Thermal insulation would be nearly perfect and radiating excess heat can be passive, so there wouldn't be energy usage from that.
For reference, if you assume 1000 sq ft per person (maybe generous, but you have to account for transportation conduits, plumbing, food production, structural reinforcement) with 10 ft height, it's about 300 cubic meters. For a trillion people, that ends up being a 70km cube. I doubt there is any durable foundation on Pluto, and any structure would melt the ground it's (axiomatic thermodynamics) without active cooling, which increases the energy requirements.
Ultimately, I don't see why anyone would want to live packed into a small cube on Pluto eating engineered food. A trillion people packed in a cube underground in a block of gas ices isn't going to experience wonder and joy of exploration.