Solar System Calculator

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Solar Panel System Sizing Calculator

System Recommendations

Recommended System Size:

Number of Panels Needed:

Est. Annual Generation:

Est. Monthly Savings:

function calculateSolar() { var monthlyKwh = parseFloat(document.getElementById('monthlyKwh').value); var sunHours = parseFloat(document.getElementById('sunHours').value); var panelWattage = parseFloat(document.getElementById('panelWattage').value); var elecRate = parseFloat(document.getElementById('elecRate').value); if (isNaN(monthlyKwh) || isNaN(sunHours) || isNaN(panelWattage) || isNaN(elecRate)) { alert("Please enter valid numbers in all fields."); return; } // Standard Efficiency Loss Factor (Derate factor) // Accounts for inverters, wiring, dust, and temperature (usually 75% to 80%) var derateFactor = 0.77; // Daily kWh needed var dailyKwh = monthlyKwh / 30; // System Size (kW) = Daily kWh / (Sun Hours * Efficiency) var systemSizeKw = dailyKwh / (sunHours * derateFactor); // Number of panels = (System Size * 1000) / Individual Panel Wattage var panelCount = Math.ceil((systemSizeKw * 1000) / panelWattage); // Real System Size based on rounded panel count var finalSystemSizeKw = (panelCount * panelWattage) / 1000; // Annual Generation = System Size * Sun Hours * 365 * derateFactor var annualGen = finalSystemSizeKw * sunHours * 365 * derateFactor; // Monthly Savings var monthlySavings = (annualGen / 12) * elecRate; document.getElementById('resSystemSize').innerHTML = finalSystemSizeKw.toFixed(2) + " kW"; document.getElementById('resPanelCount').innerHTML = panelCount + " Panels"; document.getElementById('resAnnualGen').innerHTML = Math.round(annualGen).toLocaleString() + " kWh"; document.getElementById('resSavings').innerHTML = "$" + monthlySavings.toFixed(2); document.getElementById('solarResult').style.display = "block"; }

How to Use the Solar System Calculator

Designing a residential solar energy system requires precision to ensure your investment covers your energy needs without overspending on excess capacity. This calculator helps you determine the physical and electrical scale of the solar installation required for your home.

Understanding the Key Inputs

  • Monthly Usage (kWh): Check your utility bill for your average kilowatt-hour consumption. It is best to use a 12-month average to account for seasonal spikes in heating or cooling.
  • Peak Sun Hours: This is not the total daylight hours, but the equivalent number of hours when solar intensity averages 1,000 watts per square meter. In the US, this typically ranges from 3.5 to 6 hours per day depending on geography.
  • Panel Wattage: Modern residential solar panels typically range from 350 to 450 watts. Higher wattage panels require less roof space.
  • Electricity Rate: The amount your utility company charges you per kWh. This is used to estimate your financial return on investment.

The Solar Sizing Formula

The logic used in this calculator follows industry standards for photovoltaic (PV) sizing. We apply a 0.77 derate factor, which accounts for real-world energy losses including:

  • DC to AC conversion (Inverter efficiency)
  • Electrical resistance in wiring
  • Soiling (dust, snow, or bird droppings on panels)
  • Performance degradation due to high temperatures

The math follows this sequence: Daily Target Production ÷ (Peak Sun Hours × 0.77) = Required System Capacity.

Practical Example

Suppose you live in a region with 5.0 Peak Sun Hours and consume 1,000 kWh per month. You plan to use 400W panels.

  1. Daily need: 1,000 kWh / 30 days = 33.33 kWh/day.
  2. System size: 33.33 / (5.0 * 0.77) = 8.65 kW.
  3. Panels needed: (8.65 * 1000) / 400 = 21.62 (Round up to 22 panels).
  4. If your rate is $0.15/kWh, your 22 panels will save you roughly $150 per month.

Important Considerations

While this calculator provides a highly accurate estimate, remember that roof orientation (azimuth) and shading from trees or nearby buildings can significantly impact actual performance. A south-facing roof in the northern hemisphere is ideal. If your roof faces East or West, you may need to increase the system size by 15-20% to achieve the same production targets.

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