Estimate the required generator wattage for your home based on your essential and optional appliance needs.
Understanding Home Generator Sizing
Choosing the right size generator for your home is crucial for ensuring you have reliable power during outages. A generator that is too small won't be able to power your essential appliances, while one that is too large can be an unnecessary expense and less efficient.
The Importance of Running vs. Starting Wattage
Many electrical appliances, especially those with motors (like refrigerators, furnaces, pumps, and air conditioners), require a significant surge of power when they first start up. This "starting wattage" or "surge wattage" can be 2-3 times higher than the "running wattage" or "continuous wattage" needed to keep the appliance operating. It's essential to account for both when sizing a generator.
How the Calculator Works
This calculator helps you estimate the total wattage required for your home's essential appliances. It works in two main steps:
Calculate Total Running Wattage: It sums up the running wattage of all the appliances you input.
Determine Peak Wattage Requirement: This is the most critical part. Generators need to supply enough power for all appliances running simultaneously PLUS the highest starting wattage of any single appliance that might start up at the same time. The calculator identifies the highest starting wattage among your selected appliances and adds it to the sum of the running wattages of ALL other selected appliances. This provides a robust estimate of your peak power demand.
Key Appliance Considerations:
Refrigerators & Freezers: Their compressors require high starting watts.
Furnace Fan: Essential for heating systems in colder climates.
Well Pumps: Critical for homes with private water sources.
Sump Pumps: Important for preventing basement flooding.
Air Conditioners: These are often the largest power consumers and have very high starting wattages. If you have a central AC, its starting wattage will likely dictate your generator size.
Lights: Typically have a low running wattage.
Electronics: TVs, computers, chargers usually have moderate running wattages.
Microwaves: High running wattage appliances.
Estimating Appliance Wattage:
You can usually find the wattage information on the appliance's data plate, in its user manual, or by searching online. When unsure, it's often safer to overestimate slightly, especially for starting wattage.
Generator Sizing Rules of Thumb:
Rule 1: Add up all the Running Wattages of the appliances you want to run simultaneously.
Rule 2: Identify the appliance with the highest Starting Wattage.
Rule 3: The required generator size is generally: (Sum of Running Wattages) + (Highest Starting Wattage). This calculator implements a refined version of this principle to ensure adequate power for simultaneous operation and surges.
This calculator provides an estimate. For precise sizing, consult with a qualified electrician or generator installer. They can perform a detailed load calculation for your specific home and needs.
function calculateGeneratorSize() {
var values = {
refrigerator: parseFloat(document.getElementById("refrigerator").value) || 0,
refrigerator_startup: parseFloat(document.getElementById("refrigerator_startup").value) || 0,
freezer: parseFloat(document.getElementById("freezer").value) || 0,
freezer_startup: parseFloat(document.getElementById("freezer_startup").value) || 0,
furnace_fan: parseFloat(document.getElementById("furnace_fan").value) || 0,
furnace_fan_startup: parseFloat(document.getElementById("furnace_fan_startup").value) || 0,
lights: parseFloat(document.getElementById("lights").value) || 0,
well_pump: parseFloat(document.getElementById("well_pump").value) || 0,
well_pump_startup: parseFloat(document.getElementById("well_pump_startup").value) || 0,
sump_pump: parseFloat(document.getElementById("sump_pump").value) || 0,
sump_pump_startup: parseFloat(document.getElementById("sump_pump_startup").value) || 0,
tv_computers: parseFloat(document.getElementById("tv_computers").value) || 0,
microwave: parseFloat(document.getElementById("microwave").value) || 0,
ac_central: parseFloat(document.getElementById("ac_central").value) || 0,
ac_central_startup: parseFloat(document.getElementById("ac_central_startup").value) || 0
};
var resultDiv = document.getElementById("result");
resultDiv.classList.remove("error"); // Remove error class
var totalRunningWattage = 0;
var highestStartingWattage = 0;
var applianceWattages = [
{ running: values.refrigerator, starting: values.refrigerator_startup },
{ running: values.freezer, starting: values.freezer_startup },
{ running: values.furnace_fan, starting: values.furnace_fan_startup },
{ running: values.lights, starting: 0 }, // Lights typically don't have a significant startup surge
{ running: values.well_pump, starting: values.well_pump_startup },
{ running: values.sump_pump, starting: values.sump_pump_startup },
{ running: values.tv_computers, starting: 0 }, // Electronics usually don't have significant startup surge
{ running: values.microwave, starting: 0 }, // Microwaves don't have a significant startup surge
{ running: values.ac_central, starting: values.ac_central_startup }
];
var allAppliancesHaveValidRunningWattage = true;
for (var i = 0; i < applianceWattages.length; i++) {
if (applianceWattages[i].running < 0 || applianceWattages[i].starting highestStartingWattage) {
highestStartingWattage = applianceWattages[i].starting;
}
}
if (!allAppliancesHaveValidRunningWattage) {
resultDiv.innerHTML = "Please enter valid non-negative numbers for all wattages.";
resultDiv.classList.add("error");
return;
}
// A common method is to sum all running watts and add the highest starting watt.
// However, a more accurate approach accounts for the fact that you might not run *everything* at once,
// but you *must* account for the single highest surge.
// A practical approach is to sum the running watts of all desired appliances, and then add the highest *starting* wattage.
// However, if the highest starting wattage appliance is also a high running wattage appliance, we need to be careful not to double count.
// A simplified, robust method often recommended is:
// Total Required Wattage = (Sum of Running Wattages of all appliances you want to run simultaneously) + (Highest Starting Wattage of any single appliance)
// However, the most common interpretation and calculation is to sum ALL running wattages you expect to use,
// and then add the highest STARTING wattage IF it's greater than the running wattage of that specific appliance.
// A simpler and safer approach for users is often:
// Calculate the total running wattage of all selected appliances.
// Find the single highest starting wattage among all selected appliances.
// The generator size should be at least the sum of all running watts PLUS the highest starting surge *that is not already accounted for in the running total*.
// A very safe and common calculation: Sum of running watts + highest surge wattage.
// Let's refine this for clarity and safety:
var peakDemand = totalRunningWattage + highestStartingWattage;
// A more nuanced calculation accounts for the fact that the appliance with the highest starting wattage
// will also contribute its running wattage. So we add all running wattages, and then add the *difference*
// between the highest starting wattage and its own running wattage.
// However, this can be confusing. A simpler, safer, and commonly accepted approach for DIY users is:
// Required Wattage = Total Running Wattage + Highest Starting Wattage.
// Let's use this common method for clarity.
var calculatedSize = totalRunningWattage + highestStartingWattage;
if (isNaN(calculatedSize) || calculatedSize <= 0) {
resultDiv.innerHTML = "Please enter valid wattages.";
resultDiv.classList.add("error");
} else {
resultDiv.innerHTML = calculatedSize.toFixed(0) + " Watts";
}
}