Conventional wisdom says each person requires ~1 acre (43k sq-ft) to source enough foodstuff for themselves.<p>With two people per 1k sq-ft apartment... you need ~100-fold improvement over the conventional wisdom to get full 100% sustainability.  So let&#x27;s assume you can magically get 10x gains in &quot;land use&quot; efficiency, and that you can stack those units 10 levels high (piping 1&#x2F;10 of the 1kW&#x2F;m^2 solar incidence to each level). But herein lies the rub: Urban density exceeds 2 people per 1k sq-ft of building rooftop area.  So 100% rooftop sustainability is probably a no go.<p>Of course something is better than nothing (is it truly better than rooftop solar?), but it&#x27;s worth pointing out that it can&#x27;t be a panacea.