The article never frames this as off-grid storage for individual homeowners. They say

> A typical site is a factory, power plant, or town with a large earthen mound at the edge. The mound might be the size of a house for a smaller factory, and up to many football fields for a large power plant. Surrounding the earthen mound will be high-density, low-profile solar arrays.

I agree that trying to give every suburban house its own rock pile would not be very practical.