A building owner can use a realistic energy model to understandthe future building's energy performance. But what happens when the design energy model predicted energy use and the utility bills have a significant variance? Can adjusting the energy model to match the utility bill, or calibrating it, successfully identify the cause of the variance? Will this show strategies to bring the actual energy use back in line with the predicted energy use? This paper reviews the calibration effort of a medical office building where the actual energy use was significantly above the predicted energy use.