Estimate the energy performance and environmental impact of your dwelling.
Mains Gas
Electricity (Grid)
Heating Oil
LPG
Estimated Energy Rating
SAP Score (0-100+)–
EPC Band–
Annual CO2 Emissions (kg/year)–
Primary Energy Demand (kWh/m²/yr)–
Understanding SAP Calculations for Dwellings
The Standard Assessment Procedure (SAP) is the UK government's methodology for assessing and comparing the energy and environmental performance of dwellings. Its primary purpose is to provide accurate and reliable assessments of dwelling energy performances that are needed to underpin energy and environmental policy initiatives.
How SAP Scores are Calculated
SAP calculations are not based on actual energy bills. Instead, they rely on the physical properties of the building. This tool provides a simplified estimate based on the key variables used in the SAP 10.2 framework:
U-Values: This measures the rate of heat loss through building elements (walls, roofs, windows). The lower the U-value, the better the insulation.
Thermal Bridging: Heat loss at junctions between building components.
Solar Gains: Heat gained through windows from the sun.
Heating Efficiency: How effectively your boiler or heat pump converts fuel into warmth.
SAP Rating vs. EPC Banding
The SAP score is converted into an Energy Performance Certificate (EPC) band. Ratings usually range from 1 to 100, though highly efficient homes with renewable exports can exceed 100.
SAP Score
EPC Band
92 – 100+
A (Very Efficient)
81 – 91
B
69 – 80
C
55 – 68
D
39 – 54
E
21 – 38
F
1 – 20
G (Poor Efficiency)
Why SAP Calculations Matter
If you are building a new home, extending a property, or converting a building in the UK, a SAP calculation is a legal requirement under Building Regulations (Part L). It ensures the building meets carbon emission targets and fabric energy efficiency standards. A high SAP score often translates to lower energy bills and a higher property market value.
3 Ways to Improve Your SAP Rating
Upgrade Insulation: Reducing the U-value of your walls and roof is the most effective way to retain heat.
Switch to Heat Pumps: As the electrical grid de-carbonizes, air-source heat pumps significantly boost SAP scores compared to traditional gas boilers.
Air Tightness: Reducing uncontrolled draughts ensures the heat produced by your system stays inside the thermal envelope.
function calculateSAP() {
// Inputs
var area = parseFloat(document.getElementById("floorArea").value);
var wallU = parseFloat(document.getElementById("wallUValue").value);
var windowU = parseFloat(document.getElementById("windowUValue").value);
var roofU = parseFloat(document.getElementById("roofUValue").value);
var fuelFactor = parseFloat(document.getElementById("fuelType").value);
var efficiency = parseFloat(document.getElementById("heatingEfficiency").value) / 100;
// Validate
if (isNaN(area) || isNaN(wallU) || isNaN(windowU) || isNaN(roofU) || isNaN(efficiency) || area <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
// Simplified SAP Logic
// 1. Calculate an average U-value weighted for a typical dwelling geometry
var avgU = (wallU * 0.5) + (windowU * 0.2) + (roofU * 0.3);
// 2. Estimate Space Heating Demand (kWh/year)
// Formula approximation: (Heat Loss Coefficient * Degree Days * 24) / Efficiency
// Heat Loss Coefficient (approx) = Area * AvgU * 2 (assuming height/shape factors)
var heatLossCoeff = area * avgU * 1.5;
var annualEnergyDemand = (heatLossCoeff * 2100 * 24) / 1000; // 2100 is approx degree days
// 3. Adjusted for Efficiency
var fuelConsumption = annualEnergyDemand / efficiency;
// 4. CO2 Emissions
var co2Emissions = fuelConsumption * fuelFactor;
// 5. SAP Score calculation (Simplified logarithmic scale based on cost/m2)
// National average cost per kWh is approx 0.10 for gas/oil, 0.30 for elec
var costPerKWh = (fuelFactor 100) sapScore = 100;
if (sapScore = 92) { band = "A"; color = "#008055"; }
else if (sapScore >= 81) { band = "B"; color = "#2db34b"; }
else if (sapScore >= 69) { band = "C"; color = "#89c541"; }
else if (sapScore >= 55) { band = "D"; color = "#f7ec13"; }
else if (sapScore >= 39) { band = "E"; color = "#f9b915"; }
else if (sapScore >= 21) { band = "F"; color = "#eb6625"; }
else { band = "G"; color = "#df1f26"; }
// Display Results
document.getElementById("resSAPScore").innerText = sapScore;
document.getElementById("resEPCBand").innerText = band;
document.getElementById("resEPCBand").style.backgroundColor = color;
document.getElementById("resCO2").innerText = Math.round(co2Emissions).toLocaleString();
document.getElementById("resEnergyDemand").innerText = Math.round(fuelConsumption / area).toLocaleString();
document.getElementById("sapResult").style.display = "block";
}