Window U-Factor and SHGC Savings Calculator

ANA›Life Services Authority›National Calculator Authority›Window U-Factor and SHGC Savings Calculator

.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }

Window U-Factor and SHGC Savings Calculator

Estimate annual heating and cooling energy savings when upgrading to windows with improved U-Factor (heat loss) and Solar Heat Gain Coefficient (SHGC) ratings.

### Window Properties

Total Window Area (sq ft)

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

New Window U-Factor (BTU/hr·ft²·°F)

Existing Window SHGC (0–1)

New Window SHGC (0–1)

### Climate & Building

Heating Degree Days (HDD, base 65°F)

Cooling Degree Days (CDD, base 65°F)

Average Annual Solar Irradiance (BTU/hr·ft²)

Typical range: 30–80 BTU/hr·ft². US average ≈ 50.

### Energy Costs

Heating System Efficiency (AFUE or COP, e.g. 0.80 or 3.0)

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

Heating Fuel Cost ($ per 100,000 BTU = 1 therm)

Electricity Cost ($ per kWh)

Calculate Savings

function winCalc() { const err = document.getElementById('win_error'); const res = document.getElementById('win_result'); err.style.display = 'none'; res.style.display = 'none';

// --- Collect inputs --- const area = parseFloat(document.getElementById('win_area').value); const uOld = parseFloat(document.getElementById('win_u_old').value); const uNew = parseFloat(document.getElementById('win_u_new').value); const shgcOld = parseFloat(document.getElementById('win_shgc_old').value); const shgcNew = parseFloat(document.getElementById('win_shgc_new').value); const hdd = parseFloat(document.getElementById('win_hdd').value); const cdd = parseFloat(document.getElementById('win_cdd').value); const solar = parseFloat(document.getElementById('win_solar').value); const heatEff = parseFloat(document.getElementById('win_heat_eff').value); const coolEff = parseFloat(document.getElementById('win_cool_eff').value); const heatCost = parseFloat(document.getElementById('win_heat_cost').value); const elecCost = parseFloat(document.getElementById('win_elec_cost').value);

// --- Validation --- const fields = [area, uOld, uNew, shgcOld, shgcNew, hdd, cdd, solar, heatEff, coolEff, heatCost, elecCost]; if (fields.some(v => isNaN(v) || v = uOld) { err.textContent = 'New U-Factor should be lower than existing U-Factor for savings. If upgrading, the new value must be less than the old value.'; err.style.display = 'block'; return; } if (shgcOld > 1 || shgcNew > 1) { err.textContent = 'SHGC must be between 0 and 1.'; err.style.display = 'block'; return; }

// --------------------------------------------------------------- // HEATING SAVINGS (U-Factor conduction loss reduction) // Q_heat = ΔU × Area × HDD × 24 [BTU/year] // ΔU = uOld - uNew // Fuel saved = Q_heat / heatEff [BTU/year of fuel] // Cost saved = Fuel saved / 100000 × heatCost // --------------------------------------------------------------- const deltaU = uOld - uNew; const heatSavingsBTU = deltaU * area * hdd * 24; // BTU/yr const heatFuelSaved = heatSavingsBTU / heatEff; // BTU/yr of fuel input const heatCostSaved = (heatFuelSaved / 100000) * heatCost; // dollars

// --------------------------------------------------------------- // SOLAR GAIN HEATING BENEFIT (lower SHGC = less free heat in winter) // In heating season, solar gain is beneficial. // Solar heat lost due to lower SHGC: // Q_solar_heat_loss = (shgcOld - shgcNew) × area × solar × heatingHours // heatingHours ≈ HDD × 24 / avgDeltaT_heating // Using a simplified approach: heating hours ≈ HDD * 24 / 17.5 // (assuming average indoor-outdoor delta T of 17.5°F during heating season) // This solar loss partially offsets heating savings. // --------------------------------------------------------------- const heatingHours = (hdd * 24) / 17.5; const deltaSHGC = shgcOld - shgcNew; // Solar gain reduction in heating season (negative = less free heat = cost) const solarHeatLossBTU = deltaSHGC * area * solar * heatingHours; // BTU/yr const solarHeatLossFuel = solarHeatLossBTU / heatEff; const solarHeatLossCost = (solarHeatLossFuel / 100000) * heatCost;

// --------------------------------------------------------------- // COOLING SAVINGS (U-Factor conduction gain reduction) // Q_cool_U = ΔU × Area × CDD × 24 [BTU/year] // --------------------------------------------------------------- const coolSavingsU_BTU = deltaU * area * cdd * 24; // BTU/yr

