function calculateSolarROI() {
var monthlyBill = parseFloat(document.getElementById('solar_monthly_bill').value);
var systemSize = parseFloat(document.getElementById('solar_system_size').value);
var costWatt = parseFloat(document.getElementById('solar_cost_watt').value);
var sunHours = parseFloat(document.getElementById('solar_sun_hours').value);
if (isNaN(monthlyBill) || isNaN(systemSize) || isNaN(costWatt) || isNaN(sunHours)) {
alert('Please enter valid numeric values.');
return;
}
var grossCost = systemSize * 1000 * costWatt;
var federalCredit = grossCost * 0.30;
var netInvestment = grossCost – federalCredit;
// Estimating annual production: Size * hours * days * efficiency factor (0.78 is standard derate)
var annualProductionkWh = systemSize * sunHours * 365 * 0.78;
// Average US kWh cost is roughly $0.16; but let's derive it from the bill (avg usage 900kWh for $150 bill)
var estimatedPricePerKwh = 0.16;
var annualSavings = Math.min(annualProductionkWh * estimatedPricePerKwh, monthlyBill * 12);
var paybackPeriod = netInvestment / annualSavings;
// 25 year savings accounting for 2.5% electricity inflation and 0.5% panel degradation
var totalSavings25 = 0;
var currentYearSavings = annualSavings;
for (var i = 1; i <= 25; i++) {
totalSavings25 += currentYearSavings;
currentYearSavings = currentYearSavings * 1.025 * 0.995;
}
var netProfit25 = totalSavings25 – netInvestment;
document.getElementById('res_gross_cost').innerText = '$' + grossCost.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res_itc').innerText = '-$' + federalCredit.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res_net_investment').innerText = '$' + netInvestment.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res_annual_savings').innerText = '$' + annualSavings.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res_payback').innerText = paybackPeriod.toFixed(1) + ' Years';
document.getElementById('res_25yr_savings').innerText = '$' + netProfit25.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('solar_results').style.display = 'block';
}
Understanding Your Solar Investment Return
Switching to solar energy is one of the most significant financial decisions a homeowner can make. This Solar ROI Calculator helps you estimate the time it takes for your system to pay for itself and the total profit you can expect over the 25-year lifespan of the equipment.
Key Factors in the Calculation
Gross System Cost: The total price paid for equipment and installation before any incentives. This is typically calculated as "Cost per Watt." Currently, the US average ranges from $2.50 to $3.50 per watt.
Federal Tax Credit (ITC): Under the Inflation Reduction Act, homeowners can claim a 30% federal tax credit on the total cost of their solar system. This significantly reduces the net investment.
Daily Sunlight Hours: Not all states are equal. A 6kW system in Arizona will produce significantly more power than the same system in Washington due to "Peak Sun Hours."
Payback Period: This is the "break-even" point. Most residential solar installations reach this point within 6 to 10 years.
Example Calculation
Let's look at a typical scenario for a medium-sized home:
Homeowner Profile: $150 Monthly Bill, 7kW System Size.
Gross Cost: 7,000 Watts × $3.00 = $21,000
After 30% Tax Credit: $14,700 Net Cost
Estimated Payback: With $1,800 annual savings, the system breaks even in approximately 8.1 years.
Is Solar Worth It in 2024?
As utility rates continue to rise (averaging 2-4% increases annually), solar acts as a hedge against inflation. By producing your own power, you lock in your energy costs for the next two decades. Furthermore, studies by Zillow suggest that solar installations can increase home value by an average of 4.1%.
Maintenance and Longevity
Most modern solar panels come with a 25-year performance warranty. While the panels themselves have no moving parts and require minimal maintenance (mostly occasional cleaning), you should budget for a potential inverter replacement around year 12 to 15, which is factored into most long-term ROI models.