States that the computer simulation of existing buildings presents unique problems and a high level of accuracy can only be achieved through optimisation of three factors - 1) an intimate understanding of the simulation tool, 2) an intimate understanding of the building to be simulated and 3) careful analysis and critique of output data. Optimisation of these factors regularly produces computer simulations within 5% of the measured utility consumption. Presents examples of the optimisation means. In addition, discusses one mainframe-based and two spreadsheet-based simulation tools in terms of their characteristics, proper application and relative cost of use. Provides further discussion of the use of computerised simulation as a critical component of an energy services contract (shared savings/demand side management).

KEYWORDS: Buildings, calculating, models, energy supply and use, energy conservation, accuracy