Standby Generator Size Calculator
A standby generator provides automatic backup power to your home during an outage. Sizing it correctly is crucial: too small, and it won't power your essential appliances; too large, and you'll pay more for the unit and its installation than necessary. This calculator helps you estimate the generator size (in kilowatts, kW) you'll need by considering the running and starting (surge) wattage of your desired appliances.
Understanding Running vs. Starting Watts
Most electrical devices have a "running wattage" – the power they consume continuously while operating. However, motor-driven appliances (like refrigerators, air conditioners, well pumps, and furnaces) require a significantly higher "starting wattage" or "surge wattage" for a brief moment when they first turn on. This surge can be 2-3 times their running wattage. Your generator must be able to handle both the total running wattage of all simultaneously used appliances AND the highest single starting surge on top of the running load of all other appliances.
How to Use This Calculator:
- Select the appliances you wish to power during an outage.
- Adjust the quantity for each selected appliance if you have more than one.
- If an appliance isn't listed, use the "Custom Appliance" fields to add its running and starting watts.
- Click "Calculate Generator Size" to see your estimated requirement in kilowatts (kW).
Why Proper Sizing Matters
An undersized generator will struggle to start or run all your desired appliances, potentially tripping breakers or even damaging the generator or appliances. An oversized generator, while capable, costs more upfront, consumes more fuel, and may operate less efficiently, leading to "wet stacking" (unburned fuel residue) if consistently run at low loads, which can shorten its lifespan.
Factors Not Covered by This Calculator:
- Future Needs: Consider any planned additions to your home or appliances.
- Fuel Type: Natural gas, propane, or diesel generators have different efficiencies and fuel storage requirements.
- Installation Costs: Beyond the unit itself, installation by a licensed electrician is a significant cost.
- Load Management: Some homes use automatic load shedding systems to prioritize essential circuits, allowing for a smaller generator.
- Voltage Requirements: Most home generators are 240V, but ensure your specific appliances are compatible.
Always consult with a professional generator installer or electrician to confirm your specific needs and ensure safe and compliant installation.
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function calculateGeneratorSize() {
var totalRunningWatts = 0;
var appliances = []; // Stores { running: total_running_for_qty, starting: total_starting_for_qty, singleStarting: single_appliance_starting }
// Helper to get numeric value safely
function getNumericValue(id) {
var value = parseFloat(document.getElementById(id).value);
return isNaN(value) ? 0 : value;
}
// Predefined appliances
var predefinedAppliances = [
{ id: "chkRefrigerator", qtyId: "qtyRefrigerator" },
{ id: "chkFreezer", qtyId: "qtyFreezer" },
{ id: "chkSumpPump", qtyId: "qtySumpPump" },
{ id: "chkWellPump", qtyId: "qtyWellPump" },
{ id: "chkFurnaceFan", qtyId: "qtyFurnaceFan" },
{ id: "chkCentralAC", qtyId: "qtyCentralAC" },
{ id: "chkWindowAC", qtyId: "qtyWindowAC" },
{ id: "chkElectricWaterHeater", qtyId: "qtyElectricWaterHeHeater" },
{ id: "chkMicrowave", qtyId: "qtyMicrowave" },
{ id: "chkLightsLED", qtyId: "qtyLightsLED" },
{ id: "chkTelevision", qtyId: "qtyTelevision" },
{ id: "chkComputer", qtyId: "qtyComputer" },
{ id: "chkCoffeeMaker", qtyId: "qtyCoffeeMaker" },
{ id: "chkWashingMachine", qtyId: "qtyWashingMachine" },
{ id: "chkDryerGas", qtyId: "qtyDryerGas" }
];
for (var i = 0; i 0 && !isNaN(running) && !isNaN(starting)) {
totalRunningWatts += running * qty;
appliances.push({
running: running * qty,
starting: starting * qty, // Total starting if all of this type start at once (not how it works for surge, but useful for tracking)
singleStarting: starting // The actual single appliance starting wattage
});
}
}
}
// Custom appliance
var customQty = getNumericValue("customQuantity");
var customRunning = getNumericValue("customRunningWatts");
var customStarting = getNumericValue("customStartingWatts");
if (customQty > 0 && customRunning > 0) { // Custom appliance must have running watts
// If starting watts not provided for custom, assume it's same as running (no surge)
var actualCustomStarting = (customStarting > 0) ? customStarting : customRunning;
totalRunningWatts += customRunning * customQty;
appliances.push({
running: customRunning * customQty,
starting: actualCustomStarting * customQty,
singleStarting: actualCustomStarting
});
}
var maxPotentialSurgeLoad = totalRunningWatts; // Initialize with total running watts
// Calculate the highest single surge scenario
for (var j = 0; j maxPotentialSurgeLoad) {
maxPotentialSurgeLoad = potentialSurge;
}
}
var requiredWatts = Math.max(totalRunningWatts, maxPotentialSurgeLoad);
var requiredKW = requiredWatts / 1000;
var resultElement = document.getElementById("resultOutput");
if (requiredKW > 0) {
resultElement.innerHTML = requiredKW.toFixed(1) + " kW";
} else {
resultElement.innerHTML = "Please select appliances and quantities.";
}
}