Detached House
Semi-Detached House
Mid-Terrace House
Flat (Mid-Floor/Top Floor)
Building Fabric U-values (W/m²K)
Lower U-values indicate better insulation.
Air Tightness & Heating System
Mains Gas
Electricity (Direct)
Heating Oil
Air Source Heat Pump
For ASHP, this can be >100% (e.g., 300% for COP 3.0).
Renewable Energy
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max-width: 700px;
margin: 30px auto;
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text-align: center;
margin-bottom: 25px;
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margin-bottom: 15px;
font-size: 1.3em;
border-bottom: 1px solid #eee;
padding-bottom: 5px;
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.calc-input-group {
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flex-direction: column;
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.calc-input-group label {
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width: 100%;
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.calc-input-group input[type="number"]:focus,
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.calc-input-group .calc-description {
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color: #666;
margin-top: 5px;
line-height: 1.4;
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.calc-button {
display: block;
width: 100%;
padding: 12px 20px;
background-color: #28a745;
color: white;
border: none;
border-radius: 6px;
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font-weight: bold;
cursor: pointer;
transition: background-color 0.3s ease, transform 0.2s ease;
margin-top: 25px;
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.calc-button:hover {
background-color: #218838;
transform: translateY(-2px);
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.calc-button:active {
transform: translateY(0);
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.calc-result {
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.calc-result p {
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.sap-band-A { background-color: #4CAF50; color: white; padding: 5px 10px; border-radius: 5px; font-weight: bold; }
.sap-band-B { background-color: #8BC34A; color: white; padding: 5px 10px; border-radius: 5px; font-weight: bold; }
.sap-band-C { background-color: #CDDC39; color: #333; padding: 5px 10px; border-radius: 5px; font-weight: bold; }
.sap-band-D { background-color: #FFEB3B; color: #333; padding: 5px 10px; border-radius: 5px; font-weight: bold; }
.sap-band-E { background-color: #FFC107; color: #333; padding: 5px 10px; border-radius: 5px; font-weight: bold; }
.sap-band-F { background-color: #FF9800; color: white; padding: 5px 10px; border-radius: 5px; font-weight: bold; }
.sap-band-G { background-color: #F44336; color: white; padding: 5px 10px; border-radius: 5px; font-weight: bold; }
function calculateSapEstimate() {
// Input values
var dwellingType = document.getElementById("dwellingType").value;
var floorArea = parseFloat(document.getElementById("floorArea").value);
var wallUValue = parseFloat(document.getElementById("wallUValue").value);
var roofUValue = parseFloat(document.getElementById("roofUValue").value);
var floorUValue = parseFloat(document.getElementById("floorUValue").value);
var windowUValue = parseFloat(document.getElementById("windowUValue").value);
var airPermeability = parseFloat(document.getElementById("airPermeability").value);
var mainHeatingFuel = document.getElementById("mainHeatingFuel").value;
var heatingEfficiency = parseFloat(document.getElementById("heatingEfficiency").value);
var solarPV_kWp = parseFloat(document.getElementById("solarPV_kWp").value);
// Validate inputs
if (isNaN(floorArea) || floorArea <= 0 ||
isNaN(wallUValue) || wallUValue <= 0 ||
isNaN(roofUValue) || roofUValue <= 0 ||
isNaN(floorUValue) || floorUValue <= 0 ||
isNaN(windowUValue) || windowUValue <= 0 ||
isNaN(airPermeability) || airPermeability <= 0 ||
isNaN(heatingEfficiency) || heatingEfficiency <= 0 ||
isNaN(solarPV_kWp) || solarPV_kWp ACH
// A more common simplification for ACH from air permeability is to divide by 20 for a typical house.
// Let's use a simpler factor for demonstration:
var effectiveACH = airPermeability / 20; // Very rough estimate for ACH
if (effectiveACH < 0.5) effectiveACH = 0.5; // Minimum ventilation rate
var qVentilation = dwellingVolume * effectiveACH * airDensitySpecificHeat;
// 3. Total Heat Loss Coefficient (W/K)
var qTotalLossCoeff = qFabric + qVentilation;
// 4. Annual Space Heating Demand (kWh/year)
var annualHeatingDemand_kWh = (qTotalLossCoeff * degreeDays * 24) / 1000;
// 5. Hot Water Demand (kWh/year)
var annualHotWaterDemand_kWh = hotWaterDemandFactor_kWh_per_year;
// 6. Total Gross Energy Demand (before efficiency & renewables)
var totalGrossDemand_kWh = annualHeatingDemand_kWh + annualHotWaterDemand_kWh;
// 7. Energy Supplied by Renewables (Solar PV)
var solarPVGeneration_kWh = solarPV_kWp * solarPV_kWh_per_kWp;
var netDemand_kWh = totalGrossDemand_kWh – solarPVGeneration_kWh;
if (netDemand_kWh < 0) netDemand_kWh = 0; // Cannot have negative demand
// 8. Actual Energy Consumption (after heating system efficiency)
var actualConsumption_kWh = netDemand_kWh / (heatingEfficiency / 100);
// 9. Annual Energy Cost (£)
var currentFuelCost_pence_per_kWh = fuelCosts[mainHeatingFuel];
var annualCost = (actualConsumption_kWh * currentFuelCost_pence_per_kWh) / 100; // Convert pence to pounds
// 10. Annual CO2 Emissions (kgCO2)
var currentCO2Factor_kg_per_kWh = co2Factors[mainHeatingFuel];
var annualCO2 = actualConsumption_kWh * currentCO2Factor_kg_per_kWh;
// 11. Infer SAP Band (Very rough mapping based on annual energy cost)
// This is a highly simplified mapping and not a direct SAP score calculation.
