The Process Explained: How EPC Rating is Calculated
The Process Explained: How EPC Rating is Calculated
The Process Explained: How EPC Rating is Calculated

Property owners, landlords, and anyone looking to improve their home’s energy efficiency should understand how an Energy Performance Certificate (EPC) rating is calculated. An EPC measures a building’s energy performance and highlights opportunities for improvement, making it a key tool in reducing energy waste and carbon emissions. The process involves a detailed analysis guided by the Standard Assessment Procedure (SAP), from evaluating insulation and heating systems to assessing renewable energy installations.
This guide will take you step-by-step through how EPC ratings are determined, helping you make informed decisions about your property’s energy efficiency and compliance.
Key Takeaways
- EPC Assessment Factors: An EPC rating is calculated based on various factors, including insulation, heating systems, ventilation, renewable energy installations, and the building's overall energy performance. These are evaluated through an on-site inspection by accredited Domestic Energy Assessors.
- Standard Assessment Procedure (SAP): The EPC calculation uses the UK government's SAP methodology, which simulates energy use, estimates carbon emissions, and assigns a numerical score. This score is converted into a grade (A-G) to represent the property's energy efficiency.
- Grading and Recommendations: EPC ratings are visually represented on a color-coded scale, from A (most efficient) to G (least efficient). The certificate also includes tailored recommendations to improve energy performance, highlighting potential savings and environmental benefits.
What Is an EPC Rating and Why Is It Important?
An Energy Performance Certificate (EPC) rating measures a property's energy efficiency, assigning a grade from A (most efficient) to G (least efficient). It’s an essential tool for homeowners, landlords, and buyers to evaluate energy performance and identify potential improvements.
EPC ratings help reduce energy costs, lower carbon footprints and can enhance property value. They also ensure compliance with legal requirements, particularly for rental properties. By taking steps to improve your property’s EPC rating, you can not only save money but also contribute to sustainability.

How Is Data Collected During an EPC Assessment?
During a domestic energy assessment, data is collected through a detailed on-site inspection of the property. Here's how accredited assessors gather the necessary information:
Building Fabric Inspection
A domestic energy assessor will examine the walls, roof, windows, and floors to evaluate insulation levels and thermal efficiency. This includes assessing the type and thickness of insulation, glazing types, and construction materials.
Room Measurements
Dimensions of rooms and overall property size are measured to calculate heating and energy needs accurately. This information is crucial for determining the property's energy requirements.
Heating and Energy Systems Check
Boilers, heat pumps, radiators, and thermostats are inspected to assess their efficiency and performance. The age, type, and efficiency rating of heating systems are recorded.
Ventilation Analysis
Ventilation systems, including natural ventilation and mechanical systems like extractor fans, are reviewed for their ability to maintain air quality while considering their impact on energy efficiency.
Lighting Assessment
The types of light fittings and the proportion of low-energy lighting are noted as part of the overall energy efficiency analysis.
Renewable Energy Systems
If present, renewable energy systems such as solar panels or wind turbines are documented and factored into the assessment.
Visual Inspection and Standard Assumptions
The assessment is primarily based on visual inspection and measurements. Assessors use standard assumptions about occupancy and building use, not actual energy consumption patterns.
Data Recording
Assessors must retain all data used for the EPC calculation and evidence that the data relates to the assessed property. This information is typically input into specialised software that calculates the EPC rating.
It's important to note that assessors must visit each dwelling for which an EPC is produced. In some cases, they may use previously collected data if they have recently visited the property, but they must justify this approach and consider any potential changes since the last visit.
The entire process is designed to provide a standardised assessment of the property's energy efficiency, resulting in an EPC rating from A (most efficient) to G (least efficient).
It's important to note that the EPC rating calculated focuses on the potential energy efficiency of the property rather than actual energy consumption patterns. The assessment is carried out by an accredited Domestic Energy Assessor using specialised software that follows the Standard Assessment Procedure (SAP) for residential properties or Simplified Building Energy Model (SBEM) for non-domestic buildings.
What Is the Standard Assessment Procedure (SAP), and How Is It Used?
The Standard Assessment Procedure (SAP) is the UK government's methodology for calculating a building's energy efficiency and environmental impact for Energy Performance Certificates (EPCs). Here's how it works:
Data Input
Accredited assessors input detailed property data into SAP software. This includes information about the building's dimensions, insulation levels, heating systems, ventilation, and building fabric.
Energy Use Simulation
The SAP software simulates the property's energy use based on standardised occupancy and usage patterns. It considers factors such as the property's size, design, heating efficiency, and thermal characteristics.
Carbon Emissions Calculation
SAP calculates both the Dwelling Emission Rate (DER) and the Environmental Impact Rating (EIR). These metrics estimate the building's carbon dioxide emissions, helping determine its environmental impact.
Energy Costs Estimation
The methodology evaluates the property's estimated energy costs to gauge overall efficiency. However, it's important to note that SAP uses static baseline energy prices set in 2012, which may not reflect current energy costs.
Performance Scoring
SAP generates a numerical score between 1-100, which is then converted into an EPC grade from A (most efficient) to G (least efficient). This score reflects the property's overall energy efficiency.
Additional Key Points:
- SAP is used not only for EPCs but also to demonstrate compliance with Part L of the Building Regulations.
- For new dwellings and conversions, full SAP calculations are required. Existing homes use a slightly simplified version called RdSAP (Reduced Data SAP).
- SAP calculations only include 'regulated energy' use (heating, hot water, ventilation, and lighting) and don't account for unregulated energy use like appliances.
- SAP assessments can be performed at different stages of construction, including design stage and as-built stage.
The SAP methodology provides a standardised approach to assessing energy efficiency, allowing for consistent comparisons between properties across the UK.

