Window U-Factor and Solar Heat Gain Coefficient Savings Calculator

ANA›Life Services Authority›National Calculator Authority›Window U-Factor and Solar Heat Gain Coefficient Savings Calculator

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Window U-Factor and Solar Heat Gain Coefficient Savings Calculator

Estimate annual heating and cooling energy savings when upgrading windows to lower U-Factor and SHGC values.

Total Window Area (sq ft)

Existing Window U-Factor (BTU/hr·ft²·°F)

New Window U-Factor (BTU/hr·ft²·°F)

Existing SHGC (0–1)

New SHGC (0–1)

Heating Degree Days (°F·days/year)

Cooling Degree Days (°F·days/year)

Annual Solar Irradiance on Windows (BTU/ft²/year)

Heating System Efficiency (fraction, e.g. 0.80 for 80% furnace)

Cooling System COP (e.g. 3.0 for SEER 10 AC)

Heating Energy Cost ($/MMBtu)

Electricity Cost ($/kWh)

Calculate Savings

function winCalc() { var resultDiv = document.getElementById('win-result');

var area = parseFloat(document.getElementById('win-area').value); var uOld = parseFloat(document.getElementById('win-u-old').value); var uNew = parseFloat(document.getElementById('win-u-new').value); var shgcOld = parseFloat(document.getElementById('win-shgc-old').value); var shgcNew = parseFloat(document.getElementById('win-shgc-new').value); var hdd = parseFloat(document.getElementById('win-hdd').value); var cdd = parseFloat(document.getElementById('win-cdd').value); var solar = parseFloat(document.getElementById('win-solar').value); var heatEff = parseFloat(document.getElementById('win-heat-eff').value); var coolCOP = parseFloat(document.getElementById('win-cool-eff').value); var heatCost = parseFloat(document.getElementById('win-heat-cost').value); var elecCost = parseFloat(document.getElementById('win-elec-cost').value);

// --- Validation --- var errors = []; if (isNaN(area) || area 0."); if (isNaN(uOld) || uOld 2.0) errors.push("Existing U-Factor must be between 0.10 and 2.00."); if (isNaN(uNew) || uNew 2.0) errors.push("New U-Factor must be between 0.10 and 2.00."); if (!isNaN(uOld) && !isNaN(uNew) && uNew >= uOld) errors.push("New U-Factor should be less than existing U-Factor for savings."); if (isNaN(shgcOld) || shgcOld 1) errors.push("Existing SHGC must be between 0 and 1."); if (isNaN(shgcNew) || shgcNew 1) errors.push("New SHGC must be between 0 and 1."); if (isNaN(hdd) || hdd = 0."); if (isNaN(cdd) || cdd = 0."); if (isNaN(solar) || solar 0."); if (isNaN(heatEff) || heatEff 1.0) errors.push("Heating efficiency must be between 0.50 and 1.00."); if (isNaN(coolCOP) || coolCOP 8.0) errors.push("Cooling COP must be between 1.0 and 8.0."); if (isNaN(heatCost) || heatCost 0."); if (isNaN(elecCost) || elecCost 0.");

if (errors.length > 0) { resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }

// --------------------------------------------------------------- // HEATING SAVINGS (Conduction) // Q_heat_old = U_old * Area * HDD * 24 [BTU/year] // Q_heat_new = U_new * Area * HDD * 24 [BTU/year] // Conduction heat loss reduction [BTU/year] // --------------------------------------------------------------- var heatCondOld = uOld * area * hdd * 24; // BTU/year var heatCondNew = uNew * area * hdd * 24; // BTU/year var heatCondSaving = heatCondOld - heatCondNew; // BTU/year

// --------------------------------------------------------------- // HEATING SAVINGS (Solar Gain — lower SHGC means less free solar heat in winter) // In heating season, solar gain is beneficial; lower SHGC reduces it. // Solar heat gain = SHGC * Area * Solar_irradiance [BTU/year] // Fraction of solar in heating season ≈ HDD/(HDD+CDD) (simplified) // --------------------------------------------------------------- var totalDD = hdd + cdd; var heatFrac = (totalDD > 0) ? hdd / totalDD : 0.5; var coolFrac = (totalDD > 0) ? cdd / totalDD : 0.5;

var solarGainOld_heat = shgcOld * area * solar * heatFrac; // BTU/year (beneficial) var solarGainNew_heat = shgcNew * area * solar * heatFrac; // BTU/year (beneficial) // Negative means we lose free heat → increases heating load var solarHeatPenalty = solarGainOld_heat - solarGainNew_heat; // BTU/year lost

// Net heating BTU saved = conduction saving - solar penalty var netHeatSavingBTU = heatCondSaving - solarHeatPenalty;

// Convert to source energy (divide by efficiency) var sourceHeatSavingBTU = netHeatSavingBTU / heatEff;

// --------------------------------------------------------------- // COOLING SAVINGS (Conduction) // Heat gain through windows in summer increases cooling load // Q_cool_cond_old = U_old * Area * CDD * 24 [BTU/year] // --------------------------------------------------------------- var coolCondOld = uOld * area * cdd * 24; var coolCondNew = uNew * area * cdd * 24; var coolCondSaving = coolCondOld - coolCondNew; // BTU/year reduction in cooling load

