Britain’s Department of Energy and Climate Change have made a fun, Sim City-style app to fiddle with energy production and consumption choices. Your goal is to reduce carbon emissions by 80%. You can view my choices here.
Sadly, drastic densification of human habitat isn’t among the sliders. I will really have to get down to doing the numbers about that.
More energy needed: lighting, ventilation, cooling. presently about 6 kWh/d according to Without Hot Air. (Britain)
Less energy needed: transportation of people and goods, heating. about 75 kWh/d according to same.
If we can reduce the latter by as little as 20% and have to increase the first by 100%, we still end up with net energy savings.