function calculateSolarROI() {
var cost = parseFloat(document.getElementById('solar_total_cost').value);
var size = parseFloat(document.getElementById('solar_system_size').value);
var sunHours = parseFloat(document.getElementById('solar_sun_hours').value);
var rate = parseFloat(document.getElementById('solar_elec_rate').value);
if (isNaN(cost) || isNaN(size) || isNaN(sunHours) || isNaN(rate) || cost <= 0 || size <= 0 || sunHours <= 0 || rate <= 0) {
alert('Please enter valid positive numbers for all fields.');
return;
}
// Calculation Logic
// Monthly Generation: Size (kW) * Daily Sun Hours * 30 days * 0.78 (Standard 22% efficiency loss due to inverter/dust/wiring)
var monthlyGen = size * sunHours * 30 * 0.78;
var monthlySavings = monthlyGen * rate;
var annualSavings = monthlySavings * 12;
var paybackYears = cost / annualSavings;
var totalLifetimeSavings = (annualSavings * 25) – cost;
// Display Results
document.getElementById('res_monthly_gen').innerText = monthlyGen.toFixed(0) + ' kWh';
document.getElementById('res_monthly_save').innerText = '$' + monthlySavings.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res_payback').innerText = paybackYears.toFixed(1) + ' Years';
document.getElementById('res_lifetime').innerText = '$' + totalLifetimeSavings.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('solar-results-box').style.display = 'block';
}
Understanding Your Solar Investment
Switching to solar energy is one of the most effective ways for homeowners to reduce their carbon footprint while locking in long-term financial savings. This calculator helps you estimate the Return on Investment (ROI) by analyzing your local environment and system specifications.
Key Factors in Solar Calculations
System Size (kW): Most residential systems range from 5kW to 10kW. A larger system generates more power but comes with a higher upfront cost.
Daily Sun Hours: This is not the total time the sun is up, but the "peak" sunlight hours your panels receive. For example, Arizona may average 6 hours, while Washington may average 3.5.
Efficiency Loss: Our calculator assumes a 22% loss for real-world factors like inverter inefficiency, wiring resistance, and dust accumulation on the panels.
The Payback Period: This is the time it takes for your electricity bill savings to equal the initial cost of the installation. In most US states, this ranges from 6 to 10 years.
Example Scenario: A 6kW System
Imagine a homeowner in California installing a 6kW system for a net cost of $14,000 (after tax credits). If they receive 5.5 sun hours a day and pay $0.22/kWh for electricity:
"With 6kW of panels and 5.5 hours of peak sun, the system produces roughly 772 kWh per month. At $0.22 per kWh, that is $169 in monthly savings. The system pays for itself in just under 7 years, leading to over $35,000 in net profit over the 25-year lifespan of the panels."
Don't Forget the Federal Tax Credit (ITC)
When entering your Total System Cost, remember to subtract the Federal Solar Tax Credit. As of 2023-2024, the government allows you to deduct 30% of your solar installation costs from your federal taxes. If your quote is $20,000, your effective cost for this calculator should be $14,000.