[soneil]

Car batteries simply aren't designed for deep-discharge. It actually damages (sulphates) them. They're designed to provide thousands of cycles at 2-5% discharge. Using them for deep discharge, you're likely to see them increasingly unusable (or at least, holding nowhere near the designed charge) after 30-150 cycles.

So that 420Wh battery is most likely designed to actually provide about 21Wh. Past that it's a good assumption it's lifespan is being degraded faster than design.

To match a 7kW powerwall with these figures, he'd need roughly 350 such batteries (nearly $23k) to stay within a 2-5% discharge cycle.

So his setup may be capable of delivering 420Wh cheap. Or it may be capable of lasting 1-2000 cycles. But not both.

(Deep-discharge SLA are available. They're usually marketed as marine batteries rather than car batteries. They'll survive such usage better - a couple of years, rather than a couple of months in this scenario. But still not tesla's claimed 5000 cycles.)