If it charges and discharges once per day how long will it last?

Usually lithium batteries lose 20% of their capacity after only 1000 charges, so logically it will last less than 3 years but this is energy storage not an EV and capacity is not fundamental even if you store less.
But what concerns me is "2 billion power packs could supply energy to the entire world, transportation included": There is a requirement of around 320 g of lithium per kWh. Using the smallest pack at 7 kWh, we need 448 billion kg or 448 million tonnes of lithium, however reserves are estimated at around 39 millions tonnes...
Looks like there will be a significant lack of it so you'll have to choose between storage or transportation.

Sorry , made a mistake in my comment, 2 billions 7 kWh pack will need 4.48 million tonnes, while 2 billions 100 kWh pack which are more likely to be useful in real world applications requires 64 millions tonnes.

Musk for president! This man certainly has big ideas.
Payback time for the unit has to be taken into consideration, especially for domestic use. As solar panels are DC, I don't understand the point about a DC to DC converter. Is this an error?
I wonder if any research is being done into finding another metal apart from lithium. Aluminium might be possible. This is much more abundant.

@Jeremy:
Apparently Tesla is offering a 10-year warranty on this battery pack. Which isn't really much of a surprise considering the warranty they offer on their car's pack is also 10 years.
I'm very impressed with the price.

I really am impressed by both the technology and the price. I am certainly going to be talking them up with friends and family.
The only thing that won't work, at least here in crime ridden San Francisco is the idea of mounting them on the outside walls of homes. Seems to me that the folks currently swiping copper wiring would find them very, very tempting.

Lol our local power company is now pushing for a min charge for people using less power! Wth i am so happy to be off grid

@windykites1:
Yes this should be an error, solar is 12V DC and your plug is 110/220V AC (depending on where you live) so this should actually be an AC/DC inverter. The problem with lithium is that it's the best metal for making battery because of its lightness and highest electric potential of 3V, but it comes at the price of relative scarcity. There is also the fact that South America holds 80% of the reserves and in a way, we will be switching from Middle Eastern oil to Bolivian lithium (of course lithium is recyclable but you still need to buy and extract it in the first place). There are commercial sodium-ion battery made by Aquion Energy and it seems potassium-ion are also available, but both don't have lithium's performance for the reasons I mentioned.
@Milton:
The lifetime I mentioned was for deep charge/discharge cycles (a process that is very demanding on batteries), you usually don't do that with this kind of home storage, you just top off what is missing, so the lifespan should be much higher. This is why, I think, they can warranty them for 10 years. The price is actually much higher than conventional batteries, this battery comes at a pricetag of 500$/kWh whereas others are usually around 250$/kWh, but this is explained by the fact there is not only a battery but an inverter, a cooling system and a software. They also don't mention the installation costs which will add to the cost.

Maybe this would help http://www.gizmag.com/graphene-based-supercapacitor/28579/

Expensive and worthless... Don't be suckered...