Standby Generator Sizing Calculator
Select the appliances you want your generator to power and their quantities:
Major Appliances
Kitchen & Other Common Appliances
Custom Appliance (Optional)
Enter your appliance loads and click 'Calculate' to see your recommended generator size.
function calculateGeneratorSize() {
var totalRunningWatts = 0;
var maxSurgeWatts = 0;
var loads = [];
// Define common appliances with their typical running and starting watts
var appliances = [
{ id: "acQuantity", running: 5000, starting: 15000 }, // Central AC (3-ton)
{ id: "wellPumpQuantity", running: 1000, starting: 3000 }, // Well Pump (1 HP)
{ id: "waterHeaterQuantity", running: 4500, starting: 0 }, // Electric Water Heater
{ id: "electricRangeQuantity", running: 8000, starting: 0 }, // Electric Range (assuming not all burners/oven at once)
{ id: "electricDryerQuantity", running: 5000, starting: 0 }, // Electric Dryer
{ id: "furnaceFanQuantity", running: 800, starting: 2400 }, // Furnace Fan
{ id: "refrigeratorQuantity", running: 800, starting: 2400 }, // Refrigerator
{ id: "freezerQuantity", running: 500, starting: 1500 }, // Freezer
{ id: "microwaveQuantity", running: 1500, starting: 0 }, // Microwave
{ id: "sumpPumpQuantity", running: 800, starting: 2400 }, // Sump Pump
{ id: "lightingOutletsQuantity", running: 1000, starting: 0 } // General Lighting & Outlets
];
// Process common appliances
for (var i = 0; i < appliances.length; i++) {
var quantityInput = document.getElementById(appliances[i].id);
var quantity = parseFloat(quantityInput.value);
if (isNaN(quantity) || quantity < 0) {
quantity = 0;
quantityInput.value = 0; // Reset invalid input
}
var runningLoad = appliances[i].running * quantity;
var startingLoad = appliances[i].starting * quantity;
totalRunningWatts += runningLoad;
loads.push({ running: runningLoad, starting: starting: startingLoad, singleRunning: appliances[i].running, singleStarting: appliances[i].starting, quantity: quantity });
}
// Process custom appliance
var customRunningWattsInput = document.getElementById("customRunningWatts");
var customStartingWattsInput = document.getElementById("customStartingWatts");
var customQuantityInput = document.getElementById("customQuantity");
var customRunning = parseFloat(customRunningWattsInput.value);
var customStarting = parseFloat(customStartingWattsInput.value);
var customQuantity = parseFloat(customQuantityInput.value);
if (isNaN(customRunning) || customRunning < 0) { customRunning = 0; customRunningWattsInput.value = 0; }
if (isNaN(customStarting) || customStarting < 0) { customStarting = 0; customStartingWattsInput.value = 0; }
if (isNaN(customQuantity) || customQuantity < 0) { customQuantity = 0; customQuantityInput.value = 0; }
var customRunningLoad = customRunning * customQuantity;
var customStartingLoad = customStarting * customQuantity;
totalRunningWatts += customRunningLoad;
loads.push({ running: customRunningLoad, starting: customStartingLoad, singleRunning: customRunning, singleStarting: customStarting, quantity: customQuantity });
// Calculate maximum surge watts
// The generator must handle the largest single starting surge while other loads are running.
