function calculateSolarPayback() {
// 1. Get input values
var rawCost = document.getElementById('systemCost').value;
var rawCredit = document.getElementById('taxCredit').value;
var rawBill = document.getElementById('monthlyBill').value;
var rawIncrease = document.getElementById('rateIncrease').value;
// 2. Parse values
var cost = parseFloat(rawCost);
var creditPercent = parseFloat(rawCredit);
var monthlyBill = parseFloat(rawBill);
var annualIncrease = parseFloat(rawIncrease);
// 3. Validation
if (isNaN(cost) || isNaN(creditPercent) || isNaN(monthlyBill) || isNaN(annualIncrease) || cost <= 0 || monthlyBill <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
// 4. Calculate Net Cost
var taxCreditAmount = cost * (creditPercent / 100);
var netCost = cost – taxCreditAmount;
// 5. Calculate Payback Period and Savings
var cumulativeSavings = 0;
var paybackYears = 0;
var paybackFound = false;
var currentAnnualBill = monthlyBill * 12;
var totalSavings25Years = 0;
// Loop through 25 years (standard solar lifespan)
for (var year = 1; year = netCost) {
// Calculate fractional year for precision
var previousCumulative = cumulativeSavings – currentAnnualBill;
var remainingCost = netCost – previousCumulative;
var fraction = remainingCost / currentAnnualBill;
paybackYears = (year – 1) + fraction;
paybackFound = true;
}
// Increase electricity cost for next year due to inflation
currentAnnualBill = currentAnnualBill * (1 + (annualIncrease / 100));
}
// 6. Calculate ROI
// ROI = (Net Profit / Net Cost) * 100
var netProfit = totalSavings25Years – netCost;
var roi = (netProfit / netCost) * 100;
// 7. Format and Display Results
document.getElementById('resNetCost').innerText = "$" + netCost.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
if (paybackFound) {
document.getElementById('resPayback').innerText = paybackYears.toFixed(1) + " Years";
} else {
document.getElementById('resPayback').innerText = "25+ Years";
}
document.getElementById('resSavings').innerText = "$" + netProfit.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('resROI').innerText = roi.toFixed(1) + "%";
// Show results div
document.getElementById('results').style.display = "block";
}
Is Solar Worth It? Understanding Your ROI
Switching to solar energy is one of the most significant home improvement investments a homeowner can make. Unlike a kitchen renovation or new flooring, a solar panel system generates a direct financial return. Use our Solar Panel Payback Calculator above to determine exactly how long it will take for your electricity savings to cover the cost of installation.
What is the Solar Payback Period?
The solar payback period is the amount of time it takes for your solar system to "pay for itself" through savings on your electric bill. Once you pass this break-even point, every kilowatt-hour of energy your panels produce is effectively free money for the remainder of the system's lifespan (typically 25-30 years).
The average payback period in the United States ranges between 6 to 10 years, depending on system costs, local electricity rates, and available incentives.
Key Factors Affecting Your Solar Savings
Total System Cost: The gross price of equipment and installation. Prices generally range from $2.50 to $3.50 per watt.
Federal Tax Credit (ITC): The Investment Tax Credit currently allows you to deduct 30% of the cost of installing a solar energy system from your federal taxes. This significantly lowers your Net System Cost.
Electricity Rates: Homeowners with high monthly electric bills (over $150) or those living in areas with high utility rates usually see a faster ROI.
Utility Price Inflation: Electricity prices historically rise by about 2.5% to 4% annually. As utility rates go up, the value of the energy your solar panels produce increases, accelerating your payback timeline.
How to Interpret Your Results
Net System Cost: This is your actual out-of-pocket expense after applying the federal tax credit. This is the "hurdle" your savings need to clear.
Total 25-Year Savings: This represents the estimated net profit of your system over a standard 25-year warranty period. It accounts for the money you would have otherwise paid to the utility company, adjusted for inflation.
ROI (Return on Investment): A solar system often provides a better return than the stock market. An ROI of 10% or higher is generally considered excellent for a low-risk asset attached to your home.
Example Scenario
Consider a home in a sunny state installing a 10kW system:
System Cost: $25,000
Federal Credit (30%): -$7,500
Net Cost: $17,500
Avg Monthly Bill: $200
If utility rates rise by 3% annually, this homeowner might see a payback period of just under 7 years and total lifetime savings exceeding $60,000.