How to Calculate Solar System Size

Solar System Size Calculator

Enter your average daily electricity usage in kilowatt-hours.

Enter the average number of peak sun hours for your location (e.g., 4-6 hours).

Account for inefficiencies (inverter, wiring, temperature, dust). Typical range is 10-25%.

Enter the wattage of a single solar panel you plan to use (e.g., 300W – 450W).

function calculateSolarSize() { var dailyKWH = parseFloat(document.getElementById('dailyKWH').value); var peakSunHours = parseFloat(document.getElementById('peakSunHours').value); var systemLoss = parseFloat(document.getElementById('systemLoss').value); var panelWattage = parseFloat(document.getElementById('panelWattage').value); if (isNaN(dailyKWH) || dailyKWH <= 0) { alert('Please enter a valid positive number for Average Daily Energy Consumption.'); return; } if (isNaN(peakSunHours) || peakSunHours <= 0) { alert('Please enter a valid positive number for Average Daily Peak Sun Hours.'); return; } if (isNaN(systemLoss) || systemLoss = 100) { alert('Please enter a valid System Loss Factor (0-99).'); return; } if (isNaN(panelWattage) || panelWattage <= 0) { alert('Please enter a valid positive number for Individual Solar Panel Wattage.'); return; } // Step 1: Convert daily kWh to daily Wh var totalDailyWhNeeded = dailyKWH * 1000; // Step 2: Calculate effective peak sun hours after accounting for system losses var effectiveSunHours = peakSunHours * (1 – (systemLoss / 100)); // Step 3: Calculate total panel wattage required (Wp) var totalPanelWattageRequired = totalDailyWhNeeded / effectiveSunHours; // Step 4: Calculate required system size in kW var requiredSystemSizeKW = totalPanelWattageRequired / 1000; // Step 5: Calculate the number of panels needed (round up) var numPanels = Math.ceil(totalPanelWattageRequired / panelWattage); document.getElementById('requiredSystemSizeKW').innerText = requiredSystemSizeKW.toFixed(2) + ' kW'; document.getElementById('numPanels').innerText = numPanels + ' panels'; }

Understanding How to Calculate Solar System Size

Determining the right size for your solar panel system is a crucial first step in going solar. An accurately sized system ensures you generate enough electricity to meet your needs, maximizing your savings and environmental impact. This calculator helps you estimate the total solar panel capacity and the number of panels required based on your specific energy consumption and local conditions.

Key Factors in Solar System Sizing:

1. Average Daily Energy Consumption (kWh)

This is the most critical input. It represents how much electricity your household or business uses on an average day. You can typically find this information on your monthly electricity bills, often listed as "kWh used" or "energy consumption." For example, if your bill shows 600 kWh for a 30-day period, your average daily consumption is 20 kWh (600 kWh / 30 days).

Why it matters: The primary goal of a solar system is to offset your electricity usage. Knowing your daily consumption allows us to calculate how much energy your panels need to produce.

2. Average Daily Peak Sun Hours

Peak sun hours (also known as solar insolation) refer to the equivalent number of hours per day when solar irradiance averages 1,000 watts per square meter. This isn't the same as the total daylight hours; it's a measure of the intensity and duration of effective sunlight for solar energy production. This value varies significantly by geographic location, season, and even roof orientation.

Why it matters: More peak sun hours mean your panels can generate more electricity in a shorter amount of time, potentially requiring fewer panels or a smaller system to meet the same energy demand.

Typical ranges:

• Northern US: 3-5 hours
• Southern US: 4-6 hours
• Equatorial regions: 5-7 hours

You can find specific data for your location from resources like the National Renewable Energy Laboratory (NREL) or local solar installers.

3. System Loss Factor (%)

No solar system operates at 100% efficiency. Various factors contribute to energy loss between the panels and your home's electrical system. These include:

• Inverter efficiency: Converts DC power from panels to AC power for your home.
• Wiring losses: Resistance in cables.
• Temperature losses: Panels become less efficient at higher temperatures.
• Dust and shading: Accumulation on panels or partial shading.
• Module mismatch: Slight variations in panel performance.

Why it matters: Accounting for these losses ensures your system is sized to produce the *net* energy you need, not just the theoretical maximum. A typical system loss factor ranges from 10% to 25%.

4. Individual Solar Panel Wattage (W)

This refers to the power output rating of a single solar panel under standard test conditions (STC). Modern residential solar panels typically range from 300 watts to 450 watts or more.

Why it matters: Knowing the wattage of individual panels allows the calculator to determine how many physical panels you'll need to achieve the total required system capacity.

How the Calculation Works (Simplified):

1. Total Daily Watt-hours Needed: Your daily kWh consumption is converted to Watt-hours (kWh * 1000).
2. Effective Sun Hours: Your peak sun hours are adjusted downwards by the system loss factor to reflect real-world performance.
3. Total Panel Wattage Required (Wp): The total daily Watt-hours needed is divided by the effective sun hours. This gives you the total peak wattage capacity your solar array must have.
4. Required System Size (kW): The total panel wattage required is converted to kilowatts (Wp / 1000).
5. Number of Panels: The total panel wattage required is divided by the wattage of a single panel, and the result is rounded up to ensure sufficient coverage.

Example Scenario:

Let's say your average daily energy consumption is 20 kWh. Your location gets an average of 4.5 peak sun hours per day. You estimate a 15% system loss factor, and you plan to use solar panels rated at 400 W each.

• Total Daily Wh Needed: 20 kWh * 1000 = 20,000 Wh
• Effective Sun Hours: 4.5 hours * (1 – 0.15) = 4.5 * 0.85 = 3.825 hours
• Total Panel Wattage Required: 20,000 Wh / 3.825 hours = 5228.75 Wp
• Required System Size: 5228.75 Wp / 1000 = 5.23 kW
• Number of Panels: 5228.75 Wp / 400 W/panel = 13.07 panels. Rounded up, you would need 14 panels.

This calculator provides a strong estimate, but for a precise system design, always consult with a professional solar installer who can conduct a detailed site assessment, considering factors like roof space, shading, and local building codes.