maxSurgeWatts = totalRunningWatts; // Initialize with total running watts
for (var j = 0; j 0 && loads[j].singleStarting > loads[j].singleRunning) { // Only consider loads with a significant starting surge
// Calculate the running watts of all other appliances
var otherRunningWatts = totalRunningWatts – loads[j].running;
// Add the starting watts of the current appliance
var currentSurgeScenario = otherRunningWatts + (loads[j].singleStarting * loads[j].quantity);
if (currentSurgeScenario > maxSurgeWatts) {
maxSurgeWatts = currentSurgeScenario;
}
}
}
// Determine the required generator capacity
var requiredWatts = Math.max(totalRunningWatts, maxSurgeWatts);
// Add a safety factor (e.g., 20% headroom)
var safetyFactor = 1.20;
requiredWatts = requiredWatts * safetyFactor;
var requiredKW = Math.ceil(requiredWatts / 1000); // Round up to the nearest whole kW
var resultDiv = document.getElementById("result");
if (requiredKW === 0) {
resultDiv.innerHTML = "Please enter the quantity for at least one appliance to calculate the generator size.";
resultDiv.style.backgroundColor = "#fff3cd"; // Warning color
resultDiv.style.borderColor = "#ffeeba";
} else {
resultDiv.innerHTML =
"Recommended Generator Size: " + requiredKW + " kW" +
"Estimated Total Running Watts: " + totalRunningWatts.toFixed(0) + " W" +
"Estimated Maximum Starting (Surge) Watts: " + maxSurgeWatts.toFixed(0) + " W" +
"(Includes a 20% safety margin and rounds up to the nearest kW)";
resultDiv.style.backgroundColor = "#e9f7ee"; // Success color
resultDiv.style.borderColor = "#d0e9d0";
}
}
Understanding Standby Generator Sizing
A standby generator is an essential investment for ensuring your home or business remains powered during outages. However, choosing the right size is critical. An undersized generator won't be able to power all your necessary appliances, while an oversized one will be more expensive to purchase, install, and operate inefficiently.
Why Accurate Sizing Matters
- Prevent Overload: An undersized generator can trip breakers or even suffer permanent damage if it's consistently overloaded.
- Cost Efficiency: Larger generators cost more upfront, consume more fuel, and have higher installation costs. Sizing correctly saves money.
- Optimal Performance: A properly sized generator runs more efficiently, extending its lifespan and ensuring stable power delivery.
- Fuel Consumption: Generators are most fuel-efficient when operating at 50-75% of their rated capacity. An oversized generator running at low load will waste fuel.
Running Watts vs. Starting (Surge) Watts
This is the most crucial concept in generator sizing:
- Running Watts (Rated Watts): This is the continuous power an appliance needs to operate once it's started. For example, a refrigerator might need 800 running watts.
- Starting Watts (Surge Watts): Many motor-driven appliances (like refrigerators, air conditioners, well pumps, and furnace fans) require a brief burst of extra power to start their motors. This surge can be 2 to 7 times their running wattage. For instance, that same refrigerator might need 2400 starting watts for a fraction of a second.
Your generator must be able to handle the combined running watts of all appliances you want to power simultaneously, PLUS the starting watts of the single largest motor that might kick on while everything else is running. Our calculator takes this into account by identifying the maximum potential surge scenario.
How to Use the Calculator
- Identify Essential Loads: Go through your home or business and decide which appliances you absolutely need to run during a power outage. This might include your refrigerator, freezer, well pump, furnace fan, some lighting, and perhaps a central AC unit.
- Enter Quantities: For each listed appliance, enter the quantity you wish to power. If you have a specific appliance not listed, use the "Custom Appliance" section.
- Review Watts: The calculator provides typical running and starting wattages for common appliances. If you know the exact wattage for your specific model (often found on a label or in the owner's manual), you can adjust the custom fields accordingly.
- Calculate: Click the "Calculate Recommended Generator Size" button.
- Interpret Results: The calculator will provide an estimated total running watts, maximum starting watts, and a recommended generator size in kilowatts (kW), including a safety margin.
Important Considerations
- Fuel Type: Generators run on natural gas, propane, or diesel. Consider availability and cost in your area.
- Automatic Transfer Switch (ATS): For standby generators, an ATS is crucial. It automatically detects power outages, starts the generator, and transfers your home's electrical load to the generator, then reverses the process when utility power returns.
- Professional Installation: Standby generators involve complex electrical and fuel line connections. Always use a licensed electrician and plumber for installation to ensure safety and compliance with local codes.
- Future Needs: Consider any potential future additions to your home or business that might increase your power demands. It's often wise to have a little extra capacity.
- Power Factor: Generators are often rated in kVA (kilovolt-amps) as well as kW (kilowatts). kW is the "real power" used by appliances, while kVA is "apparent power." For most residential applications, assuming a power factor of 0.8, a 10 kW generator would be roughly 12.5 kVA. Our calculator focuses on kW, which is the more practical measure for sizing based on appliance loads.
This calculator provides an estimate. For precise sizing and installation, always consult with a qualified generator dealer or electrician who can perform an on-site load assessment.