Factsheet: Estimating Energy Use

Estimating annual energy use manually from fuel bills, meter readings or smart meter data

A simple assessment of energy use can be estimated using metered information or fuel bills, manually adjusted and weather-corrected using the CIBSE Degree Days method modelling. This method can be relatively quick and inexpensive and may be adequate for simple buildings, but it does not provide an accurate estimating tool for a specific building’s energy use.

As this is based on total metered data, this will include both regulated and unregulated loads. Depending on the metered information available the split between these loads is likely to need manual estimation using eg CIBSE degree day calculations.

• Regulated loads - Energy used for fixed building services, such as heating, cooling, ventilation, hot water, lighting, and fixed appliances. Typically covered in energy performance assessments like EPCs and subject to efficiency standards such as Building Regulations Part L.
• Unregulated loads - Energy used by systems and appliances that are not subject to mandatory energy efficiency standards or regulations. This includes plug-in appliances, office equipment, kitchen appliances, lifts, and occupant devices. Unregulated energy can constitute a large proportion of overall energy consumption, and are strongly influenced by occupant behaviour.

Energy Use Intensity (EUI)

The energy use intensity (EUI), is measured in kWh per square metre of floor area (GIA) per year (kWh/m²/yr). This is a metric that can be verified from meter readings when the building is in use. The EUI includes all energy used by the building, but excludes contributions from renewable generation. This ensures that energy used by the building is reduced as far as possible. EUI is most valuable when used to monitor progress within the same building or when compared to sector-specific benchmarks. It should not be used in isolation to judge performance across very different building types and uses.

Display Energy Certificate (DEC) software

Display Energy Certificate (DEC) software provides an estimate of annual energy use from fuel bills, with benchmarks that are adjusted for use, occupancy data, weather and climate.

The method follows CIBSE TM47 and provides a more formalised approach including a rating that enables comparison against other buildings. DEC software may be applied to buildings of any size but cannot easily be used to support an improvement option evaluation.

Quasi steady state models

These models tend to be the simplest and quickest to run. The information required to run these models is often averaged over larger periods of time (months or years) or space (i.e. less zoning) when they are compared to dynamic models. Steady State Models are very useful when a building has simple controls i.e. not reliant on complex interdependent systems or controls to maintain the required environmental conditions.

Simplified Building Energy Modelling (SBEM)

Simplified Building Energy Method (SBEM) software, e.g. iSBEM, is the methodology used for Energy Performance Certificate and Building Regulations compliance assessments, which can be applied to most non-domestic buildings. This is a simplified thermal model, which takes into account an approximation of building geometries and fabric performance; internal and external heat gains; and static assumptions around occupant behaviour and other conditions.

Only regulated loads are included in the calculations.

Passivhaus Planning Package (PHPP)

Passive House Planning Package (PHPP) software (available from the Passive House Institute), is a spreadsheet-based (static) energy balance tool that calculates a building’s annual energy demand based on inputs relating to the building’s physical characteristics and local climate.

PHPP was developed primarily for high-performing domestic residential buildings, but can also be used to verify the energy performance of existing buildings, including EnerPhit retrofits. EnerPhit is a Passivhaus refurbishment standard, which enables certification of a quality-approved energy retrofit with Passive House components in existing buildings.

Both regulated and unregulated loads can be incorporated in the calculations.

Dynamic Simulation Modelling (DSM)

For complex buildings or where a greater level of accuracy is required, Dynamic Simulation Modelling (DSM) can be used to simulate building performance over time. As they model the building behaviour at a granular level, they can explore the performance and interactions between occupancy and more complicated heating, cooling and ventilation systems. DSM models incorporate localised future weather files and can be used for future proofing and resilience considerations in line with anticipated changes in climate such as overheating.

DSM models incorporate more detailed information about the building’s geometry, materials,  systems, and use to provide hour-by-hour dynamic simulations of building performance. This can be used to demonstrate energy compliance, overheating analysis, daylighting analysis and ventilation.  

DSM models should be the most accurate method of estimation, but is more data intensive, costly and time-consuming to build and validate. The use of Dynamic Simulation Modelling is increasingly integrated into the early stages of the design process and depending on the scale and complexity of the project, can prove to be more cost effective and time efficient.

Both regulated and unregulated loads can be incorporated in the calculations.

Evaluating operational energy performance of buildings

TM54 is a methodology published by the Chartered Institute of Building Services Engineers (CIBSE), which provides best practice methods to evaluate the operational energy use of buildings. TM54 can be used to provide a framework for carrying out calculations, but is not prescriptive over which methods above are used.