Check the appliances you want to power and adjust quantities. Typical running and starting wattages are pre-filled.
2. Add Custom Appliances (Optional)
3. Your Generator Size Recommendation
Please select appliances to calculate.
This calculation provides an estimate. It's always recommended to add a 10-20% buffer to your total wattage for safety and future needs.
var applianceData = [
{ id: 'appliance1', running: 700, starting: 2200 }, // Refrigerator
{ id: 'appliance2', running: 500, starting: 1500 }, // Freezer
{ id: 'appliance3', running: 1000, starting: 2000 }, // Sump Pump
{ id: 'appliance4', running: 800, starting: 1600 }, // Furnace Fan
{ id: 'appliance5', running: 1000, starting: 1500 }, // Microwave
{ id: 'appliance6', running: 100, starting: 100 }, // Lights
{ id: 'appliance7', running: 200, starting: 200 }, // TV
{ id: 'appliance8', running: 300, starting: 300 }, // Computer
{ id: 'appliance9', running: 1200, starting: 3600 }, // Window AC
{ id: 'appliance10', running: 1000, starting: 1000 } // Coffee Maker
];
var customApplianceCount = 1;
function addCustomAppliance() {
customApplianceCount++;
var container = document.getElementById('custom-appliances-container');
var newApplianceDiv = document.createElement('div');
newApplianceDiv.className = 'custom-appliance-item';
newApplianceDiv.style = "border: 1px solid #e0e0e0; padding: 10px; border-radius: 5px; background-color: #fdfdfd; margin-bottom: 10px;";
newApplianceDiv.innerHTML = `
`;
container.appendChild(newApplianceDiv);
calculateGeneratorSize();
}
function calculateGeneratorSize() {
var totalRunningWatts = 0;
var maxSingleStartingWatts = 0;
var maxSingleStartingWattsRunningComponent = 0; // Running watts of the appliance causing the max surge
// Process pre-defined appliances
for (var i = 0; i < applianceData.length; i++) {
var checkId = applianceData[i].id + '_check';
var qtyId = applianceData[i].id + '_qty';
var runningId = applianceData[i].id + '_running';
var startingId = applianceData[i].id + '_starting';
var isChecked = document.getElementById(checkId).checked;
var qty = parseFloat(document.getElementById(qtyId).value);
var running = parseFloat(document.getElementById(runningId).value);
var starting = parseFloat(document.getElementById(startingId).value);
if (isNaN(qty) || qty < 0) qty = 0;
if (isNaN(running) || running < 0) running = 0;
if (isNaN(starting) || starting 0) {
totalRunningWatts += (running * qty);
var currentApplianceStartingWatts = starting * qty;
if (currentApplianceStartingWatts > maxSingleStartingWatts) {
maxSingleStartingWatts = currentApplianceStartingWatts;
maxSingleStartingWattsRunningComponent = running * qty;
}
}
}
// Process custom appliances
for (var i = 1; i <= customApplianceCount; i++) {
var customQtyId = 'custom_qty_' + i;
var customRunningId = 'custom_running_' + i;
var customStartingId = 'custom_starting_' + i;
var qty = parseFloat(document.getElementById(customQtyId).value);
var running = parseFloat(document.getElementById(customRunningId).value);
var starting = parseFloat(document.getElementById(customStartingId).value);
if (isNaN(qty) || qty < 0) qty = 0;
if (isNaN(running) || running < 0) running = 0;
if (isNaN(starting) || starting 0 && (running > 0 || starting > 0)) {
totalRunningWatts += (running * qty);
var currentApplianceStartingWatts = starting * qty;
if (currentApplianceStartingWatts > maxSingleStartingWatts) {
maxSingleStartingWatts = currentApplianceStartingWatts;
maxSingleStartingWattsRunningComponent = running * qty;
}
}
}
var totalSurgeWatts = totalRunningWatts – maxSingleStartingWattsRunningComponent + maxSingleStartingWatts;
var recommendedGeneratorWatts = Math.max(totalRunningWatts, totalSurgeWatts);
var resultDiv = document.getElementById('generatorResult');
if (recommendedGeneratorWatts === 0) {
resultDiv.innerHTML = "Please select appliances to calculate.";
resultDiv.style.color = '#007BFF';
resultDiv.style.borderColor = '#007BFF';
} else {
