Estimate the number of panels needed to power your home.
Check your utility bill for average kWh.
U.S. average is approx. 4-5 hours.
Common panels range from 300W-450W.
Accounts for wiring/inverter losses (75% is standard).
Your Solar Estimate:
0 Panels
Required System Size
0 kW
Estimated Roof Space
0 sq. ft.
Understanding Your Solar Panel Requirements
Switching to solar power is a significant step toward energy independence and reducing your carbon footprint. However, the most common question homeowners ask is: "How many solar panels do I actually need?" Determining the correct system size is crucial to ensure you cover 100% of your energy needs without overpaying for equipment you don't use.
The Three Key Factors
Annual Energy Consumption: This is the total kilowatt-hours (kWh) your home uses. Our calculator uses monthly data to find your daily average.
Solar Irradiance (Peak Sun Hours): Not every hour of daylight is equal. Peak sun hours represent the intensity of sunlight in your specific geographic location. Arizona will have more peak sun hours than Washington state.
Solar Panel Wattage: Modern residential panels typically range from 320 to 450 watts. Higher wattage panels mean you need fewer panels overall to achieve the same energy output.
The Calculation Formula
To find the number of panels, we use the following scientific approach:
1. (Daily kWh / Sun Hours) / Efficiency Factor = Required System kW
2. (System kW * 1000) / Panel Wattage = Total Panels Required
A Real-World Example
If your home uses 900 kWh per month and you live in an area with 5 peak sun hours per day:
Your daily usage is 30 kWh (900 / 30).
Accounting for 75% system efficiency (to cover losses in DC to AC conversion), you need a system that can produce 40 kWh daily in ideal conditions.
40 kWh / 5 sun hours = an 8 kW system.
If you use 400W panels, you would need 20 panels (8000W / 400W).
Roof Space Considerations
On average, a residential solar panel is about 17.5 square feet. If our calculator suggests you need 20 panels, you will require approximately 350 square feet of clear, unshaded, south-facing roof space to maximize your investment.
function calculateSolar() {
var monthlyUsage = parseFloat(document.getElementById('monthlyUsage').value);
var sunHours = parseFloat(document.getElementById('sunHours').value);
var panelWattage = parseFloat(document.getElementById('panelWattage').value);
var efficiency = parseFloat(document.getElementById('efficiency').value) / 100;
if (isNaN(monthlyUsage) || isNaN(sunHours) || isNaN(panelWattage) || isNaN(efficiency) || sunHours <= 0 || panelWattage <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
// 1. Calculate daily energy target in kWh
var dailyKwh = monthlyUsage / 30;
// 2. Calculate system size in kW required
// Formula: (Daily Usage / Sun Hours) / Efficiency
var rawKw = dailyKwh / sunHours;
var systemSizeKw = rawKw / efficiency;
// 3. Calculate number of panels
// Formula: (System Watts) / Panel Wattage
var totalWattsNeeded = systemSizeKw * 1000;
var numberOfPanels = Math.ceil(totalWattsNeeded / panelWattage);
// 4. Calculate approximate roof space (avg 17.5 sq ft per panel)
var sqFt = numberOfPanels * 17.5;
// Display Results
document.getElementById('panelCount').innerText = numberOfPanels;
document.getElementById('systemSize').innerText = systemSizeKw.toFixed(2) + " kW";
document.getElementById('roofSpace').innerText = Math.round(sqFt) + " sq. ft.";
document.getElementById('solarResult').style.display = 'block';
// Smooth scroll to result
document.getElementById('solarResult').scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}