How Are Energy Performance Certificate Ratings Graded and Presented?
EPC ratings provide a clear measure of a property's energy efficiency. Here's how they are determined and presented:
Grading Scale
Properties are assigned a grade from A to G based on their calculated energy efficiency. An A rating indicates exceptional performance, while G denotes poor energy efficiency.
Energy Efficiency Score
Each rating corresponds to a numeric score based on the Standard Assessment Procedure (SAP), with higher numbers indicating better energy efficiency. The specific ranges are:
- A rating: 92-100 SAP points
- B rating: 81-91 SAP points
- C rating: 69-80 SAP points
- D rating: 55-68 SAP points
- E rating: 39-54 SAP points
- F rating: 21-38 SAP points
- G rating: 1-20 SAP points
Visual Representation
The EPC certificate includes a color-coded bar chart, making it easy to compare the property's current rating with its potential rating if recommended improvements are implemented.
Recommended Improvements
The EPC lists recommendations for boosting efficiency, such as adding insulation or upgrading heating systems. Each recommendation includes estimated costs, potential savings, and the impact on the EPC rating.
Environmental Impact
A separate section shows the property's current and potential CO2 emissions, illustrating the environmental benefits of suggested upgrades.
Additional Key Points:
- EPCs are valid for 10 years from the date of issue.
- The average EPC score for UK households is grade D (60).
- EPCs use 'standard occupancy' assumptions, which may differ from actual usage patterns.
How Long Does the EPC Calculation Process Take?
The time required for an EPC assessment varies based on the property's size and complexity. Here's a breakdown of what to expect:
On-Site Inspection Time
A typical inspection for a standard property usually takes between 30 minutes to 1 hour. Larger or more complex buildings may require additional time, potentially up to 2 hours, for a thorough evaluation.
Data Analysis and Calculation
After the inspection, the assessor inputs collected data into specialised software to calculate the energy efficiency score. While the exact time for this step can vary, it's part of the overall process that leads to certificate issuance.
Certificate Issuance
Once calculations are complete, the EPC is generated and provided to the property owner. The entire process, from assessment to receiving the certificate, typically takes 1-3 days, though it can sometimes be up to a week. Some providers offer expedited services for faster delivery.
Preparation Tips
You can help streamline the process by preparing for your energy assessment. Ensure easy access to key areas like lofts, boilers, and visible insulation. Remember, the assessment is non-intrusive, so there's no need to move furniture or provide access within walls or under flooring.
Additional Key Points:
- The overall duration can vary based on factors like assessor availability and property complexity.
- The assessment is designed to be quick and minimally disruptive to occupants.
With proper preparation, you can receive your EPC promptly, helping you stay compliant and informed about your property's energy efficiency. Keep in mind that while the process is usually quick, allowing for some flexibility in timing can ensure a thorough and accurate assessment.