Estimate how long it will take for your solar energy system to pay for itself through electricity savings.
Total quote amount before incentives.
Currently 30% in the US (ITC).
Cash rebates or grants.
Average bill amount you expect to offset.
Historical average is ~3-4%.
Estimated yearly upkeep (often $0 early on).
Calculation Results
Net System Cost–
Payback Period–
10-Year Savings–
25-Year Total ROI–
*Estimates assume the system offsets 100% of the entered monthly bill amount.
ROI (Return on Investment) represents total net savings over 25 years divided by the net system cost.
function calculateSolarPayback() {
// 1. Get input values
var grossCost = parseFloat(document.getElementById('spGrossCost').value);
var taxCreditPercent = parseFloat(document.getElementById('spTaxCredit').value);
var rebates = parseFloat(document.getElementById('spOtherRebates').value);
var monthlyBill = parseFloat(document.getElementById('spMonthlyBill').value);
var inflationRate = parseFloat(document.getElementById('spEnergyInflation').value);
var maintenance = parseFloat(document.getElementById('spMaintenance').value);
// 2. Validate inputs
var errorDiv = document.getElementById('spErrorMessage');
var resultsDiv = document.getElementById('spResults');
errorDiv.innerHTML = "";
if (isNaN(grossCost) || isNaN(monthlyBill) || grossCost <= 0 || monthlyBill <= 0) {
errorDiv.innerHTML = "Please enter valid numbers for System Cost and Monthly Bill.";
resultsDiv.style.display = "none";
return;
}
// 3. Logic – Net Cost
// Tax credit applies to gross cost usually, rebates might be pre- or post-tax depending on locale,
// but for simplicity here: Net = Gross – (Gross * Credit%) – Rebates.
var taxCreditAmount = grossCost * (taxCreditPercent / 100);
var netCost = grossCost – taxCreditAmount – rebates;
if (netCost <= 0) {
netCost = 0; // Edge case: free system
}
// 4. Logic – Payback Loop
var cumulativeSavings = 0;
var yearsPassed = 0;
var paybackFound = false;
var finalPaybackYear = 0;
// Variables for long term calculation
var annualSavings = monthlyBill * 12;
var totalSavings25 = 0;
var savingsAt10 = 0;
// Loop up to 50 years to find payback
for (var i = 1; i = netCost) {
// Calculate fractional year for precision
// Previous cumulative = cumulativeSavings – yearNetBenefit
var previousCumulative = cumulativeSavings – yearNetBenefit;
var remainingNeeded = netCost – previousCumulative;
var fraction = remainingNeeded / yearNetBenefit;
finalPaybackYear = (i – 1) + fraction;
paybackFound = true;
}
// Capture 10 year snapshot
if (i === 10) {
savingsAt10 = cumulativeSavings – netCost; // Net profit at year 10
}
// Capture 25 year snapshot
if (i <= 25) {
totalSavings25 = cumulativeSavings;
}
// Inflate energy prices for next year
annualSavings = annualSavings * (1 + (inflationRate / 100));
// Inflate maintenance? Let's keep maintenance flat for simplicity or inflate it at inflationRate
maintenance = maintenance * (1 + (0.02)); // Assume 2% inflation on maintenance
}
// 5. Calculate ROI
// ROI % = (Total Net Profit / Net Investment) * 100
var netProfit25 = totalSavings25 – netCost;
var roi = (netProfit25 / netCost) * 100;
// 6. Display Results
document.getElementById('resNetCost').innerText = "$" + netCost.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
if (paybackFound) {
document.getElementById('resPaybackYears').innerText = finalPaybackYear.toFixed(1) + " Years";
} else {
document.getElementById('resPaybackYears').innerText = "25+ Years";
}
document.getElementById('res10YearSavings').innerText = "$" + savingsAt10.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('res25YearROI').innerText = roi.toFixed(0) + "%";
resultsDiv.style.display = "block";
}
Understanding Your Solar Payback Period
Investing in solar panels is one of the most significant home improvement decisions a homeowner can make. Unlike a kitchen renovation, however, a solar energy system provides a tangible financial return. The Solar Payback Period is the amount of time it takes for the electricity savings generated by your system to equal the initial cost of installation.
How the Calculator Works
This calculator determines your break-even point by analyzing three main factors:
Net System Cost: This is your upfront investment after subtracting the Federal Solar Investment Tax Credit (ITC) and any local rebates. The ITC currently allows you to deduct 30% of the cost of installing a solar energy system from your federal taxes.
Energy Offset: We estimate your annual savings based on your current monthly electric bill. The calculator assumes your solar system is sized to offset 100% of this usage.
Energy Inflation: Electricity prices rarely stay static. This tool factors in an annual increase in utility rates (typically 3-4%), which accelerates your payback period because the value of the energy you produce increases over time.
What is a Good Solar Payback Period?
While results vary based on sunlight availability and local electricity rates, a payback period between 6 to 10 years is considered excellent. Since most solar panels are warranted for 25 years, a system paying for itself in 8 years offers 17 years of essentially free electricity.
Improving Your ROI
To shorten your payback period, ensure you maximize the Federal Tax Credit and investigate State-specific Renewable Energy Credits (SRECs). Additionally, reducing your energy consumption before sizing your system can lower the gross cost, leading to a faster return on investment.