function calculateSolarPower() {
var bill = parseFloat(document.getElementById('monthlyBill').value);
var rate = parseFloat(document.getElementById('kwhRate').value);
var sun = parseFloat(document.getElementById('sunHours').value);
var offset = parseFloat(document.getElementById('offset').value) / 100;
var panelW = parseFloat(document.getElementById('panelWatts').value);
if (isNaN(bill) || isNaN(rate) || isNaN(sun) || isNaN(offset) || isNaN(panelW) || rate <= 0 || sun <= 0) {
alert("Please enter valid positive numbers in all fields.");
return;
}
// 1. Calculate Monthly kWh consumption
var monthlyKwh = bill / rate;
// 2. Calculate daily target kWh
var dailyKwhTarget = (monthlyKwh / 30) * offset;
// 3. System Size (DC kW)
// Formula: (Daily kWh / Sun Hours) / Derate Factor (typically 0.8 to account for system losses)
var derateFactor = 0.8;
var systemSizeKw = (dailyKwhTarget / sun) / derateFactor;
// 4. Number of panels
var numberOfPanels = Math.ceil((systemSizeKw * 1000) / panelW);
// 5. Annual Generation
var annualKwh = systemSizeKw * sun * 365 * derateFactor;
// 6. Estimated Cost ($3 per watt is a standard national average)
var costPerWatt = 3.00;
var totalCost = systemSizeKw * 1000 * costPerWatt;
// 7. Payback Period (simple)
var annualSavings = (monthlyKwh * 12 * offset) * rate;
var paybackPeriod = totalCost / annualSavings;
// Display Results
document.getElementById('systemSize').innerHTML = systemSizeKw.toFixed(2) + " kW";
document.getElementById('panelCount').innerHTML = numberOfPanels + " Panels";
document.getElementById('annualGen').innerHTML = Math.round(annualKwh).toLocaleString() + " kWh";
document.getElementById('estCost').innerHTML = "$" + Math.round(totalCost).toLocaleString();
document.getElementById('payback').innerHTML = paybackPeriod.toFixed(1) + " Years";
document.getElementById('solarResults').style.display = 'block';
}
Complete Guide to Solar Power Calculation: Planning Your Energy Independence
Switching to renewable energy is a significant financial and environmental decision. To understand if solar is right for your home, you must accurately calculate the system size required to meet your specific energy demands. Our Solar Power Calculator simplifies this complex physics-based calculation into actionable data.
How the Solar System Size is Calculated
The math behind solar energy relies on several variables. To determine how many kilowatts (kW) your roof needs, we use the following formula:
System Size (kW) = (Monthly kWh Usage ÷ 30 Days) ÷ (Daily Peak Sun Hours × Efficiency Factor)
The "Efficiency Factor" (often 0.80) accounts for real-world losses such as wiring resistance, inverter conversion, and dust on the panels. Without this adjustment, your system would be undersized and fail to cover your bill.
Key Factors Influencing Solar Production
Peak Sun Hours: This is not the total daylight time, but the intensity of sunlight equivalent to 1,000 watts per square meter. Most regions in the US range between 3.5 and 6.0 peak sun hours per day.
Electricity Consumption: Measured in kilowatt-hours (kWh), your bill reflects your total energy "burn rate." Most residential solar systems are designed to offset 100% of this usage.
Panel Efficiency & Wattage: Modern panels typically range from 350W to 450W. Higher wattage panels require less roof space to achieve the same total system output.
Understanding Your Return on Investment (ROI)
When you calculate solar ROI, you are comparing the upfront cost of the hardware and installation against the cumulative savings on your utility bills over 25 years. Most homeowners see a payback period between 6 and 10 years. After this point, the electricity generated is essentially free for the remainder of the system's life.
Example Solar Calculation
Imagine a home in Phoenix, Arizona:
Monthly Bill: $200
Electricity Rate: $0.13 per kWh
Peak Sun Hours: 5.8 hours
Result: This homeowner would need approximately a 10.5 kW system consisting of roughly 26 panels (at 400W each) to achieve a 100% energy offset.
Environmental Impact
Beyond the financial savings, a standard residential solar installation reduces approximately 100,000 pounds of carbon dioxide emissions over 20 years. This is equivalent to planting over 2,000 trees or driving 100,000 fewer miles in a gasoline-powered vehicle.