function calculateSolar() {
// 1. Get Input Values
var monthlyBill = parseFloat(document.getElementById('monthlyBill').value);
var costPerKwh = parseFloat(document.getElementById('costPerKwh').value);
var sunHours = parseFloat(document.getElementById('sunHours').value);
var panelWattage = parseFloat(document.getElementById('panelWattage').value);
var installCostPerWatt = parseFloat(document.getElementById('installCostPerWatt').value);
var taxCreditPercent = parseFloat(document.getElementById('taxCredit').value);
// 2. Validate Inputs
if (isNaN(monthlyBill) || monthlyBill <= 0) {
alert("Please enter a valid monthly electric bill.");
return;
}
if (isNaN(costPerKwh) || costPerKwh <= 0) {
alert("Please enter a valid electricity rate.");
return;
}
// 3. Perform Calculations
// Monthly usage in kWh = Bill / Rate
var monthlyKwhUsage = monthlyBill / costPerKwh;
// Daily usage in kWh
var dailyKwhUsage = monthlyKwhUsage / 30.41; // 30.41 avg days per month
// Required System Size (kW) = Daily Usage / Sun Hours
// We add a 25% buffer (1.25) for system inefficiencies (inverter loss, wiring, heat, dust)
var inefficiencyFactor = 1.25;
var requiredSystemSizeKw = (dailyKwhUsage / sunHours) * inefficiencyFactor;
// Number of Panels = (System Size in kW * 1000) / Panel Wattage
var numberOfPanels = Math.ceil((requiredSystemSizeKw * 1000) / panelWattage);
// Adjusted System Size based on integer panel count
var actualSystemSizeKw = (numberOfPanels * panelWattage) / 1000;
// Financials
var grossCost = actualSystemSizeKw * 1000 * installCostPerWatt;
var taxCreditAmount = grossCost * (taxCreditPercent / 100);
var netCost = grossCost – taxCreditAmount;
// Payback Period
// Annual bill savings = Monthly Bill * 12 (assuming 100% offset)
var annualSavings = monthlyBill * 12;
var breakEvenYears = netCost / annualSavings;
// 25 Year Savings (Account for utility inflation)
// Assume utility rates rise 3% per year
var totalUtilityCost25Years = 0;
var currentAnnualBill = annualSavings;
var utilityInflation = 0.03; // 3%
for (var i = 0; i < 25; i++) {
totalUtilityCost25Years += currentAnnualBill;
currentAnnualBill = currentAnnualBill * (1 + utilityInflation);
}
var totalSavings25Years = totalUtilityCost25Years – netCost;
// 4. Update UI
document.getElementById('res-systemSize').innerText = actualSystemSizeKw.toFixed(2) + " kW";
document.getElementById('res-panels').innerText = numberOfPanels + " Panels";
document.getElementById('res-grossCost').innerText = "$" + grossCost.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('res-taxCredit').innerText = "-$" + taxCreditAmount.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('res-netCost').innerText = "$" + netCost.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('res-breakEven').innerText = breakEvenYears.toFixed(1) + " Years";
document.getElementById('res-savings').innerText = "$" + totalSavings25Years.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
// Show results div
document.getElementById('solar-results').style.display = "block";
}
How to Calculate Your Solar Panel Needs
Switching to solar energy is a major financial decision that involves understanding your current energy consumption, the solar potential of your roof, and the upfront costs versus long-term savings. Our Solar Panel Cost & Savings Calculator uses industry-standard formulas to help you estimate the system size required to offset your electric bill.
1. Determining Your Energy Usage
The first step in sizing a solar system is determining how many kilowatt-hours (kWh) you use. We calculate this by taking your average monthly bill and dividing it by your local electricity rate (Cost per kWh). For example, if your bill is $150 and you pay $0.16/kWh, you consume approximately 937 kWh per month.
2. Peak Sun Hours Explained
Not all sunlight creates equal energy. "Peak Sun Hours" refers to the specific number of hours per day where the solar intensity is strong enough (1,000 watts per square meter) to generate peak power. While the sun may be in the sky for 12 hours, you might only get 4 to 5 "peak" hours depending on your latitude and local weather patterns. Our calculator uses this metric to determine how large your system needs to be to generate enough daily power.
3. Calculating System Size
The formula for solar system sizing is:
Daily kWh Usage = Monthly Usage / 30
System Size (kW) = (Daily kWh / Peak Sun Hours) × 1.25 (Inefficiency Factor)
We apply a 1.25 inefficiency factor to account for energy losses in the inverter, wiring, and environmental factors like dust or heat, ensuring your system is robust enough to meet your needs.
4. The Federal Solar Tax Credit (ITC)
One of the most significant factors in solar ROI is the Federal Investment Tax Credit (ITC). As of 2024, this credit allows you to deduct 30% of the cost of installing a solar energy system from your federal taxes. This is a dollar-for-dollar reduction in the income tax you owe, significantly lowering the "Net Cost" of your installation and accelerating your break-even period.
5. Estimated 25-Year Savings
Solar panels typically have a warranty of 25 years. To calculate your long-term savings, we compare the net cost of the solar system against what you would have paid the utility company over that same period. Utility rates historically rise by about 3% annually. By locking in your energy costs now with solar, the gap between your fixed solar cost and rising utility rates grows every year, maximizing your return on investment.