A PVWatts calculator is an essential tool for homeowners, solar installers, and engineers to estimate the energy production and cost of grid-connected photovoltaic energy systems. Developed based on logic from the National Renewable Energy Laboratory (NREL), this calculator accounts for various physical and environmental factors that impact solar efficiency.
Key Input Factors Explained
DC System Size: This is the total nameplate capacity of your solar panels in kilowatts (kW). For example, twenty 300W panels equal a 6kW system.
Module Type: Standard modules use crystalline silicon. Premium modules have higher efficiency and better temperature coefficients, while thin-film modules are often lighter but less efficient per square meter.
Array Type: How the panels are mounted. "Roof Mount" typically yields slightly less energy than "Open Rack" due to higher operating temperatures from restricted airflow. Tracking systems follow the sun to increase yield but are more mechanically complex.
System Losses: This accounts for energy lost to wiring, inverter inefficiency, soiling (dirt/dust), snow, and aging. A standard default is 14%.
Tilt & Azimuth: Tilt is the angle from the horizontal. Azimuth is the compass direction the panels face (180° is South in the Northern Hemisphere).
Example Production Scenario
If you have a 5kW system in a location with 5.5 peak sun hours per day, using Standard modules on a Roof Mount with 14% losses:
Annual Production ≈ 5kW × 5.5 hours × 365 days × 0.86 (efficiency) × 0.95 (mount factor) ≈ 8,200 kWh per year.
Why Calculate Production?
Calculating your expected output helps determine your "Solar ROI" (Return on Investment). By comparing the estimated annual value to the installation cost, you can determine how many years it will take for the system to pay for itself through utility bill savings.
function calculatePVWatts() {
var dcCapacity = parseFloat(document.getElementById('dcCapacity').value);
var moduleType = parseFloat(document.getElementById('moduleType').value);
var arrayType = parseFloat(document.getElementById('arrayType').value);
var systemLosses = parseFloat(document.getElementById('systemLosses').value);
var tilt = parseFloat(document.getElementById('tilt').value);
var azimuth = parseFloat(document.getElementById('azimuth').value);
var sunHours = parseFloat(document.getElementById('sunHours').value);
var costPerKwh = parseFloat(document.getElementById('electricityCost').value);
if (isNaN(dcCapacity) || isNaN(sunHours) || isNaN(systemLosses)) {
alert("Please enter valid numerical values.");
return;
}
// Basic PVWatts logic: Capacity * Sun Hours * Days * Efficiency * Module/Array Multipliers
var derateFactor = (1 – (systemLosses / 100));
// Adjusting for tilt/azimuth efficiency (simplified geometric approximation for general tool)
// Most solar peaks at 180 azimuth and latitude-matching tilt.
var tiltEfficiency = 1.0;
if (tilt 50) tiltEfficiency -= 0.05;
var azimuthRad = (azimuth – 180) * (Math.PI / 180);
var azimuthEfficiency = Math.cos(azimuthRad);
// Constrain azimuth efficiency so it doesn't go negative or unrealistic for a simple tool
azimuthEfficiency = Math.max(0.7, (azimuthEfficiency + 1) / 2);
var annualKwh = dcCapacity * sunHours * 365 * derateFactor * moduleType * arrayType * tiltEfficiency * azimuthEfficiency;
var monthlyKwh = annualKwh / 12;
var annualDollarValue = annualKwh * costPerKwh;
document.getElementById('annualEnergy').innerText = Math.round(annualKwh).toLocaleString();
document.getElementById('monthlyEnergy').innerText = Math.round(monthlyKwh).toLocaleString();
document.getElementById('annualValue').innerText = '$' + annualDollarValue.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('pvResult').style.display = 'block';
}