I'd imagine in this case they are looking at the economy of scale they'll be able to gain when producing these out of the gigafactory. I'd be interested to see the $/Wh cost difference once it comes fully online.
Tesla batteries retail for $30K@85KWh, so approximately $350/Wh, Lead-acid deep discharge is about $10000/100KWh, so approximately $100/Wh. That's a pretty large gap.
Keep in mind that $30K for 85KWh is pre-gigafactory and is also for a vehicle. There are many more safety factors involved in making a battery that is going to moving around at 70mph. Much of the cost of the vehicle based batteries, may be able to be skipped, if you are not as concerned with mobile safety and weight.
Fair enough. But the pros/cons of each tech would need to be balanced against the $/Wh cost. Once the gigafactory is fully online I thought I'd read they were shooting to drop the pack cost in half? If that were true then it'd be 175 vs 100 and the pro/con list would be much more important, right?
No, the costs are better expressed in terms of price divided by total KWh stored and subsequently recovered over the lifetime of the battery.
So even if they can get the 'installed' price down to < 30% of what it is today (some corners can be cut for stationary applications) then there is still another barrier to be crossed.
All in all this is extremely exciting because manufacturing batteries at this scale will surely lead to economies unseen before but Lead-Acid has an 80 year head-start and is very hard to beat when weight and density are not a major factor.
After all the one reason why we have Lithium-ion is because of weight and power density.
Laptop and vehicles have a lot in common that houses versus laptops and vehicles do not.
The Tesla Model 3 car, due in a few years with a fully online gigafactory, is supposed to retail for $35k I think. This certainly means that Elon is expecting a price drop in the (car) batteries by quite a significant amount - perhaps even down to a price competitive with Lead-acid for the (home) batteries discussed here.
Might want to check your numbers there. $10,000 / 100 kWH == $0.10 per Wh.
Which you absolute cannot get in any market. If you could, everyone would do it because the supply cost per kWh including replacement would beat regular power company power.
EDIT: Just went back over my old calculations for this. The basic problem is that you trade off against the cost of peak electricity, not your solar.
So you can essentially assume off-peak and shoulder power is used for charging, and then you use that to offset your most expensive period. The question is then "how efficient is charging" and "how many cycles do you get from the battery before replacement".
Even at $100/kWh, the math is a near miss rather then a clear win as far as I can tell still.