Selecting the right size generator for your home is crucial to ensure you have reliable backup power during outages.
This calculator helps estimate the generator capacity you'll need based on your home's electrical demands. It considers your typical running load, the extra power required for appliances with motors (starting surge), and allows for an optional calculation for essential circuits only.
How the Calculation Works
The generator size is determined by the highest power demand. The calculation involves several key factors:
Peak Electrical Load (Watts): This is the sum of the wattage of all the electrical devices you anticipate running simultaneously during a power outage. Think about your essential appliances like refrigerators, lights, medical equipment, and electronics.
Appliance Starting Surge (Watts): Many appliances, particularly those with motors (like refrigerators, air conditioners, pumps), require a significantly higher amount of power for a brief moment when they first start up. This is known as the starting surge or inrush current. If not accounted for, a generator that's only sized for the running load might fail to start these appliances.
Critical Loads Only (Optional): Sometimes, you might only want to power a select few essential appliances. This input allows you to specify a lower wattage requirement if you're not aiming to power your entire home.
Generator Power Factor: Generators are rated in Volt-Amps (VA) and Watts (W). The relationship between them is Watts = VA × Power Factor. A common power factor for generators is 0.8, meaning the true power (Watts) is 80% of the apparent power (VA). This input is used to convert the required Watts to VA, which is often how generators are marketed.
The calculator determines the required generator size by taking the higher of:
Your estimated peak running load.
Your peak running load PLUS the starting surge of your most demanding appliance (if provided and it exceeds the peak load).
This sum is then divided by the generator's power factor to provide an estimated generator size in Volt-Amps (VA), which is a common industry standard for generator output.
Example Calculation
Let's say you estimate your Peak Electrical Load to be 4000 Watts.
You have a refrigerator that requires an additional 2500 Watts for its starting surge.
Your Generator Power Factor is 0.8.
Since 6500 Watts (peak + surge) is greater than 4000 Watts (peak load), the generator must be sized to handle at least 6500 Watts.
To convert this to Volt-Amps (VA):
Required VA = Watts / Power Factor
Required VA = 6500 Watts / 0.8 = 8125 VA
Therefore, you would need a generator with an output of at least 8125 VA. It's often recommended to select a generator with a slightly higher capacity (e.g., 8500 VA or 10000 VA) to provide a buffer and ensure longevity.
When to Use This Calculator
Planning for a new home standby generator.
Purchasing a portable generator for emergencies.
Evaluating if your current generator is adequately sized.
Determining power needs for specific appliances during an outage.
Remember, these are estimates. Consulting with a qualified electrician or generator professional is always recommended for a precise assessment tailored to your specific home and electrical system.
function calculateGeneratorSize() {
var peakLoadInput = document.getElementById("peakLoad");
var surgeRequirementInput = document.getElementById("surgeRequirement");
var criticalLoadsInput = document.getElementById("criticalLoads");
var powerFactorInput = document.getElementById("powerFactor");
var resultDiv = document.getElementById("result");
// Clear previous results and error messages
resultDiv.innerHTML = "";
// Get values and convert to numbers
var peakLoad = parseFloat(peakLoadInput.value);
var surgeRequirement = parseFloat(surgeRequirementInput.value);
var criticalLoads = parseFloat(criticalLoadsInput.value);
var powerFactor = parseFloat(powerFactorInput.value);
// Validate inputs
if (isNaN(peakLoad) || peakLoad <= 0) {
resultDiv.innerHTML = "Please enter a valid Peak Electrical Load (Watts).";
return;
}
if (isNaN(surgeRequirement) || surgeRequirement < 0) {
resultDiv.innerHTML = "Please enter a valid Appliance Starting Surge (Watts), or 0 if none.";
return;
}
if (isNaN(criticalLoads) || criticalLoads < 0) {
criticalLoads = 0; // Treat as 0 if invalid or empty
}
if (isNaN(powerFactor) || powerFactor 1) {
resultDiv.innerHTML = "Please enter a valid Power Factor between 0.1 and 1.0.";
return;
}
var runningLoadToConsider = peakLoad;
if (criticalLoads > 0) {
runningLoadToConsider = Math.max(peakLoad, criticalLoads);
}
var requiredWatts = runningLoadToConsider;
var surgePowerNeeded = runningLoadToConsider + surgeRequirement;
// Determine the highest wattage demand
if (surgePowerNeeded > runningLoadToConsider) {
requiredWatts = surgePowerNeeded;
}
// Calculate required generator size in VA
var requiredVA = requiredWatts / powerFactor;
// Round up to nearest common generator size or a sensible increment
// Common generator sizes: 2000, 3500, 5000, 7500, 10000, 12500, 15000, 20000, etc.
// We'll round up to the nearest 500 VA for a practical result.
var roundedUpVA = Math.ceil(requiredVA / 500) * 500;
resultDiv.innerHTML = "Recommended Generator Size: " + roundedUpVA.toFixed(0) + " VA";
}