Backup Generator Size Calculator
Calculation Results:
Enter your appliance selections and click "Calculate" to see the estimated generator size.
Understanding Backup Generator Sizing
A backup generator provides essential power during outages, keeping your home or business running smoothly. However, choosing the right size is crucial. An undersized generator won't be able to power all your necessary appliances, while an oversized one will cost more upfront, consume more fuel, and operate less efficiently.
Running Watts vs. Starting Watts
When sizing a generator, you need to consider two types of wattage:
- Running Watts (Continuous Watts): This is the power an appliance needs to operate continuously once it's started. For most appliances, this is the wattage listed on their label.
- Starting Watts (Surge Watts): Many appliances, especially those with electric motors (like refrigerators, air conditioners, well pumps, and sump pumps), require a brief burst of extra power to start up. This surge can be 2-3 times their running wattage. Your generator must be able to handle this momentary surge.
The calculator above takes both into account. It sums the running watts of all selected appliances and then determines the highest possible surge demand when the largest motor-driven appliance starts while everything else is already running.
How to Use This Calculator
- Identify Essential Appliances: Think about what you absolutely need to run during a power outage. This might include your refrigerator, freezer, furnace fan, well pump, some lights, and perhaps a TV or computer.
- Select Appliances and Quantities: Use the checkboxes to select the appliances from the list that you want to power. Adjust the quantity for each selected item.
- Click "Calculate": The calculator will provide two key figures: the total continuous running watts and the peak starting (surge) watts required.
- Choose a Generator: Look for a generator that meets or exceeds both of these wattage requirements. Generator specifications typically list both "Rated Watts" (continuous) and "Surge Watts" (maximum starting capacity).
Important Considerations Beyond the Calculator
- Future Needs: Consider if you plan to add more appliances in the future that might require generator power.
- Fuel Type: Generators run on various fuels (gasoline, propane, natural gas, diesel). Each has pros and cons regarding storage, availability, and cost.
- Portability vs. Standby: Portable generators are manually operated and moved, while standby generators are permanently installed, automatically turn on, and connect directly to your home's electrical system via an automatic transfer switch.
- Installation: Standby generators require professional installation, including a concrete pad, fuel line, and electrical connections.
- Noise Levels: Generators vary in noise output. Consider this, especially if you have close neighbors.
- Maintenance: Like any engine, generators require regular maintenance to ensure they operate reliably when needed.
Safety First!
Always operate generators safely. Never run a portable generator indoors or in an attached garage, as exhaust fumes contain deadly carbon monoxide. Ensure proper ventilation and follow all manufacturer guidelines for operation and maintenance.
function calculateGeneratorSize() {
var appliances = [
{ id: "chkRefrigerator", qtyId: "qtyRefrigerator", running: 150, starting: 600 },
{ id: "chkFreezer", qtyId: "qtyFreezer", running: 100, starting: 400 },
{ id: "chkSumpPump", qtyId: "qtySumpPump", running: 800, starting: 1300 },
{ id: "chkWellPump", qtyId: "qtyWellPump", running: 1000, starting: 2000 },
{ id: "chkFurnaceFan", qtyId: "qtyFurnaceFan", running: 800, starting: 1200 },
{ id: "chkMicrowave", qtyId: "qtyMicrowave", running: 1500, starting: 0 },
{ id: "chkCoffeeMaker", qtyId: "qtyCoffeeMaker", running: 1000, starting: 0 },
{ id: "chkToaster", qtyId: "qtyToaster", running: 1200, starting: 0 },
{ id: "chkTV", qtyId: "qtyTV", running: 100, starting: 0 },
{ id: "chkComputer", qtyId: "qtyComputer", running: 300, starting: 0 },
{ id: "chkLights", qtyId: "qtyLights", running: 100, starting: 0 },
{ id: "chkWindowAC", qtyId: "qtyWindowAC", running: 1200, starting: 1800 },
{ id: "chkCentralAC", qtyId: "qtyCentralAC", running: 3500, starting: 7000 },
{ id: "chkWaterHeater", qtyId: "qtyWaterHeater", running: 4500, starting: 0 },
{ id: "chkHairDryer", qtyId: "qtyHairDryer", running: 1500, starting: 0 }
];
var totalRunningWatts = 0;
var maxSurgeWatts = 0;
var largestMotorRunningWatts = 0;
var largestMotorStartingWatts = 0;
var selectedAppliances = [];
for (var i = 0; i < appliances.length; i++) {
var appliance = appliances[i];
var checkbox = document.getElementById(appliance.id);
var quantityInput = document.getElementById(appliance.qtyId);
if (checkbox && checkbox.checked) {
var quantity = parseFloat(quantityInput.value);
if (isNaN(quantity) || quantity < 0) {
document.getElementById("result").innerHTML = "Please enter a valid positive number for all quantities.";
return;
}
var currentApplianceRunningWatts = appliance.running * quantity;
var currentApplianceStartingWatts = appliance.starting * quantity;
totalRunningWatts += currentApplianceRunningWatts;
selectedAppliances.push({
running: appliance.running,
starting: appliance.starting,
quantity: quantity
});
}
}
// Calculate max surge watts
// The surge calculation is: (Total Running Watts – Running Watts of Largest Motor) + Starting Watts of Largest Motor
// We need to find the single largest starting wattage appliance among the selected ones.
