Model calibration

Overview of the model calibration process. (Source: IBPSA-USA BEM Workshop)

Energy model calibration is the process of adjusting energy model inputs such that the outputs align more closely with energy consumption measured in the real (operational) building. The calibration process consists of gathering data on the actual energy use in the real building after it has been operational, and using the data as the basis for adjusting the energy model inputs. Depending on the level of detail of collected operational data, the model input adjustment process and accuracy may vary.

Use cases

Model calibration can be important to ensure the model is useful for various purposes:

• Existing building retrofit analysis
• Measurement & verification
• Optimize performance

Questions that calibrated models can help answer

Calibrated models help provide an understanding of:

• How good are my modeling assumptions?
• Are the savings estimates reliable?
• Did the building perform as expected?
• Are there opportunities for operational improvements?

Calibration process

Model calibration is an iterative process that consists of:

• Developing an initial version of the existing building energy model
• Gathering information from the existing building to inform modifications to the inputs
• Updating the model based on the gathered information
• Running a simulation and checking whether the results more closely represent the actual performance of the existing building
• Repeating this process until the results are satisfactory

Data gathering sources

Information about the existing building may be gathered from a number of sources:

• Utility data
• Actual weather data
• As-built documents
• Submittals
• Operator interview
• Occupant survey data
• Site audit
• Building automation system
• Trend logs
• Energy submeters
• Short-term monitoring

This information should then be compared against inputs in the model such as:

• Actual occupancy schedules
• HVAC start/stop times
• VAV box minimum airflow setpoints
• Supply air temperature control method
• Set point temperatures
• Economizer operation
• Actual outdoor air ventilation rate
• Day, night, and weekend plug loads and lighting loads
• Exterior lighting, other miscellaneous loads

Example calibration process

The following images present a theoretical project where an energy model used to predict energy savings during the design process is calibrated post-occupancy to determine if savings were achieved, and to identify potential operation savings opportunities.

Fig. 1—During the design phase, a series of efficient design choices were made. The design model estimated that the building would achieve 19% savings compared to the code baseline. (Source: IBPSA-USA BEM Workshop)

Fig. 2—Post-occupancy, the modeled energy consumption estimates were compared against actual energy consumption in the building. This image shows a monthly comparison of electricity consumption, peak electric demand, and natural gas consumption. The charts indicate that the model underestimated electricity consumption and demand, and overestimated natural gas consumption. (Source: IBPSA-USA BEM Workshop)

Fig. 3—By reviewing detailed hourly data (modeled vs. actual), this chart shows that the actual building is consistently using more electricity during off-hours (unoccupied periods) compared to the estimates used in the original model. (Source: IBPSA-USA BEM Workshop)

Fig. 4—This image shows post-calibration comparison of the hourly electric demand. The calibration process involved making modifications to better represent electrical equipment in specialty spaces, and adjustments to off-hour loads and schedules. The calibrated model predictions now closely represent the actual energy consumption. (Source: IBPSA-USA BEM Workshop)

Fig. 5—After completing the calibration process, the estimated savings were recalculated. While the savings compared to the code baseline are reduced slightly, this exercise identified there there is a large potential for energy savings during unoccupied periods by turning off equipment at night. (Source: IBPSA-USA BEM Workshop)