A standby generator is a crucial investment for ensuring uninterrupted power during outages. Sizing it correctly is paramount to guarantee it can meet your electrical needs without being oversized and wasteful, or undersized and ineffective. This calculator helps you determine the appropriate generator capacity based on your expected power consumption and surge requirements.
How the Calculator Works
The calculator primarily focuses on two key figures:
Total Required Wattage (Running Watts): This represents the continuous power your appliances and devices will draw when running simultaneously. To estimate this, sum the running wattage of all essential appliances you want to power, such as refrigerators, lights, sump pumps, medical equipment, and HVAC systems (note that HVAC systems often have a much higher surge requirement).
Maximum Surge Wattage (Starting Watts): Many appliances, especially those with electric motors (like refrigerators, air conditioners, pumps, and power tools), require a significantly higher amount of power for a brief moment when they first start up. This is known as surge wattage. You need to identify the appliance with the highest surge requirement and add it to your total running wattage, or simply input the highest surge wattage of any single appliance you intend to run. The calculator uses the higher of the total running watts and the highest surge wattage plus a buffer to determine the generator's capacity.
Fuel Types and Considerations
The calculator also considers the fuel type, as this can influence generator efficiency and availability:
Natural Gas: Offers an almost limitless supply as long as utility lines are intact, but requires a gas line connection.
Propane: Stored in tanks, offering flexibility but requiring refills. Tank size and fuel consumption over run hours are key considerations.
Diesel: Known for efficiency and power, but can be more expensive and require fuel storage.
Gasoline: Common for smaller portable generators, readily available, but fuel can degrade over time and storage requires care.
Interpreting the Results
The calculator provides a recommended minimum generator size in Watts. It's generally advisable to select a generator that is slightly larger than the minimum requirement to allow for future additions, fluctuating power demands, and to ensure the generator operates efficiently without being constantly at its maximum load.
Recommendation: Always consult with a qualified electrician or generator installer. They can perform a professional load calculation for your specific home or business, taking into account all circuits, potential future needs, and local codes for a precise and safe installation.
function calculateGeneratorSize() {
var totalWatts = parseFloat(document.getElementById("totalWatts").value);
var surgeWatts = parseFloat(document.getElementById("surgeWatts").value);
var runHours = parseFloat(document.getElementById("runHours").value);
var fuelType = document.getElementById("fuelType").value;
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
if (isNaN(totalWatts) || totalWatts <= 0) {
resultDiv.innerHTML = "Please enter a valid Total Required Wattage.";
return;
}
if (isNaN(surgeWatts) || surgeWatts <= 0) {
resultDiv.innerHTML = "Please enter a valid Maximum Surge Wattage.";
return;
}
if (isNaN(runHours) || runHours <= 0) {
resultDiv.innerHTML = "Please enter a valid Average Run Hours per Day.";
return;
}
var requiredWatts = totalWatts;
var surgeRequirement = surgeWatts;
// The primary determinant of generator size is the higher of total running watts or the surge watts.
// We also add a small buffer.
var calculatedSize = Math.max(requiredWatts, surgeRequirement);
calculatedSize = calculatedSize * 1.10; // Add a 10% buffer
// Round up to the nearest common generator size increment (e.g., 500W, 1000W)
calculatedSize = Math.ceil(calculatedSize / 500) * 500;
var resultHtml = "Your estimated minimum standby generator size is: " + calculatedSize.toLocaleString() + " Watts";
// Additional considerations based on fuel type can be added here,
// but for basic sizing, the wattage is the primary output.
// For example, fuel consumption over run hours is complex and depends on load.
resultDiv.innerHTML = resultHtml;
}