maxSurgeWatts = totalRunningWatts; // Initialize with total running watts, as this is the baseline if no motors start.
for (var j = 0; j 0) { // Only consider appliances with starting watts (motors)
// Calculate the surge if this specific appliance were the last one to start
var surgeForThisAppliance = totalRunningWatts – (selectedApp.running * selectedApp.quantity) + (selectedApp.starting * selectedApp.quantity);
if (surgeForThisAppliance > maxSurgeWatts) {
maxSurgeWatts = surgeForThisAppliance;
}
}
}
// If no motor appliances are selected, maxSurgeWatts will just be totalRunningWatts.
// This is correct as there's no additional surge.
var resultHtml = "
Estimated Continuous Running Watts Required: " + totalRunningWatts.toLocaleString() + " Watts";
resultHtml += "
Estimated Peak Starting (Surge) Watts Required: " + maxSurgeWatts.toLocaleString() + " Watts";
resultHtml += "
Your generator should have a continuous running wattage capacity of at least " + totalRunningWatts.toLocaleString() + " Watts and a surge (starting) wattage capacity of at least " + maxSurgeWatts.toLocaleString() + " Watts.";
document.getElementById("result").innerHTML = resultHtml;
}
function resetCalculator() {
var appliances = [
{ id: "chkRefrigerator", qtyId: "qtyRefrigerator" },
{ id: "chkFreezer", qtyId: "qtyFreezer" },
{ id: "chkSumpPump", qtyId: "qtySumpPump" },
{ id: "chkWellPump", qtyId: "qtyWellPump" },
{ id: "chkFurnaceFan", qtyId: "qtyFurnaceFan" },
{ id: "chkMicrowave", qtyId: "qtyMicrowave" },
{ id: "chkCoffeeMaker", qtyId: "qtyCoffeeMaker" },
{ id: "chkToaster", qtyId: "qtyToaster" },
{ id: "chkTV", qtyId: "qtyTV" },
{ id: "chkComputer", qtyId: "qtyComputer" },
{ id: "chkLights", qtyId: "qtyLights" },
{ id: "chkWindowAC", qtyId: "qtyWindowAC" },
{ id: "chkCentralAC", qtyId: "chkCentralAC" },
{ id: "chkWaterHeater", qtyId: "qtyWaterHeater" },
{ id: "chkHairDryer", qtyId: "qtyHairDryer" }
];
for (var i = 0; i < appliances.length; i++) {
var checkbox = document.getElementById(appliances[i].id);
var quantityInput = document.getElementById(appliances[i].qtyId);
if (checkbox) {
checkbox.checked = false;
}
if (quantityInput) {
quantityInput.value = "0";
}
}
// Set default values for common items
document.getElementById("chkRefrigerator").checked = true;
document.getElementById("qtyRefrigerator").value = "1";
document.getElementById("chkTV").checked = true;
document.getElementById("qtyTV").value = "1";
document.getElementById("chkLights").checked = true;
document.getElementById("qtyLights").value = "1";
document.getElementById("result").innerHTML = "Enter your appliance selections and click \"Calculate\" to see the estimated generator size.";
}
// Initialize with default selections on load
document.addEventListener('DOMContentLoaded', function() {
resetCalculator(); // Call reset to set initial values and clear results
});