The use of the bin method for energy calculations in heated and cooled buildings entails a compromise between the calculation time taken and the increased accuracy of using a larger numbers of bins. A deterministic expression has been developed which can be used to predict the error attributable to different degrees of discretization. The expression was tested using two typical calculation problems, which were run using both binned input data and the parent time-series data files. The expression was found to give a satisfactory prediction of the error due to discretization.