// --------------------------------------------------------------- // COOLING SAVINGS (Solar Gain — lower SHGC reduces unwanted solar heat in summer) // --------------------------------------------------------------- var solarGainOld_cool = shgcOld * area * solar * coolFrac; // BTU/year (unwanted) var solarGainNew_cool = shgcNew * area * solar * coolFrac; // BTU/year (unwanted) var solarCoolSaving = solarGainOld_cool - solarGainNew_cool; // BTU/year reduction

// Total cooling load reduction [BTU/year] var netCoolSavingBTU = coolCondSaving + solarCoolSaving;

// Convert cooling BTU to kWh: 1 kWh = 3412 BTU; divide by COP for electricity var coolElecSavingKWh = netCoolSavingBTU / 3412 / coolCOP;

// --------------------------------------------------------------- // COST SAVINGS // Heating: source BTU → MMBtu → cost // --------------------------------------------------------------- var heatCostSaving = (sourceHeatSavingBTU / 1e6) * heatCost; var coolCostSaving = coolElecSavingKWh * elecCost; var totalCostSaving = heatCostSaving + coolCostSaving;

// --------------------------------------------------------------- // CO2 Savings (approximate) // Heating: assume natural gas ~117 lbs CO2/MMBtu // Cooling: grid electricity ~0.386 kg CO2/kWh = 0.851 lbs/kWh // --------------------------------------------------------------- var co2HeatLbs = (sourceHeatSavingBTU / 1e6) * 117; var co2CoolLbs = coolElecSavingKWh * 0.851; var co2TotalLbs = co2HeatLbs + co2CoolLbs;

// --------------------------------------------------------------- // Format helpers // --------------------------------------------------------------- function fmt(n, d) { return n.toLocaleString('en-US', {minimumFractionDigits: d, maximumFractionDigits: d}); } function fmtSign(n) { return n >= 0 ? '+' : ''; }

var heatNote = netHeatSavingBTU Note: The SHGC reduction reduces beneficial winter solar gain more than the U-Factor improvement saves — net heating load increases slightly. Consider a higher SHGC for cold climates.'
'';

resultDiv.innerHTML = '### Results ' + '' + 'MetricValue' +

'Conduction Heat Loss Reduction (Heating)' + '' + fmt(heatCondSaving/1e6,3) + ' MMBtu/yr' +

'Solar Gain Change — Heating Season' + '' + fmtSign(-solarHeatPenalty) + fmt(-solarHeatPenalty/1e6,3) + ' MMBtu/yr' +

'Net Heating Load Reduction' + '' + fmt(netHeatSavingBTU/1e6,3) + ' MMBtu/yr' +

'Conduction Cooling Load Reduction' + '' + fmt(coolCondSaving/1e6,3) + ' MMBtu/yr' +

'Solar Gain Reduction — Cooling Season' + '' + fmt(solarCoolSaving/1e6,3) + ' MMBtu/yr' +

'Net Cooling Load Reduction' + '' + fmt(netCoolSavingBTU/1e6,3) + ' MMBtu/yr (' + fmt(coolElecSavingKWh,0) + ' kWh/yr)' +

'Annual Heating Cost Savings' + '$' + fmt(heatCostSaving,2) + '' +

'Annual Cooling Cost Savings' + '$' + fmt(coolCostSaving,2) + '' +

'Total Annual Cost Savings' + '$' + fmt(totalCostSaving,2) + '' +

'Estimated CO₂ Reduction' + '' + fmt(co2TotalLbs,0) + ' lbs/yr (' + fmt(co2TotalLbs/2204.6,2) + ' metric tons/yr)' +

'' + heatNote; }

#### Formulas Used

Conduction Heat Transfer (Heating & Cooling):

Qcond = U × A × DD × 24  [BTU/year]

where U = U-Factor (BTU/hr·ft²·°F), A = window area (ft²), DD = degree days (°F·days), 24 = hours/day

Solar Heat Gain:

Qsolar = SHGC × A × Isolar × Season Fraction  [BTU/year]

where Isolar = annual solar irradiance (BTU/ft²/year), Season Fraction = HDD/(HDD+CDD) for heating, CDD/(HDD+CDD) for cooling

Net Heating Savings:

ΔQheat = (Uold − Unew) × A × HDD × 24 − (SHGCold − SHGCnew) × A × Isolar × HeatFrac

Source energy saved = ΔQheat / ηheating

Net Cooling Savings:

ΔQcool = (Uold − Unew) × A × CDD × 24 + (SHGCold − SHGCnew) × A × Isolar × CoolFrac

Electricity saved = ΔQcool / (3412 × COP)  [kWh/year]

Cost Savings:

Heating savings ($) = (Source BTU saved / 1,000,000) × Heating cost ($/MMBtu)

Cooling savings ($) = kWh saved × Electricity cost ($/kWh)

#### Assumptions & References

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