// --------------------------------------------------------------- // COOLING SAVINGS (SHGC solar gain reduction) // Q_cool_SHGC = ΔSHGC × area × solar × coolingHours // coolingHours ≈ CDD * 24 / 15 (avg delta T ~15°F in cooling season) // --------------------------------------------------------------- const coolingHours = (cdd * 24) / 15; const coolSavingsSHGC_BTU = deltaSHGC * area * solar * coolingHours; // BTU/yr

const totalCoolSavingsBTU = coolSavingsU_BTU + coolSavingsSHGC_BTU; // Convert BTU to kWh: 1 kWh = 3412 BTU; divide by COP for electricity used const coolElecSaved_kWh = totalCoolSavingsBTU / (3412 * coolEff); const coolCostSaved = coolElecSaved_kWh * elecCost;

// --------------------------------------------------------------- // NET ANNUAL SAVINGS // --------------------------------------------------------------- const netHeatSavings = heatCostSaved - solarHeatLossCost; const netCoolSavings = coolCostSaved; const totalSavings = netHeatSavings + netCoolSavings;

// --------------------------------------------------------------- // Energy quantities for display // --------------------------------------------------------------- const heatSavedTherms = heatFuelSaved / 100000; const solarLossTherms = solarHeatLossFuel / 100000; const coolSavedKwh = coolElecSaved_kWh;

// Format helpers const fmt = (v, d=2) => v.toLocaleString('en-US', {minimumFractionDigits:d, maximumFractionDigits:d}); const fmtC = (v) => '$' + fmt(Math.abs(v));

res.innerHTML = ` ### Results

Category Energy Annual Savings

Heating – Conduction Reduction ${fmt(heatSavedTherms,1)} therms +${fmtC(heatCostSaved)}

Heating – Solar Gain Reduction (offset) ${fmt(solarLossTherms,1)} therms lost −${fmtC(solarHeatLossCost)}

Net Heating Savings — =0?'#27ae60':'#c0392b'};">${netHeatSavings>=0?'+':'−'}${fmtC(netHeatSavings)}

Cooling – Conduction Reduction ${fmt(coolSavingsU_BTU/3412,0)} kWh +${fmtC((coolSavingsU_BTU/(3412coolEff))elecCost)}

Cooling – Solar Gain Reduction (SHGC) ${fmt(coolSavingsSHGC_BTU/3412,0)} kWh +${fmtC((coolSavingsSHGC_BTU/(3412coolEff))elecCost)}

Net Cooling Savings ${fmt(coolSavedKwh,0)} kWh +${fmtC(netCoolSavings)}

Total Annual Savings — ${totalSavings>=0?'+':'−'}${fmtC(totalSavings)}/yr

Key Metrics: ΔU-Factor: ${fmt(deltaU,3)} BTU/hr·ft²·°F | ΔSHGC: ${fmt(deltaSHGC,3)} | Window Area: ${fmt(area,0)} ft²

`; res.style.display = 'block'; }

#### Formulas Used

Heating Conduction Savings (BTU/yr): Qheat = (Uold − Unew) × Area × HDD × 24

Heating Fuel Saved: Fuelheat = Qheat / ηheat → Cost = (Fuel / 100,000) × Rateheat

Solar Gain Offset in Heating Season (BTU/yr): Qsolar,heat = (SHGCold − SHGCnew) × Area × Isolar × (HDD × 24 / 17.5)

Cooling Conduction Savings (BTU/yr): Qcool,U = (Uold − Unew) × Area × CDD × 24

Cooling Solar Gain Savings (BTU/yr): Qcool,SHGC = (SHGCold − SHGCnew) × Area × Isolar × (CDD × 24 / 15)

Cooling Electricity Saved (kWh/yr): kWhcool = (Qcool,U + Qcool,SHGC) / (3,412 × COPcool)

Net Annual Savings: Savings = Net Heating Savings + Net Cooling Savings

#### Assumptions & References

Average indoor–outdoor temperature difference assumed as 17.5°F during heating season and 15°F during cooling season to estimate seasonal hours.
Solar irradiance (Isolar) represents average incident solar radiation on vertical glazing surfaces. US average ≈ 50 BTU/hr·ft²; varies by orientation and climate.
Heating efficiency (ηheat): use AFUE for gas/oil furnaces (e.g., 0.80), or COP for heat pumps (e.g., 2.5–3.5).

Window U-Factor and SHGC Savings Calculator

Window U-Factor and SHGC Savings Calculator

More Calculators

Read Next

References

Window U-Factor and SHGC Savings Calculator

Window U-Factor and SHGC Savings Calculator

More Calculators

Read Next

References

Related Authorities