Use this calculator to determine the appropriate generator size (in Watts) needed to power your essential appliances during an outage or for off-grid use. Input the running watts, starting watts, and quantity for each appliance you plan to connect.
Appliance Name
Running Watts
Starting Watts
Quantity
Action
Generator Sizing Recommendations:
var rowCounter = 5; // Start counter after initial rows
function addApplianceRow() {
rowCounter++;
var tableBody = document.getElementById("applianceTableBody");
var newRow = tableBody.insertRow();
newRow.innerHTML = `
`;
}
function removeApplianceRow(button) {
var row = button.parentNode.parentNode;
row.parentNode.removeChild(row);
}
function calculateGeneratorSize() {
var totalContinuousWatts = 0;
var maxIndividualStartingWatts = 0; // This will store the highest (Starting Watts * Quantity) for any single appliance
var runningWattsOfMaxSurgeAppliance = 0; // Running watts of the appliance causing maxIndividualStartingWatts
var hasValidInput = false;
var tableBody = document.getElementById("applianceTableBody");
var rows = tableBody.getElementsByTagName("tr");
for (var i = 0; i < rows.length; i++) {
var row = rows[i];
var inputs = row.getElementsByTagName("input");
// Assuming order: name, running, starting, quantity
var runningWattsInput = inputs[1];
var startingWattsInput = inputs[2];
var quantityInput = inputs[3];
var runningW = parseFloat(runningWattsInput.value);
var startingW = parseFloat(startingWattsInput.value);
var qty = parseInt(quantityInput.value);
if (isNaN(runningW) || runningW < 0) {
alert("Please enter a valid positive number for Running Watts for all appliances.");
return;
}
if (isNaN(startingW) || startingW < 0) {
alert("Please enter a valid positive number for Starting Watts for all appliances.");
return;
}
if (isNaN(qty) || qty maxIndividualStartingWatts) {
maxIndividualStartingWatts = currentApplianceTotalStartingWatts;
runningWattsOfMaxSurgeAppliance = runningW * qty; // Store its running watts
}
}
if (!hasValidInput) {
document.getElementById("generatorResults").style.display = "none";
alert("Please add at least one appliance with valid wattage and quantity.");
return;
}
// Calculate the actual peak surge needed for the generator
// This is the sum of all other appliances running continuously, plus the full starting surge of the largest starting appliance.
var actualPeakGeneratorSurgeNeeded = (totalContinuousWatts – runningWattsOfMaxSurgeAppliance) + maxIndividualStartingWatts;
// Apply a safety margin for continuous running watts
var recommendedRunningWatts = totalContinuousWatts * 1.20; // 20% safety margin
document.getElementById("totalContinuousPower").innerHTML = "Total Continuous Power Required: " + totalContinuousWatts.toFixed(0) + " Watts";
document.getElementById("peakStartingPower").innerHTML = "Peak Starting Power (Highest Individual Surge): " + maxIndividualStartingWatts.toFixed(0) + " Watts";
document.getElementById("recommendedRunningWatts").innerHTML = "Recommended Generator Running Watts: " + recommendedRunningWatts.toFixed(0) + " Watts (includes 20% safety margin)";
document.getElementById("recommendedSurgeWatts").innerHTML = "Recommended Generator Surge Watts: " + actualPeakGeneratorSurgeNeeded.toFixed(0) + " Watts (to handle the highest starting load while others run)";
document.getElementById("safetyMarginNote").innerHTML = "Note: It's crucial that your generator's continuous rating meets the 'Recommended Generator Running Watts' and its surge rating meets the 'Recommended Generator Surge Watts'.";
document.getElementById("generatorResults").style.display = "block";
}
Understanding Generator Sizing
Choosing the right size generator is crucial for ensuring your essential appliances run smoothly during power outages or for off-grid applications. An undersized generator won't be able to power your equipment, while an oversized one might be an unnecessary expense and less fuel-efficient.
Running Watts vs. Starting Watts
The most important distinction when sizing a generator is between Running Watts (also known as continuous watts) and Starting Watts (also known as surge watts).
Running Watts: This is the power an appliance needs to operate continuously once it's already running. For example, a refrigerator might need 700 running watts to keep its compressor going.
Starting Watts: Many appliances, especially those with electric motors (like refrigerators, air conditioners, sump pumps, and power tools), require a brief burst of extra power to start up. This surge can be 2 to 3 times their running wattage. For instance, that same refrigerator might need 2200 starting watts for a few seconds to get its compressor going, before settling back down to 700 running watts.
Your generator must be able to supply enough running watts for all the appliances you want to operate simultaneously, AND it must have enough surge capacity to handle the starting watts of the single largest motor-driven appliance that might start while other items are already running.
How to Use This Calculator
List Your Appliances: Identify all the essential appliances you want to power with your generator.
Find Wattage Information: Look for the running and starting wattage on the appliance's label, owner's manual, or the manufacturer's website. If only running watts are listed, a common rule of thumb is to estimate starting watts as 2-3 times the running watts for motor-driven appliances. For electronics and resistive loads (like lights, heaters without fans), starting watts are usually equal to running watts.
Enter Details: Input the appliance name, its running watts, starting watts, and the quantity you plan to run.
Add More Appliances: Use the "Add Another Appliance" button to include all your items.
Calculate: Click "Calculate Generator Size" to get your recommendations.
Interpreting the Results
The calculator provides two key figures:
Total Continuous Power Required: This is the sum of all running watts for your selected appliances. Your generator's continuous (running) wattage rating should meet or exceed this value.
Peak Starting Power (Highest Individual Surge): This is the highest starting wattage requirement from any single appliance you listed. Your generator's surge (starting) wattage rating must be able to handle this momentary peak, in addition to the running watts of other active appliances.
It's generally recommended to choose a generator that provides a little extra capacity (e.g., 10-20% safety margin) beyond your calculated needs to account for unforeseen circumstances or future additions.