highly energy consuming as most of them? If yes, then their environmental benefits might be questionable.
The part about the day-night gap between demand and supply of solar energy is very similar to what I discuss in my presentations. Ascent Systems Technologies addresses it differently however. It uses vacuum tube solar thermal collectors which are more efficient then even the best PV modules, a thermal storage as a battery, and an air-to-water heat pump as a booster. The resulting integrated system is much more cost-effective comparing to a PV-battery system.