The interior of a NEST is surprisingly bright. That somehow makes it worse. At least if its dark, you're always wary, and searching. During the day you can get complacent - or worse yet, miss something in the shadows that are cast.
A large part of energy efficiency, is not needing energy in the first place. Most of the arcos are not one monolithic block, but a series of smaller inter-linked towers, each of which is narrow enough that natural light can penetrate all - or at least most of - the way into the central core. Some use mirrors, some fiber optics, and there are a few that really are pitch-black in many spots.
Given the massive expense and time it takes to create these things - it becomes evident that shortcuts and stop gaps were made at various times, so be aware that some levels aren't always the most logical.
Anything that can be acquired by a quick "smash and grab" run, probably has. Instead, the endeavor must be approached like climbing a mountain. Clear a base camp, and set up stopping points along the way. If you can, get a guide. Just like on Everest - oxygen tanks might not be a bad idea - there are all sorts of internal pollutants and rot. Rappelling gear can help avoid certain areas for those who don't mind scaling objects two hundred metres or so above ground.
Reanimates can hibernate when no prey is around. It might be hard for the lesser variants to chase people into an arco and navigate the labyrinth, but there are still huge numbers just waiting to wake up.
Given the danger and complications of mounting a several day - or even several week long - expedition into an arco - most of them are still quite full of untouched supplies and artifacts. Food is probably going to be the limiting factor on any given adventure, since anything stocked in the building has probably expired. Aside from that, artifacts in the interior areas, up to and including national guard armories and nano-vac hospital stocks may very well be still in place.
A possible way to start a game is to either have players as survivors who are trying to get out of an Arco, they've been living is, now that all the supplies in nearby apartments have been used up. Another would be a large group of reanimates chasing them into the structure - leaving the players with little choice but to go up and find a way out that isn't blocked.