content: Firstly, it's not about making them net exporter, it's about lowering their total usage, which when combined with modern building techniques creates a significant amount of energy use reduction thus improving their efficiency. Secondly, inverse square law, which governs things like vector fields at a distance from a source, doesn't apply here since the building is far enough from the sun that, for intents and purposes, all points on its surface are effectively equidistant from the source. Third, there is no such law (or a rule of thumb) governing the size of buildings and their efficiency. Heating and cooling load estimating is based on square footage (ventilation), surface area solar exposure (which depends on location and site conditions), psychrometric conditions (yes that's a real word), construction materials, lighting, paint (impacts radiation due to emissivity of surface), room usage, number of occupants, and even internal room physical layout.  Buildings are complex thermodynamic and mechanical systems that you can't simply boil down to a simple estimating metric. Obviously larger constructs will have more surface and floor area, necessitating some level of increased power usage, but careful design and planning mitigates these factors. Arguably the greatest cause for energy loss in a building isn't in its construction, but it's maintenance and upkeep, as things within the building will change over time from their original intended design.  Even with the increase in size of a building and its accompanying increase in power usage, applying thin film solar cells, or solar cells in general, to the outside of the building will still provide mitigation to power usage due to the increased surface area. Perhaps there's some level of diminishing returns, but until it is mathematically modeled we can only rely on the science and engineering we know.