Solar Panel Output Calculator

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Solar Panel Output Calculator

Estimate the daily and annual energy output of a solar panel system based on panel wattage, quantity, peak sun hours, and system efficiency losses.

Panel Wattage (W)

Rated power output of each panel (typically 250–600 W)

Number of Panels

Total number of solar panels in the system

Peak Sun Hours (hours/day)

Average daily peak sun hours for your location (1–7 typical; use 4–5 for most of the US)

System Efficiency (%)

Overall system efficiency accounting for inverter, wiring, temperature, and soiling losses (typically 75–85%)

Electricity Rate (¢/kWh) (optional)

Your utility rate in cents per kWh — used to estimate annual savings

Calculate

function solCalc() { const resultDiv = document.getElementById('sol-result'); resultDiv.style.display = 'none'; resultDiv.innerHTML = '';

const panelW = parseFloat(document.getElementById('sol-panel-wattage').value); const numPanels = parseFloat(document.getElementById('sol-num-panels').value); const psh = parseFloat(document.getElementById('sol-peak-sun-hours').value); const effPct = parseFloat(document.getElementById('sol-system-efficiency').value); const rateCents = parseFloat(document.getElementById('sol-electricity-rate').value);

// --- Validation --- const errors = []; if (isNaN(panelW) || panelW 1000) errors.push("Panel wattage must be between 1 and 1,000 W."); if (isNaN(numPanels) || numPanels 10000) errors.push("Number of panels must be between 1 and 10,000."); if (isNaN(psh) || psh 12) errors.push("Peak sun hours must be between 0.5 and 12."); if (isNaN(effPct) || effPct 100) errors.push("System efficiency must be between 1% and 100%."); if (!isNaN(rateCents) && (rateCents 200)) errors.push("Electricity rate must be between 0 and 200 ¢/kWh.");

if (errors.length > 0) { resultDiv.style.display = 'block'; resultDiv.innerHTML = '' + errors.map(e => '').join('') + ''; return; }

const eff = effPct / 100;

// --- Core Formula --- // System DC capacity (kW) const systemKw = (panelW * numPanels) / 1000;

// Daily AC energy output (kWh/day) // E_daily = P_system_kW × PSH × η_system const dailyKwh = systemKw * psh * eff;

// Annual energy output (kWh/year) const annualKwh = dailyKwh * 365;

// Monthly average (kWh/month) const monthlyKwh = annualKwh / 12;

// Performance Ratio check (eff is the PR here) // Capacity Factor = E_annual / (P_system_kW × 8760) const capacityFactor = annualKwh / (systemKw * 8760) * 100;

// CO₂ offset: US average grid emission factor ≈ 0.386 kg CO₂/kWh (EPA 2023) const co2KgPerKwh = 0.386; const annualCo2Kg = annualKwh * co2KgPerKwh; const annualCo2Tonnes = annualCo2Kg / 1000;

// Annual savings (optional) let savingsHTML = ''; if (!isNaN(rateCents) && rateCents > 0) { const annualSavings = annualKwh * (rateCents / 100); savingsHTML = 'Estimated Annual Savings$' + annualSavings.toLocaleString('en-US', {minimumFractionDigits:2, maximumFractionDigits:2}) + ''; }

// --- Output --- resultDiv.style.display = 'block'; resultDiv.innerHTML = '### Results ' + '' + 'System DC Capacity' + systemKw.toFixed(2) + ' kW' + 'Daily Energy Output' + dailyKwh.toFixed(2) + ' kWh/day' + 'Monthly Energy Output' + monthlyKwh.toFixed(1) + ' kWh/month' + 'Annual Energy Output' + annualKwh.toFixed(0) + ' kWh/year' + 'Capacity Factor' + capacityFactor.toFixed(1) + '%' + 'Annual CO₂ Offset' + annualCo2Tonnes.toFixed(2) + ' metric tons CO₂' + savingsHTML + ''; }

#### Formula

System DC Capacity (kW)

Psystem = (Panel Wattage × Number of Panels) ÷ 1,000

Daily AC Energy Output (kWh/day)

Edaily = Psystem × PSH × ηsystem

Annual Energy Output (kWh/year)

Eannual = Edaily × 365

Capacity Factor (%)

CF = Eannual ÷ (Psystem × 8,760) × 100

CO₂ Offset (metric tons/year)

CO₂ = Eannual × 0.386 kg/kWh ÷ 1,000

Where: PSH = Peak Sun Hours (equivalent full-sun hours per day); ηsystem = overall system efficiency (inverter + wiring + temperature + soiling losses).

#### Assumptions & References

Peak Sun Hours (PSH) represent the number of hours per day when solar irradiance averages 1,000 W/m² (1 kW/m²). This is location-dependent and can be looked up via NREL PVWatts or the Global Solar Atlas.
System efficiency (Performance Ratio) of 75–85% is typical for modern grid-tied systems, accounting for inverter losses (~4%), DC wiring losses (~2%), temperature derating (~5%), soiling (~2%), and mismatch losses (~2%). Source: NREL, 2014.
CO₂ emission factor of 0.386 kg CO₂/kWh is the US average grid emission factor per the EPA eGRID 2022 national average.
Annual output assumes uniform daily production (Edaily × 365). Actual output varies seasonally.

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