What energy modelling and simulation actually involves

Energy modelling uses validated software tools to predict how a building will consume energy under defined operating conditions, simulating the interaction between climate, envelope performance, mechanical and electrical systems, occupancy patterns, and internal gains. Models are used at design stage to test specification choices before construction, and for existing buildings to identify where performance deviates from design intent or where retrofits would have the greatest impact. The output is typically an energy use intensity figure (kWh/m²/year), a breakdown by end use, and a comparative analysis of design alternatives or retrofit scenarios.

Why it's harder in practice than it looks

Model quality depends on input data quality

An energy model is only as reliable as its inputs. Inaccurate occupancy assumptions, underspecified HVAC systems, or incorrect fabric performance values produce models that look precise but are not. Design-stage models frequently assume theoretical performance values that are not achieved in practice, a gap known as the performance gap.

The performance gap between design and built reality is significant and persistent

Post-occupancy studies consistently show that real buildings perform 30 to 50 percent worse than predicted by design-stage energy models. This gap arises from poor commissioning, occupant behaviour diverging from design assumptions, and equipment operating differently in practice than in the simulation. Addressing the performance gap requires operational monitoring data, not just design models.

Regulatory compliance models and performance models serve different purposes

Building Regulations compliance modelling (SAP for dwellings, SBEM for commercial buildings) uses standardised assumptions that do not reflect actual occupancy or operation. Using compliance models for operational performance benchmarking or green finance purposes without understanding this distinction produces misleading results.

Software tool choice affects methodology and results

Different modelling tools (EnergyPlus, IES-VE, DesignBuilder, TAS) have different methodological approaches, default assumptions, and user interfaces. Results are not directly comparable between tools, and choosing the appropriate tool for the building type and assessment purpose requires technical expertise.

What good looks like

High-quality energy modelling is underpinned by rigorous data collection, uses validated inputs calibrated against actual or typical performance data, clearly distinguishes between regulatory compliance models and operational performance models, and is supported by sensitivity analysis that shows how results change under different assumptions. Models used for investment decisions or green finance purposes should be independently reviewed by a qualified energy modeller. Post-construction, design models are updated with as-built data and calibrated against metered performance data.

When to bring in external support

Energy modelling requires specialist software expertise and deep knowledge of building physics that is typically not available within generalist facilities or property teams. Leafr's network includes qualified energy modellers with experience across commercial, residential, and industrial buildings, providing models that support BREEAM assessments, LEED compliance, EPC production, and decarbonisation roadmap development.

Frequently asked questions

What software tools are used for energy modelling?

Common tools include IES-VE, DesignBuilder, EnergyPlus, and TAS for detailed dynamic simulation. SBEM (Simplified Building Energy Model) and SAP (Standard Assessment Procedure) are the UK's regulatory compliance calculation methods for commercial and residential buildings respectively. Each tool has different capabilities, appropriate use cases, and levels of complexity, tool selection should be driven by the specific assessment purpose and building type.

What is dynamic thermal simulation and when is it required?

Dynamic thermal simulation (DTS) models building energy performance hour by hour over a full year, accounting for thermal mass, solar geometry, and the dynamic interaction between climate and the building envelope. It provides more accurate results than simplified steady-state methods but takes longer to produce. DTS is typically required for BREEAM Excellent and Outstanding ratings, complex buildings with atypical geometries or systems, and for overheating assessments under UK Building Regulations Part O.

What is the performance gap in building energy modelling?

The performance gap is the difference between predicted energy use from design-stage modelling and actual measured energy use in operation. Studies consistently show actual buildings consuming 30 to 50 percent more energy than design models predict. Common causes include commissioning failures, occupancy patterns diverging from design assumptions, and equipment operating inefficiently. Post-occupancy monitoring and calibrated models are used to identify and address the gap.

How is energy modelling used in green building certification?

BREEAM requires energy modelling to score credits in the Energy category, including energy performance targets and renewable energy assessments. LEED's Energy and Atmosphere category requires whole-building energy simulation to demonstrate performance against a baseline model. Both schemes require modelling by a suitably qualified professional with documentation supporting the model inputs and methodology.

Can energy modelling support decarbonisation planning for existing buildings?

Yes. Calibrated models of existing buildings, built using actual metering data and operational records, can be used to evaluate retrofit scenarios, quantify emissions reductions from different interventions, and support investment decisions. This is particularly valuable for large property portfolios where decisions about heat pump installations, fabric improvements, or renewable energy systems need to be prioritised across multiple sites with limited capital budget.