Interesting metric. I recently came across this team [1] in Seattle that is building what you mention. Disclaimer: after coming across the team I decided to invest in their equity crowdfunding effort.
However the form factor is a definite winner, along with the stacking. I;m not sure about the integration into solar. But for what it is, a portable power block, its very nice.
On a side note, I would love to see some weather proof alternative to a tesla powerwall. I dislike Tesla as a company, and it pains me to see that they only decent product out there being owned by them