var recommendedKW = (recommendedGeneratorWatts / 1000).toFixed(1);
resultDiv.innerHTML = `
Total Running Watts: ${totalRunningWatts.toLocaleString()} Watts
Peak Starting (Surge) Watts: ${totalSurgeWatts.toLocaleString()} Watts
Recommended Generator Size: ${recommendedGeneratorWatts.toLocaleString()} Watts (${recommendedKW} kW)
`;
resultDiv.style.color = '#28a745';
resultDiv.style.borderColor = '#28a745';
}
}
// Initial calculation on page load
window.onload = calculateGeneratorSize;
Understanding Generator Sizing for Your Home
When the power goes out, a home generator can be a lifesaver, keeping essential appliances running and maintaining comfort. But choosing the right size generator is crucial. Too small, and it won't power what you need; too large, and you'll overspend on purchase and fuel. This guide and calculator will help you determine the ideal generator size for your household needs.
Running Watts vs. Starting Watts (Surge Watts)
This is the most critical concept in generator sizing:
Running Watts (Rated Watts): This is the continuous power an appliance needs to operate once it's already on. For example, a refrigerator might need 700 running watts to keep its compressor going.
Starting Watts (Surge Watts): Many appliances, especially those with electric motors (like refrigerators, freezers, air conditioners, and well pumps), require a brief burst of extra power to start up. This surge can be 2-3 times their running wattage. A generator must be able to handle this momentary surge without tripping or stalling.
Your generator needs to meet two criteria: it must supply enough running watts for all appliances you want to operate simultaneously, AND it must be able to handle the highest single starting wattage of any appliance you plan to turn on while others are already running.
How to Use the Calculator
Our calculator simplifies this process:
Select Appliances: Check the boxes next to the common household appliances you intend to power during an outage.
Adjust Quantities: For each selected appliance, specify how many you'll be running.
Add Custom Appliances: If an appliance isn't listed, use the "Add Custom Appliance" section to input its running and starting watts (usually found on the appliance's label or in its manual).
View Recommendation: The calculator will automatically sum your total running watts and determine the peak starting (surge) watts required. It then recommends a generator size in watts and kilowatts.
Highest Single Starting Wattage: The refrigerator has the highest starting wattage at 2200 Watts. When it starts, the other items (Furnace Fan, Lights, TV) are already running.
Peak Surge Requirement: (Running Watts of other items) + (Starting Watts of Refrigerator) = (800 + 100 + 200) + 2200 = 1100 + 2200 = 3300 Watts.
The generator needs to handle 1800 running watts continuously and be able to surge to 3300 watts momentarily. Therefore, the recommended generator size would be 3300 Watts.
Factors to Consider Beyond Wattage:
Buffer Capacity: Always add a 10-20% buffer to your calculated wattage. This accounts for efficiency losses, aging appliances, and potential future needs.
Fuel Type: Generators run on gasoline, propane, natural gas, or diesel. Consider availability, storage, and run time.
Noise Level: Generators can be noisy. Check decibel ratings, especially if you have close neighbors. Inverter generators are generally quieter.
Portability vs. Standby:
Portable Generators: More affordable, can be moved, but require manual setup and refueling.
Standby Generators: Permanently installed, automatically turn on during outages, more expensive but offer ultimate convenience.
Transfer Switch: For safety and compliance, a transfer switch is essential to disconnect your home from the utility grid when using a generator.
By carefully assessing your power needs and using this calculator, you can confidently choose a generator that will keep your home comfortable and functional during unexpected power interruptions.