Generac Generator Sizing Calculator
Use this calculator to estimate the total wattage required to power your essential appliances during an outage, helping you choose the right Generac generator size for your home.
Understanding Generator Sizing for Your Home
Choosing the right size generator is crucial for ensuring your home has adequate power during an outage without overspending on an unnecessarily large unit. Generac offers a wide range of generators, from portable units for basic needs to whole-home standby generators that automatically power your entire house.
Running Watts vs. Surge Watts
- Running Watts (Rated Watts): This is the continuous power an appliance needs to operate once it's started. Most appliances list their running wattage on a label or in their manual.
- Surge Watts (Starting Watts): Many motor-driven appliances (like refrigerators, well pumps, air conditioners) require a brief burst of extra power to start up. This initial surge can be 2-3 times their running wattage. While this surge is momentary, your generator must be able to supply it.
How to Use This Calculator
This calculator helps you estimate your total power needs by summing the running watts of all appliances you want to power simultaneously, and then accounting for the highest single surge wattage. It's important to consider which appliances are truly essential during an outage. For example, you might not need to power your oven or clothes dryer, which are high-wattage appliances.
- Identify Essential Appliances: List all the appliances you absolutely need to run during a power outage.
- Find Wattage Information: Look for the wattage on the appliance's data plate, owner's manual, or manufacturer's website. If you can't find exact numbers, use the typical values provided in the calculator as a starting point.
- Enter Values: Input the running and surge watts for each appliance into the respective fields.
- Calculate: Click the "Calculate Generator Size" button to see your estimated total running watts, peak surge watts, and a recommended Generac generator size.
Factors to Consider Beyond Wattage
- Fuel Type: Generac generators run on natural gas, propane, or gasoline. Consider availability and storage.
- Automatic vs. Portable: Standby generators turn on automatically when power goes out, while portable generators require manual setup and fueling.
- Installation: Whole-home standby generators require professional installation, including electrical and fuel line connections.
- Future Needs: Consider if you plan to add more appliances or expand your home in the future.
This calculator provides an estimate. For precise sizing and installation, it's always recommended to consult with a certified Generac dealer or electrician who can assess your specific home and needs.
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function calculateGeneratorSize() {
var refrigeratorRunning = parseFloat(document.getElementById('refrigeratorRunning').value) || 0;
var refrigeratorSurge = parseFloat(document.getElementById('refrigeratorSurge').value) || 0;
var freezerRunning = parseFloat(document.getElementById('freezerRunning').value) || 0;
var freezerSurge = parseFloat(document.getElementById('freezerSurge').value) || 0;
var wellPumpRunning = parseFloat(document.getElementById('wellPumpRunning').value) || 0;
var wellPumpSurge = parseFloat(document.getElementById('wellPumpSurge').value) || 0;
var furnaceFanRunning = parseFloat(document.getElementById('furnaceFanRunning').value) || 0;
var furnaceFanSurge = parseFloat(document.getElementById('furnaceFanSurge').value) || 0;
var lightsRunning = parseFloat(document.getElementById('lightsRunning').value) || 0;
var microwaveRunning = parseFloat(document.getElementById('microwaveRunning').value) || 0;
var tvRunning = parseFloat(document.getElementById('tvRunning').value) || 0;
var computerRunning = parseFloat(document.getElementById('computerRunning').value) || 0;
var sumpPumpRunning = parseFloat(document.getElementById('sumpPumpRunning').value) || 0;
var sumpPumpSurge = parseFloat(document.getElementById('sumpPumpSurge').value) || 0;
var custom1Running = parseFloat(document.getElementById('custom1Running').value) || 0;
var custom1Surge = parseFloat(document.getElementById('custom1Surge').value) || 0;
var custom2Running = parseFloat(document.getElementById('custom2Running').value) || 0;
var custom2Surge = parseFloat(document.getElementById('custom2Surge').value) || 0;
// Calculate total running watts
var totalRunningWatts = refrigeratorRunning + freezerRunning + wellPumpRunning + furnaceFanRunning +
lightsRunning + microwaveRunning + tvRunning + computerRunning + sumpPumpRunning +
custom1Running + custom2Running;
// Identify the highest single surge wattage requirement
// We need to find the largest difference between surge and running for any single appliance
// because the running watts are already included in totalRunningWatts.
var maxSurgeDifference = 0;
var appliances = [
{ running: refrigeratorRunning, surge: refrigeratorSurge },
{ running: freezerRunning, surge: freezerSurge },
{ running: wellPumpRunning, surge: wellPumpSurge },
{ running: furnaceFanRunning, surge: furnaceFanSurge },
{ running: sumpPumpRunning, surge: sumpPumpSurge },
{ running: custom1Running, surge: custom1Surge },
{ running: custom2Running, surge: custom2Surge }
];
for (var i = 0; i maxSurgeDifference) {
maxSurgeDifference = diff;
}
}
// Total peak watts needed (running watts + the highest single surge difference)
var totalPeakWatts = totalRunningWatts + maxSurgeDifference;
// Convert to Kilowatts (kW) for common generator sizing
var recommendedKW = Math.ceil(totalPeakWatts / 1000); // Round up to the nearest whole kW
var resultDiv = document.getElementById('generatorResult');
resultDiv.innerHTML = `
Estimated Total Running Watts: ${totalRunningWatts.toLocaleString()} Watts
Estimated Peak (Surge) Watts Required: ${totalPeakWatts.toLocaleString()} Watts
Recommended Generac Generator Size: Approximately
${recommendedKW} kW or higher
This is an estimate. Always consult a professional for precise sizing.
`;
}