The PV Watts Calculator is a valuable tool for estimating the energy production and financial benefits of a solar photovoltaic (PV) system. It helps homeowners and businesses understand how much electricity their solar panels might generate and how much they could save on their electricity bills over the lifespan of the system. The underlying calculations are based on various factors related to system performance, local climate data, and energy costs.
How it Works (The Math)
While the official NREL PVWatts Calculator uses complex meteorological and system loss modeling, a simplified version for estimation focuses on key inputs:
System Size (kW DC): This is the rated capacity of the solar panels in kilowatts under direct current (DC) conditions. It's a primary driver of potential energy output.
Estimated Annual Production (kWh/year): This is a projection of how many kilowatt-hours (kWh) of alternating current (AC) electricity the system will produce annually. This accounts for DC to AC conversion losses, inverter efficiency, wiring losses, shading, soiling, and other system inefficiencies. If not provided, a rough estimate can be made based on system size and typical performance ratios for a given location (e.g., 1000-1500 kWh per kW DC per year in many sunny regions), but direct estimation is more accurate.
Total System Cost ($): The upfront investment required to purchase and install the solar PV system.
Total Incentives/Rebates ($): Any grants, tax credits, or rebates received that reduce the net cost of the system.
Cost of Electricity ($/kWh): The average price paid to the utility company for electricity. This is crucial for calculating savings.
System Lifespan (Years): The expected operational life of the solar PV system, typically 25-30 years.
Simplified Calculation Logic:
Net System Cost: This is the total cost after accounting for incentives.
Net System Cost = Total System Cost - Total Incentives/Rebates
Total Energy Production over Lifespan: The total estimated kWh the system will generate over its lifetime.
Total Energy Production = Estimated Annual Production (kWh/year) * System Lifespan (Years)
Total Electricity Bill Savings: The total amount saved by generating your own electricity instead of buying it from the grid.
Total Savings = Total Energy Production * Cost of Electricity ($/kWh)
Simple Payback Period (Years): The time it takes for the total savings to equal the net system cost.
Simple Payback Period = Net System Cost / (Estimated Annual Production * Cost of Electricity ($/kWh)) Or, more directly:
Simple Payback Period = Net System Cost / Annual Savings Where Annual Savings = Estimated Annual Production * Cost of Electricity ($/kWh)
Return on Investment (ROI) over Lifespan: The total profit over the system's life relative to the net cost.
ROI = ((Total Savings - Net System Cost) / Net System Cost) * 100%
Note: This simplified calculator provides an estimate. Actual performance can vary due to many factors, including weather, installation specifics, panel degradation, and changes in electricity rates. The NREL PVWatts Calculator offers a more sophisticated and site-specific estimation.
Use Cases:
Homeowners: Estimating the financial viability of installing rooftop solar.
Businesses: Assessing the potential cost savings and ROI for commercial solar installations.
Policy Makers: Understanding the economic impact of solar incentives.
Educators: Demonstrating the principles of solar energy economics.
function calculatePVWatts() {
var systemSize = parseFloat(document.getElementById("systemSize").value);
var annualProduction = parseFloat(document.getElementById("annualProduction").value);
var systemCost = parseFloat(document.getElementById("systemCost").value);
var incentives = parseFloat(document.getElementById("incentives").value);
var electricityRate = parseFloat(document.getElementById("electricityRate").value);
var systemLifespan = parseFloat(document.getElementById("systemLifespan").value);
var resultDiv = document.getElementById("result-value");
var resultDetailsDiv = document.getElementById("result-details");
// Clear previous results and styles
resultDiv.innerText = "–";
resultDetailsDiv.innerText = "";
resultDiv.style.color = "#28a745"; // Default to green
// Input validation
if (isNaN(systemSize) || systemSize <= 0 ||
isNaN(annualProduction) || annualProduction <= 0 ||
isNaN(systemCost) || systemCost < 0 || // System cost can be 0 if donated, though unlikely
isNaN(incentives) || incentives < 0 ||
isNaN(electricityRate) || electricityRate <= 0 ||
isNaN(systemLifespan) || systemLifespan <= 0) {
resultDetailsDiv.innerText = "Please enter valid positive numbers for all fields (except incentives and cost which can be 0).";
resultDiv.style.color = "#dc3545"; // Red for error
return;
}
var netSystemCost = systemCost – incentives;
if (netSystemCost 0) {
simplePaybackPeriod = netSystemCost / annualSavings;
} else {
simplePaybackPeriod = Infinity; // Avoid division by zero if savings are 0
}
var roi = 0;
if (netSystemCost > 0) {
roi = ((totalSavings – netSystemCost) / netSystemCost) * 100;
} else if (totalSavings > 0) {
roi = Infinity; // Infinite ROI if cost is 0 and there are savings
} else {
roi = 0; // No savings and no cost = 0 ROI
}
var resultText = "";
if (netSystemCost === 0) {
resultText = "Free System! ";
if (totalSavings > 0) {
resultText += "Excellent Savings!";
resultDiv.style.color = "#28a745";
} else {
resultText += "No Savings.";
resultDiv.style.color = "#ffc107"; // Yellow for neutral
}
} else if (simplePaybackPeriod <= systemLifespan) {
resultText = "Likely Profitable!";
resultDiv.style.color = "#28a745";
} else {
resultText = "Long Payback Period";
resultDiv.style.color = "#dc3545";
}
resultDiv.innerText = resultText;
var details = [
"Net System Cost: $" + netSystemCost.toFixed(2),
"Annual Savings: $" + annualSavings.toFixed(2),
"Total Lifetime Savings: $" + totalSavings.toFixed(2),
"Simple Payback Period: " + (simplePaybackPeriod === Infinity ? "Infinite" : simplePaybackPeriod.toFixed(2) + " years"),
"Lifetime ROI: " + (roi === Infinity ? "Infinite" : roi.toFixed(2) + "%")
];
resultDetailsDiv.innerHTML = details.join("");
}