Estimate your system size, potential savings, and break-even point.
Check your utility bill (avg is ~0.16)
US Average is 3.5 – 5.5 hours
Gross cost before tax credits
Currently 30% in the US
Estimated yearly rate hike
Rec. System Size
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Net System Cost
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First Year Savings
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Payback Period
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20-Year Lifetime Savings
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Is Solar Worth the Investment? Understanding ROI
Switching to solar energy is one of the most significant home improvement decisions a homeowner can make. While the environmental benefits are clear, the financial viability—specifically the Return on Investment (ROI)—is often the deciding factor. This calculator helps you determine how long it will take for your solar panel system to pay for itself through electricity bill savings.
Key Factors Affecting Solar Payback
The "Payback Period" is the time it takes for your cumulative energy savings to equal the net cost of your solar system. Several variables influence this timeline:
Current Electricity Rates: Homeowners with high utility rates (above $0.16/kWh) typically see a faster ROI because every kWh generated by solar offsets a more expensive purchase from the grid.
Peak Sun Hours: This is not just the hours of daylight, but the intensity of the sun. A home in Arizona (5.5+ peak hours) will generate more power with the same equipment than a home in Seattle (3.5 peak hours).
System Cost & Incentives: The gross cost of installation is reduced significantly by the Federal Investment Tax Credit (ITC), which is currently set at 30% for eligible systems. State and local rebates can further reduce the net cost.
How We Calculate System Size
To offset 100% of your energy usage, we first calculate your monthly kWh consumption based on your bill amount and rate per kWh. We then account for an efficiency loss factor (typically around 20-25% due to temperature, wiring, and inverter inefficiency) to recommend a system size (in kW) that meets your daily demand.
Long-Term Savings
Solar panels are long-term assets, typically warrantied for 25 years. Once the "Payback Period" is reached, the electricity generated is essentially free profit. Our 20-year savings calculation factors in the rising cost of utility power (inflation), demonstrating the hedge solar provides against future rate hikes.
function calculateSolar() {
// 1. Get Inputs
var bill = parseFloat(document.getElementById('monthlyBill').value);
var rate = parseFloat(document.getElementById('kwhCost').value);
var sun = parseFloat(document.getElementById('sunHours').value);
var cost = parseFloat(document.getElementById('systemCost').value);
var creditPct = parseFloat(document.getElementById('taxCredit').value);
var inflation = parseFloat(document.getElementById('annualIncrease').value);
// 2. Validate Inputs
if(isNaN(bill) || isNaN(rate) || isNaN(sun) || isNaN(cost) || isNaN(creditPct) || isNaN(inflation)) {
alert("Please fill in all fields with valid numbers.");
return;
}
if(rate === 0 || sun === 0) {
alert("Rate and Sun Hours cannot be zero.");
return;
}
// 3. Calculation Logic
// Step A: Calculate Energy Needs
var monthlyKwh = bill / rate;
var dailyKwh = monthlyKwh / 30.41; // Average days per month
// Step B: Calculate Required System Size (kW)
// Formula: Daily kWh / Sun Hours / Efficiency Factor (0.75 is conservative for system losses)
var systemEfficiency = 0.75;
var requiredSystemSize = dailyKwh / sun / systemEfficiency;
// Step C: Financials – Net Cost
var taxCreditAmount = cost * (creditPct / 100);
var netCost = cost – taxCreditAmount;
// Step D: Financials – Savings
// Annual Production in kWh = kW size * sun hours * 365 days * efficiency
var annualProductionKwh = requiredSystemSize * sun * 365 * systemEfficiency;
var firstYearSavings = annualProductionKwh * rate;
// Step E: Payback Period & 20 Year Outlook (with inflation)
var cumulativeSavings = 0;
var paybackYears = 0;
var foundPayback = false;
var currentYearSavings = firstYearSavings;
var currentRate = rate;
// Loop through 20 years to calculate cumulative savings with energy inflation
for(var i = 1; i = netCost) {
// Approximate fraction of the year
var previousTotal = cumulativeSavings – currentYearSavings;
var remainingNeeded = netCost – previousTotal;
var fraction = remainingNeeded / currentYearSavings;
paybackYears = (i – 1) + fraction;
foundPayback = true;
}
// Increase savings for next year based on energy inflation
// Assuming production degrades slightly (0.5%) and price increases
currentYearSavings = currentYearSavings * (1 + (inflation / 100)) * 0.995;
}
var lifetimeSavings = cumulativeSavings – netCost;
// 4. Update UI
document.getElementById('resSystemSize').innerHTML = requiredSystemSize.toFixed(2) + " kW";
document.getElementById('resNetCost').innerHTML = "$" + netCost.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('resAnnualSavings').innerHTML = "$" + firstYearSavings.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
if (foundPayback) {
document.getElementById('resPayback').innerHTML = paybackYears.toFixed(1) + " Years";
} else {
document.getElementById('resPayback').innerHTML = "20+ Years";
}
document.getElementById('res20Year').innerHTML = "$" + lifetimeSavings.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
// Show Results
document.getElementById('solar-results').style.display = 'block';
}