The gap between predicted and actual building energy performance - often called the performance gap in green building literature - is well documented globally, and BIM-based simulation, while a genuine improvement over older methods, hasn't closed it entirely.
| Simulation approach | Typical variance from actual post-occupancy energy use |
|---|---|
| Design-stage energy simulation (based on assumed occupancy, standard usage patterns) | Often 20% to 40% variance, typically actual use exceeding prediction |
| Calibrated simulation (adjusted using early post-occupancy monitoring data) | Narrows to 10% to 20% variance once calibrated against real usage patterns |
Why the Gap Exists Even With Sophisticated BIM-Based Simulation
Design-stage simulations assume standardised occupancy schedules and behaviour patterns that real building occupants rarely follow exactly, and the model's inputs - not its underlying calculation method - are usually the larger source of this discrepancy. Facilities management practices, specifically whether HVAC systems are actually operated and maintained the way the design assumed, diverge from design assumptions in ways that no simulation run before occupancy could realistically predict in advance.
A Scenario Showing Why Calibration Closes Much of the Gap
Picture a building where the initial design-stage energy simulation showed a genuinely large discrepancy against actual post-occupancy consumption in its first year. Rather than treating that discrepancy as a simulation failure and moving on, the project team recalibrates the simulation using real monitored usage data from that first year - the recalibrated model then predicts subsequent years with considerably tighter accuracy, illustrating why post-occupancy monitoring and calibration should be treated as a deliberate follow-up step, not a one-time design-stage exercise whose accuracy is simply accepted or rejected once.