- This paper explores the limiting energy efficiency for the energy uses in a particular office building. This limit might be viewed as the "Carnot efficiency" for the entire building. It assumes that all energy-consuming services in the building are provided at the minimum energy value that does not violate physical law. Rigorous physical limits such as the Carnot efficiency are used where applicable; in other cases, a plausible approximation has been adopted. Based on the assumptions made, it would be possible to provide all of the energy-based services currently provided in the building using approximately 0.2kWh/(ft2-yr). This limiting value is less than 1% of the energy used by a typical office building in the United States. Examination of expected advances in individual technologies suggests that it may be possible to construct a building that uses approximately 0.8kWh/(ft2-yr) within the next decade. 2011 American Institute of Physics.