function calculateSolarROI() {
var size = parseFloat(document.getElementById('sys_size').value);
var cost = parseFloat(document.getElementById('total_cost').value);
var hours = parseFloat(document.getElementById('sun_hours').value);
var rate = parseFloat(document.getElementById('elec_rate').value);
var credit = parseFloat(document.getElementById('tax_credit').value);
if (isNaN(size) || isNaN(cost) || isNaN(hours) || isNaN(rate) || isNaN(credit)) {
alert("Please enter valid numbers in all fields.");
return;
}
// Efficiency loss factor (standard is ~0.78 for system losses)
var efficiency = 0.78;
// Annual generation: size * sun_hours * days * efficiency
var annualKwh = size * hours * 365 * efficiency;
// Annual dollar savings
var annualSavings = annualKwh * rate;
// Net cost after tax credit
var netCost = cost * (1 – (credit / 100));
// Payback period
var payback = netCost / annualSavings;
// 25-year lifetime savings (assuming 0.5% degradation per year and 2% energy price inflation)
var lifetimeSavings = 0;
var currentYearSavings = annualSavings;
for (var i = 1; i <= 25; i++) {
lifetimeSavings += currentYearSavings;
currentYearSavings = currentYearSavings * 1.02 * 0.995;
}
var totalRoi = lifetimeSavings – netCost;
document.getElementById('res_net_cost').innerText = "$" + netCost.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res_annual_savings').innerText = "$" + annualSavings.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res_payback').innerText = payback.toFixed(1) + " Years";
document.getElementById('res_lifetime').innerText = "$" + totalRoi.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('solar-results').style.display = 'block';
}
Understanding Your Solar Investment Return (ROI)
Switching to solar power is one of the most significant financial and environmental decisions a homeowner can make. By using our Solar ROI Calculator, you can estimate how long it will take for your system to pay for itself and how much profit you'll generate over the 25-year lifespan of the panels.
Key Factors That Influence Solar Savings
System Size: Measured in kilowatts (kW), this is the capacity of your panels. A larger system costs more upfront but generates more electricity.
Sun Hours: This isn't just daylight; it's "peak sun hours." Regions like Arizona receive more energy per square meter than regions like Washington, impacting your total generation.
The Federal Solar Tax Credit (ITC): Currently, the U.S. federal government offers a 30% tax credit on the total cost of solar installation, significantly reducing your net investment.
Utility Rates: The more you pay for grid power (measured in $/kWh), the more you save by generating your own.
How the Calculation Works
The calculator estimates your Annual Generation by taking your system size and multiplying it by your average daily sun hours, adjusted for an industry-standard 78% system efficiency (accounting for inverter losses and wiring).
Your Payback Period is determined by dividing the net cost of the system (after tax credits) by your annual electricity savings. Most residential solar systems in the United States currently see a payback period between 6 and 10 years.
Long-term Financial Benefits
Solar panels are usually warrantied for 25 years. Our "Total ROI" calculation accounts for a modest 2% annual increase in utility electricity prices and a standard 0.5% annual degradation in panel efficiency. By the end of 25 years, a well-placed solar array can often return 3 to 4 times its original cost in energy savings.