Payback Period Calculator for Window Upgrades
ANA›Life Services Authority›National Calculator Authority›Payback Period Calculator for Window Upgrades
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Payback Period Calculator for Window Upgrades
Estimate how many years it will take to recover your window upgrade investment through reduced heating and cooling costs.
Total Upgrade Cost (materials + installation, $)
Rebates / Tax Credits Received ($)
Existing Window U-Value (BTU/hr·ft²·°F)
New Window U-Value (BTU/hr·ft²·°F)
Total Window Area (ft²)
Heating Degree Days (HDD) per Year
Cooling Degree Days (CDD) per Year
Heating System Efficiency (AFUE or COP, e.g. 0.80 for gas furnace, 3.0 for heat pump)
Cooling System EER (BTU/Wh, e.g. 12)
Heating Fuel Cost ($ per 100,000 BTU delivered)
Electricity Cost for Cooling ($ per kWh)
Annual Discount Rate / Inflation Rate (%)
Calculate Payback Period
function payCalc() { var resultDiv = document.getElementById('pay-result'); resultDiv.innerHTML = '';
// --- Read inputs --- var upgradeCost = parseFloat(document.getElementById('pay-upgrade-cost').value); var rebate = parseFloat(document.getElementById('pay-rebate').value) || 0; var oldU = parseFloat(document.getElementById('pay-old-uval').value); var newU = parseFloat(document.getElementById('pay-new-uval').value); var area = parseFloat(document.getElementById('pay-window-area').value); var hdd = parseFloat(document.getElementById('pay-hdd').value) || 0; var cdd = parseFloat(document.getElementById('pay-cdd').value) || 0; var heatEff = parseFloat(document.getElementById('pay-heat-eff').value); var coolEer = parseFloat(document.getElementById('pay-cool-eff').value); var heatCost = parseFloat(document.getElementById('pay-energy-cost-heat').value); var coolCost = parseFloat(document.getElementById('pay-energy-cost-cool').value); var discountRate = parseFloat(document.getElementById('pay-discount-rate').value) || 0;
// --- Validation --- var errors = []; if (isNaN(upgradeCost) || upgradeCost 0."); if (isNaN(newU) || newU 0."); if (!isNaN(oldU) && !isNaN(newU) && newU >= oldU) errors.push("New window U-value must be lower than the existing U-value."); if (isNaN(area) || area 0."); if (hdd === 0 && cdd === 0) errors.push("Enter at least one of Heating Degree Days or Cooling Degree Days."); if (isNaN(heatEff) || heatEff 0."); if (isNaN(coolEer) || coolEer 0."); if (isNaN(heatCost) || heatCost = 0."); if (isNaN(coolCost) || coolCost = 0."); if (discountRate 0) { resultDiv.innerHTML = ''; return; }
// --- Net Investment --- var netCost = upgradeCost - rebate; if (netCost Payback Period: Immediate — Rebates cover the full upgrade cost.'; return; }
// --- Delta U-Value --- var deltaU = oldU - newU; // BTU / (hr · ft² · °F)
// --- Annual Heat Loss Reduction --- // Q_heat = deltaU × area × HDD × 24 [BTU/yr] var heatSavedBtu = deltaU * area * hdd * 24;
// Annual heating cost savings // Fuel cost is per 100,000 BTU delivered; efficiency already in delivered heat // Cost savings = (heatSavedBtu / heatEff) / 100000 × heatCost var heatCostSavings = (heatSavedBtu / heatEff) / 100000 * heatCost;
// --- Annual Cooling Load Reduction --- // Q_cool = deltaU × area × CDD × 24 [BTU/yr] var coolSavedBtu = deltaU * area * cdd * 24;
// Convert BTU savings to kWh: 1 kWh = 3412 BTU // Electricity saved = coolSavedBtu / EER / 1000 [kWh] (EER in BTU/Wh → Wh = BTU/EER → kWh = BTU/(EER×1000)) var coolKwhSaved = coolSavedBtu / (coolEer * 1000); var coolCostSavings = coolKwhSaved * coolCost;
// --- Total Annual Savings (Year 1) --- var annualSavings = heatCostSavings + coolCostSavings;
if (annualSavings Annual savings are zero or negative. Check your inputs — the new windows may not reduce energy costs under these conditions.'; return; }
// --- Simple Payback Period --- var simplePayback = netCost / annualSavings;
// --- Discounted Payback Period --- // Find year n where sum of discounted savings >= netCost // Discounted savings year t = annualSavings / (1 + r)^t var discountedPayback = null; var r = discountRate / 100; var MAX_YEARS = 100;
if (r === 0) { discountedPayback = simplePayback; } else { var cumulative = 0; for (var t = 1; t = netCost) { // Interpolate within the year var prevCumulative = cumulative - annualSavings / Math.pow(1 + r, t); var fraction = (netCost - prevCumulative) / (annualSavings / Math.pow(1 + r, t)); discountedPayback = (t - 1) + fraction; break; } } }
// --- 10-Year NPV of Savings --- var npv10 = 0; for (var y = 1; y ' + '### Results ' + '' + 'Net Investment (after rebates)$' + netCost.toLocaleString('en-US', {minimumFractionDigits:2, maximumFractionDigits:2}) + '' + 'Annual Heating Cost Savings$' + heatCostSavings.toFixed(2) + '' + 'Annual Cooling Cost Savings$' + coolCostSavings.toFixed(2) + '' + 'Total Annual Energy Savings (Year 1)$' + annualSavings.toFixed(2) + '' + 'Simple Payback Period' + simplePayback.toFixed(1) + ' years' + 'Discounted Payback Period (' + discountRate.toFixed(1) + '% rate)' + discountedStr + '' + '10-Year NPV of Savings$' + npv10.toFixed(2) + '' + '10-Year ROI' + roi10.toFixed(1) + '%' + '' + ''; }
#### Formulas Used
Annual Heat Loss Reduction (BTU/yr): Qheat = (Uold − Unew) × Area (ft²) × HDD × 24 hr/day
Annual Heating Cost Savings ($): Savingsheat = [Qheat ÷ Heating Efficiency] ÷ 100,000 × Fuel Cost ($/100,000 BTU)
Annual Cooling Load Reduction (BTU/yr): Qcool = (Uold − Unew) × Area (ft²) × CDD × 24 hr/day
Annual Cooling Cost Savings ($): Savingscool = [Qcool ÷ (EER × 1,000)] × Electricity Cost ($/kWh)
Simple Payback Period (years): Payback = Net Investment ÷ Total Annual Savings
Discounted Payback Period: Find smallest n such that Σt=1..n [Annual Savings ÷ (1 + r)t] ≥ Net Investment
10-Year NPV of Savings: NPV = Σt=1..10 [Annual Savings ÷ (1 + r)t]
10-Year ROI: ROI = (NPV − Net Investment) ÷ Net Investment × 100%
#### Assumptions & References
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