.solar-calculator-container {
font-family: 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
max-width: 800px;
margin: 0 auto;
background: #fff;
padding: 20px;
border-radius: 8px;
box-shadow: 0 4px 15px rgba(0,0,0,0.05);
}
.solar-calculator-container h2 {
color: #2c3e50;
text-align: center;
margin-bottom: 25px;
border-bottom: 2px solid #f39c12;
padding-bottom: 10px;
}
.calc-grid {
display: grid;
grid-template-columns: 1fr 1fr;
gap: 20px;
}
@media (max-width: 768px) {
.calc-grid {
grid-template-columns: 1fr;
}
}
.input-group {
margin-bottom: 15px;
}
.input-group label {
display: block;
margin-bottom: 5px;
font-weight: 600;
color: #555;
}
.input-group input {
width: 100%;
padding: 10px;
border: 1px solid #ddd;
border-radius: 4px;
font-size: 16px;
box-sizing: border-box;
}
.input-group input:focus {
border-color: #f39c12;
outline: none;
}
.calc-btn {
grid-column: 1 / -1;
background-color: #f39c12;
color: white;
padding: 15px;
border: none;
border-radius: 4px;
font-size: 18px;
font-weight: bold;
cursor: pointer;
transition: background 0.3s;
margin-top: 10px;
width: 100%;
}
.calc-btn:hover {
background-color: #d68910;
}
#solar-results {
grid-column: 1 / -1;
background-color: #f9f9f9;
padding: 20px;
border-radius: 6px;
margin-top: 20px;
display: none;
border: 1px solid #eee;
}
.result-row {
display: flex;
justify-content: space-between;
margin-bottom: 10px;
font-size: 16px;
}
.result-row.highlight {
font-weight: bold;
color: #27ae60;
font-size: 20px;
border-top: 1px solid #ddd;
padding-top: 10px;
margin-top: 10px;
}
.content-section {
margin-top: 40px;
line-height: 1.6;
color: #444;
}
.content-section h3 {
color: #2c3e50;
margin-top: 25px;
}
.note {
font-size: 0.9em;
color: #777;
margin-top: 5px;
}
function calculateSolarPayback() {
// Get input values
var sysCost = parseFloat(document.getElementById('sysCost').value);
var taxCredit = parseFloat(document.getElementById('taxCredit').value);
var sysSize = parseFloat(document.getElementById('sysSize').value);
var sunHours = parseFloat(document.getElementById('sunHours').value);
var elecRate = parseFloat(document.getElementById('elecRate').value);
var rateIncrease = parseFloat(document.getElementById('rateIncrease').value);
// Validation
if (isNaN(sysCost) || isNaN(sysSize) || isNaN(elecRate) || sysCost <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
// 1. Calculate Net Cost
var creditAmount = sysCost * (taxCredit / 100);
var netCost = sysCost – creditAmount;
// 2. Calculate Annual Production (kWh)
// Formula: Size (kW) * Sun Hours * 365 Days * 0.75 (System Efficiency Loss Factor)
var efficiencyFactor = 0.75;
var annualProduction = sysSize * sunHours * 365 * efficiencyFactor;
// 3. Loop to find Payback and Lifetime Savings
var cumulativeSavings = 0;
var years = 0;
var paybackFound = false;
var currentRate = elecRate;
var lifetimeSavings = 0;
var paybackYears = 0;
// Loop for 25 years (standard panel lifespan)
for (var i = 1; i = netCost) {
// Calculate fractional year for precision
var previousCumulative = cumulativeSavings – yearSavings;
var remaining = netCost – previousCumulative;
var fraction = remaining / yearSavings;
paybackYears = (i – 1) + fraction;
paybackFound = true;
}
// Increase electricity rate for next year
currentRate = currentRate * (1 + (rateIncrease / 100));
// Degrade panel efficiency slightly (0.5% per year standard)
annualProduction = annualProduction * 0.995;
}
// 4. Update UI
document.getElementById('resNetCost').innerText = "$" + netCost.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('resProduction').innerText = Math.round(sysSize * sunHours * 365 * efficiencyFactor).toLocaleString() + " kWh/year";
document.getElementById('resYear1').innerText = "$" + (sysSize * sunHours * 365 * efficiencyFactor * elecRate).toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
// Net lifetime savings (Savings – Net Cost)
var netLifetime = lifetimeSavings – netCost;
document.getElementById('resLifetime').innerText = "$" + netLifetime.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
var paybackText = "";
if (paybackFound) {
paybackText = paybackYears.toFixed(1) + " Years";
} else {
paybackText = "25+ Years";
}
document.getElementById('resPayback').innerText = paybackText;
// Show results
document.getElementById('solar-results').style.display = "block";
}
Solar Panel Payback & ROI Calculator
Gross cost before incentives.
Current US Federal ITC is 30%.
Avg: 4-5 hours in US.
Net System Cost (After Tax Credit):
–
Annual Solar Production:
–
First Year Savings:
–
25-Year Lifetime Savings:
–
Payback Period:
–
Understanding Your Solar Payback Period
The solar payback period is the amount of time it takes for your solar energy system to "pay for itself" through savings on your electric bill. Unlike a car or furniture, solar panels are a financial investment that generates a return. This calculator helps homeowners estimate the financial viability of going solar by analyzing upfront costs against long-term utility savings.
Key Factors in the Calculation
- Total System Cost: The gross price of equipment and installation. The national average typically ranges between $2.50 and $3.50 per watt.
- Federal Tax Credit (ITC): The Investment Tax Credit allows you to deduct a percentage of your solar costs (currently 30% until 2032) from your federal taxes, significantly lowering the net cost.
- Peak Sun Hours: This isn't just daylight hours; it represents the intensity of sunlight your panels receive. States like Arizona may see 6+ hours, while cooler climates may see 3.5-4 hours.
- Electricity Rate & Inflation: Higher local electricity rates mean faster savings. Furthermore, utility rates historically rise by 2-3% annually, increasing the value of your solar energy over time.
What is a Good Payback Period?
Generally, a solar payback period between 6 to 9 years is considered excellent. With most solar panels warranted for 25 years, a payoff occurring in year 8 means you enjoy roughly 17 years of essentially free electricity. Even a payback period of 10-12 years can offer a better Return on Investment (ROI) than traditional savings accounts or bonds.