var sapBand = "G";
var sapBandClass = "sap-band-G";
if (annualCost <= 500) { sapBand = "A"; sapBandClass = "sap-band-A"; }
else if (annualCost <= 800) { sapBand = "B"; sapBandClass = "sap-band-B"; }
else if (annualCost <= 1200) { sapBand = "C"; sapBandClass = "sap-band-C"; }
else if (annualCost <= 1800) { sapBand = "D"; sapBandClass = "sap-band-D"; }
else if (annualCost <= 2500) { sapBand = "E"; sapBandClass = "sap-band-E"; }
else if (annualCost <= 3500) { sapBand = "F"; sapBandClass = "sap-band-F"; }
else { sapBand = "G"; sapBandClass = "sap-band-G"; }
// Display results
var resultDiv = document.getElementById("result");
resultDiv.innerHTML =
"
Estimated Energy Performance
" +
"Estimated Annual Energy Cost: £" + annualCost.toFixed(2) + "" +
"Estimated Annual CO2 Emissions: " + annualCO2.toFixed(0) + " kgCO2″ +
"Inferred SAP Band:" + sapBand + " (Based on estimated annual cost)" +
"Note: This is a highly simplified estimate and not a full SAP calculation. Actual SAP ratings involve many more variables and detailed methodologies.";
}
Understanding SAP Energy Ratings for Your Home
The Standard Assessment Procedure (SAP) is the methodology used in the UK to assess and compare the energy and environmental performance of dwellings. It's a crucial tool for demonstrating compliance with building regulations and for generating Energy Performance Certificates (EPCs).
What is a SAP Rating?
A SAP rating is a numerical score ranging from 1 to 100+ (though typically 1-100), with higher numbers indicating better energy efficiency and lower running costs. This score is then translated into an energy efficiency band from A (most efficient) to G (least efficient), which you see on an EPC. The rating considers various factors to calculate a dwelling's annual energy cost for space heating, water heating, ventilation, and lighting, as well as its CO2 emissions.
Why is Your SAP Rating Important?
Building Regulations Compliance: All new dwellings and certain conversions/extensions must achieve a minimum SAP rating to comply with Part L of the Building Regulations.
Energy Performance Certificates (EPCs): A SAP calculation is the basis for the EPC, which is legally required when a property is built, sold, or rented.
Energy Bills: A higher SAP rating generally means lower energy bills, as the property is more efficient at retaining heat and using energy.
Environmental Impact: Better SAP ratings correlate with lower carbon emissions, contributing to environmental sustainability.
Property Value: Energy-efficient homes are increasingly attractive to buyers and tenants, potentially increasing property value and marketability.
Key Factors Influencing a SAP Rating
A comprehensive SAP calculation takes into account a wide array of building characteristics and systems. Our simplified calculator focuses on some of the most impactful elements:
Building Fabric (U-values): This refers to the insulation levels of the walls, roof, floor, and windows. A U-value measures how much heat is lost through a given area of a building element. Lower U-values mean better insulation and less heat loss.
Walls: Modern cavity walls with insulation or solid walls with external/internal insulation.
Roofs: Loft insulation, warm roofs, or cold roofs.
Floors: Insulated suspended timber floors or solid concrete floors.
Windows: Single, double, or triple glazing, and the quality of the window frames.
Air Tightness (Air Permeability): This measures how much uncontrolled air leaks in and out of a building. A lower air permeability value (e.g., 3-5 m³/h.m² @50Pa for new builds) indicates a more airtight home, reducing heat loss through drafts.
Main Heating System & Fuel: The type of heating system (e.g., gas boiler, electric heating, heat pump) and the fuel it uses significantly impact both energy costs and CO2 emissions. Modern, efficient boilers and renewable heating systems like air source heat pumps (ASHP) can drastically improve a SAP rating.
Renewable Energy Generation: Systems like Solar Photovoltaic (PV) panels, which generate electricity, or solar thermal panels, which heat water, contribute positively by reducing the dwelling's reliance on grid energy.
Dwelling Type & Size: The overall floor area and the dwelling type (detached, semi-detached, flat) influence the exposed surface area and thus the potential for heat loss.
Hot Water System: The efficiency of the hot water generation and storage system.
Ventilation: While essential for indoor air quality, uncontrolled ventilation can lead to heat loss. Mechanical Ventilation with Heat Recovery (MVHR) systems can improve efficiency.
Thermal Bridging: Heat loss that occurs at junctions between different building elements (e.g., wall-floor junctions).
Limitations of This Simplified Calculator
It's important to understand that this calculator provides a directional estimate based on a few key inputs and simplified assumptions. A full SAP calculation is a complex process performed by accredited assessors using specialized software. It involves hundreds of data points, detailed geometry, monthly climate data, internal gains, shading, and much more. Therefore, the results from this tool should not be taken as an official SAP rating or EPC score, but rather as an educational guide to understand the impact of different building characteristics on energy performance.
How to Improve Your SAP Rating
Based on the factors above, here are common strategies to improve a property's SAP rating:
Increase Insulation: Upgrade wall, roof, and floor insulation to achieve lower U-values.
Improve Windows and Doors: Install high-performance double or triple glazing.
Enhance Air Tightness: Seal gaps and cracks to reduce uncontrolled air leakage.
Upgrade Heating System: Replace old, inefficient boilers with modern condensing boilers, or consider renewable heating solutions like heat pumps.
Install Renewables: Add solar PV panels or solar thermal systems.
Consider Ventilation with Heat Recovery: Install MVHR systems to maintain air quality while recovering heat.
Reduce Thermal Bridging: Ensure good design and construction practices at junctions.
By understanding these core principles, you can make informed decisions to improve the energy efficiency of your home, reduce running costs, and contribute to a greener environment.