Smart Thermostat Payback Period Calculator
ANA›Life Services Authority›National Calculator Authority›Smart Thermostat Payback Period Calculator
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Smart Thermostat Payback Period Calculator
Determine how many months or years it will take for your smart thermostat investment to pay for itself through reduced heating and cooling costs.
Smart Thermostat Cost (including installation) ($)
Current Annual Heating & Cooling Bill ($)
Expected Energy Savings (%)
Typical range: 8%–15%. EPA ENERGY STAR estimates ~8%; Nest/Ecobee report up to 15%.
Utility Rebate / Tax Credit ($)
Enter 0 if none. Many utilities offer $25–$100 rebates.
Annual Energy Price Inflation Rate (%)
U.S. historical average ~3% per year (EIA).
Calculate Payback Period
function smaCalc() { // --- gather inputs --- var deviceCost = parseFloat(document.getElementById('sma-device-cost').value); var annualBill = parseFloat(document.getElementById('sma-annual-energy-bill').value); var savingsPct = parseFloat(document.getElementById('sma-savings-pct').value); var rebate = parseFloat(document.getElementById('sma-rebate').value) || 0; var inflationPct = parseFloat(document.getElementById('sma-energy-inflation').value) || 0;
var resultDiv = document.getElementById('sma-result'); resultDiv.style.display = 'block';
// --- validation --- var errors = []; if (isNaN(deviceCost) || deviceCost 100) errors.push("Savings percentage must be between 0.1% and 100%."); if (rebate = deviceCost && !isNaN(deviceCost)) errors.push("Rebate cannot exceed or equal the device cost."); if (isNaN(inflationPct) || inflationPct 30) errors.push("Energy inflation rate must be between 0% and 30%.");
if (errors.length > 0) { resultDiv.className = 'calc-result calc-error'; resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }
// --- core calculations --- // Net upfront cost after rebate var netCost = deviceCost - rebate;
// Year-1 savings (no inflation applied yet) var annualSavingsY1 = annualBill * (savingsPct / 100);
// Simple payback (no inflation) var simplePaybackYears = netCost / annualSavingsY1; var simplePaybackMonths = simplePaybackYears * 12;
// Inflation-adjusted payback using cumulative savings with compounding: // Cumulative savings after n years = S1 * [(1+r)^n - 1] / r (r = inflation rate) // Solve for n: (1+r)^n = 1 + (netCost * r) / S1 // n = ln(1 + netCost*r/S1) / ln(1+r) var inflationAdjPaybackYears, inflationAdjPaybackMonths; var r = inflationPct / 100; if (r === 0) { inflationAdjPaybackYears = simplePaybackYears; inflationAdjPaybackMonths = simplePaybackMonths; } else { inflationAdjPaybackYears = Math.log(1 + (netCost * r) / annualSavingsY1) / Math.log(1 + r); inflationAdjPaybackMonths = inflationAdjPaybackYears * 12; }
// 10-year total savings (inflation-adjusted) var tenYearSavings; if (r === 0) { tenYearSavings = annualSavingsY1 * 10; } else { tenYearSavings = annualSavingsY1 * ((Math.pow(1 + r, 10) - 1) / r); } var tenYearNetBenefit = tenYearSavings - netCost;
// ROI over 10 years var roi10 = (tenYearNetBenefit / netCost) * 100;
// Monthly savings (Year 1) var monthlySavingsY1 = annualSavingsY1 / 12;
// --- format helpers --- function fmt(n, dec) { return n.toFixed(dec !== undefined ? dec : 2); } function fmtMoney(n) { return '$' + n.toFixed(2).replace(/\B(?=(\d{3})+(?!\d))/g, ','); } function fmtYM(months) { var y = Math.floor(months / 12); var m = Math.round(months % 12); if (m === 12) { y++; m = 0; } var parts = []; if (y > 0) parts.push(y + ' year' + (y !== 1 ? 's' : '')); if (m > 0) parts.push(m + ' month' + (m !== 1 ? 's' : '')); return parts.length ? parts.join(' and ') : 'Less than 1 month'; }
// --- build result HTML --- var html = '### ✓ Payback Period Results ';
html += ''; var rows = [ ['Net Upfront Cost (after rebate)', fmtMoney(netCost)], ['Year-1 Annual Savings', fmtMoney(annualSavingsY1)], ['Year-1 Monthly Savings', fmtMoney(monthlySavingsY1)], ['Simple Payback Period', '' + fmtYM(simplePaybackMonths) + ' (' + fmt(simplePaybackYears, 1) + ' yrs)'], ['Inflation-Adjusted Payback', '' + fmtYM(inflationAdjPaybackMonths) + ' (' + fmt(inflationAdjPaybackYears, 1) + ' yrs)'], ['10-Year Total Savings (inflation-adj.)', fmtMoney(tenYearSavings)], ['10-Year Net Benefit', fmtMoney(tenYearNetBenefit)], ['10-Year ROI', fmt(roi10, 1) + '%'], ]; rows.forEach(function(r, i) { var bg = i % 2 === 0 ? '#f9f9f9' : '#fff'; html += '' + '' + r[0] + '' + '' + r[1] + '' + ''; }); html += '';
// Contextual note var paybackNote = ''; if (inflationAdjPaybackYears
#### Formulas Used
Net Upfront Cost:
Net Cost = Device Cost − Rebate
Annual Savings (Year 1):
Annual Savings = Annual Energy Bill × (Savings % ÷ 100)
Simple Payback Period (no inflation):
Payback (years) = Net Cost ÷ Annual Savings
Inflation-Adjusted Payback Period:
Cumulative savings grow each year as energy prices rise. Solving for n years where cumulative savings equal net cost:
Cumulative Savings(n) = S₁ × [(1 + r)ⁿ − 1] ÷ r = Net Cost
n = ln(1 + Net Cost × r ÷ S₁) ÷ ln(1 + r)
where S₁ = Year-1 annual savings, r = annual energy inflation rate (decimal).
10-Year Total Savings (inflation-adjusted):
Total Savings = S₁ × [(1 + r)¹⁰ − 1] ÷ r
10-Year ROI:
ROI = (Total Savings − Net Cost) ÷ Net Cost × 100%
#### Assumptions & References
- EPA ENERGY STAR estimates smart thermostats save ~8% on heating and cooling bills on average (energystar.gov).
- Many utilities offer rebates of $25–$100; the federal Inflation Reduction Act (IRA) may provide additional tax credits — check dsireusa.org for your state.
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References
- 10 CFR § 436.23 — § 436.23 Estimated simple payback time.
- 10 CFR § 460.202 — § 460.202 Thermostats and controls.
The law belongs to the people. Georgia v. Public.Resource.Org, 590 U.S. (2020)