The most common use of building energy simulations, by far, is in the design of buildings, especially non-residential ones. It is a common perception that the simulations ought to be useful for many other applications, such as commissioning, control and diagnostics. A distinguishing feature of the latter applications is that they require linking with monitored data, and this link must be addressed before the applications can be realized. In linking with data, the following dilemma arises: only a small number of parameters can be reasonably estimated from typical building performance data whereas a realistically complex model of the building has a large number of parameters. This dilemma is resolved and the mathematical sophistication of microdynamic simulations (i.e., simulations such as DOE-2 which simulate each component) is brought to bear on the problem by a technique called PSTAR (Primary and Secondary Terms Analysis and Renormalization). This allows a simulation of the as-built building, and provides the required link with performance data. The results for a test building are presented. The role of the different heat flow terms in determining the building performance are elucidated. Applications to commissioning, control and diagnostics are outlined. Further developments necessary for field applications are pointed out.