Estimate the right Generac generator size for your home's power needs.
Recommended Generator Size:
—
Watts
Understanding Your Generac Generator Size Calculation
Choosing the right size generator is crucial for ensuring your essential appliances and systems remain powered during an outage. This calculator provides an estimated wattage requirement based on common household power needs.
How the Calculation Works:
This calculator uses a simplified model to estimate your total power demand. It considers:
Home Size: Larger homes generally require more power for general comfort (lighting, heating/cooling adjustments).
Occupants: More people often mean more simultaneous use of electronics and appliances.
Essential Appliances: These are critical systems that typically need continuous power (refrigerator, HVAC, water pumps). Each has a typical running wattage and a higher starting wattage.
Major Appliances: High-demand appliances like electric stoves, dryers, and microwaves significantly increase power needs, especially during startup.
Electronics & Lighting: This is a broad category for all other devices like TVs, computers, routers, and light bulbs.
Simplified Wattage Estimates Used:
Below are the estimated wattages factored into the calculation. Note that actual wattages can vary significantly by model and efficiency.
Home Size Factor: Approximately 1 Watt per square foot (for general comfort, lighting, etc.).
Occupant Factor: Approximately 200 Watts per occupant (for personal electronics, charging, etc.).
Electric Range/Oven: ~3000 Running Watts / 3000 Starting Watts
Electric Dryer: ~2000 Running Watts / 2000 Starting Watts
Electronics & Lighting: User-inputted value.
The calculation prioritizes covering the highest potential starting wattages (surge power needed when an appliance first turns on) plus the continuous running wattages of other appliances.
Important Considerations:
Starting vs. Running Watts: Appliances like motors (furnace, well pump) require significantly more power to start than to run. This calculator attempts to account for the highest single starting surge plus other running loads.
Simultaneous Use: This calculator assumes a reasonable level of simultaneous use. If you plan to run many high-demand appliances at once, you may need a larger unit.
Specific Models: Always check the exact wattage requirements (running and starting) for your specific appliances.
Professional Advice: For precise sizing, especially for whole-house backup, consult with a qualified electrician or Generac dealer. They can perform a detailed load calculation.
Transfer Switches: This calculation is for the generator's capacity. You'll also need an appropriate transfer switch (manual or automatic) to safely connect the generator to your home's electrical system.
function calculateGeneratorSize() {
// Get input values
var homeSize = parseFloat(document.getElementById("homeSize").value);
var numberOfOccupants = parseFloat(document.getElementById("numberOfOccupants").value);
var refrigeratorFreezer = parseFloat(document.getElementById("refrigeratorFreezer").value);
var furnaceFan = parseFloat(document.getElementById("furnaceFan").value);
var wellPump = parseFloat(document.getElementById("wellPump").value);
var sumpPump = parseFloat(document.getElementById("sumpPump").value);
var microwaveOven = parseFloat(document.getElementById("microwaveOven").value);
var electricRangeOven = parseFloat(document.getElementById("electricRangeOven").value);
var electricDryer = parseFloat(document.getElementById("electricDryer").value);
var electronics = parseFloat(document.getElementById("electronics").value);
// Basic validation
if (isNaN(homeSize) || homeSize < 0) homeSize = 0;
if (isNaN(numberOfOccupants) || numberOfOccupants < 0) numberOfOccupants = 0;
if (isNaN(refrigeratorFreezer) || refrigeratorFreezer < 0) refrigeratorFreezer = 0;
if (isNaN(furnaceFan) || furnaceFan < 0) furnaceFan = 0;
if (isNaN(wellPump) || wellPump < 0) wellPump = 0;
if (isNaN(sumpPump) || sumpPump < 0) sumpPump = 0;
if (isNaN(microwaveOven) || microwaveOven < 0) microwaveOven = 0;
if (isNaN(electricRangeOven) || electricRangeOven < 0) electricRangeOven = 0;
if (isNaN(electricDryer) || electricDryer < 0) electricDryer = 0;
if (isNaN(electronics) || electronics 0) {
essentialRunningWatts += refrigeratorFreezer * wattages.refrigerator.run;
essentialStartingWatts += wattages.refrigerator.start; // Assume only one fridge starts at a time for surge calculation
}
if (furnaceFan > 0) {
essentialRunningWatts += furnaceFan * wattages.furnace.run;
essentialStartingWatts += wattages.furnace.start; // Assume only one furnace starts at a time for surge calculation
}
if (wellPump > 0) {
essentialRunningWatts += wellPump * wattages.wellPump.run;
essentialStartingWatts += wattages.wellPump.start; // Assume only one well pump starts at a time for surge calculation
}
if (sumpPump > 0) {
essentialRunningWatts += sumpPump * wattages.sumpPump.run;
essentialStartingWatts += wattages.sumpPump.start; // Assume only one sump pump starts at a time for surge calculation
}
var majorRunningWatts = 0;
var majorStartingWatts = 0; // Major appliances often have similar start/run, but list separately for clarity
if (microwaveOven > 0) {
majorRunningWatts += microwaveOven * wattages.microwave.run;
majorStartingWatts += wattages.microwave.start;
}
if (electricRangeOven > 0) {
majorRunningWatts += electricRangeOven * wattages.electricRange.run;
majorStartingWatts += wattages.electricRange.start;
}
if (electricDryer > 0) {
majorRunningWatts += electricDryer * wattages.electricDryer.run;
majorStartingWatts += wattages.electricDryer.start;
}
// Total running watts
var totalRunningWatts = baseLoad + essentialRunningWatts + majorRunningWatts + electronics;
// Total starting watts (simplified: highest single start + all other running)
// Find the highest single starting wattage among essential and major appliances
var maxSingleStart = 0;
if (refrigeratorFreezer > 0) maxSingleStart = Math.max(maxSingleStart, wattages.refrigerator.start);
if (furnaceFan > 0) maxSingleStart = Math.max(maxSingleStart, wattages.furnace.start);
if (wellPump > 0) maxSingleStart = Math.max(maxSingleStart, wattages.wellPump.start);
if (sumpPump > 0) maxSingleStart = Math.max(maxSingleStart, wattages.sumpPump.start);
if (microwaveOven > 0) maxSingleStart = Math.max(maxSingleStart, wattages.microwave.start);
if (electricRangeOven > 0) maxSingleStart = Math.max(maxSingleStart, wattages.electricRange.start);
if (electricDryer > 0) maxSingleStart = Math.max(maxSingleStart, wattages.electricDryer.start);
// Calculate total surge demand: max single start + all other running loads
// (This is a common simplification, actual load calculations can be more complex)
var totalSurgeWatts = maxSingleStart +
(essentialRunningWatts – (refrigeratorFreezer > 0 ? wattages.refrigerator.run : 0)) + // Subtract the running watts of the appliance that might surge
(majorRunningWatts) +
electronics;
// Ensure the surge calculation doesn't go below the max single start if other loads are minimal
if (totalSurgeWatts < maxSingleStart) {
totalSurgeWatts = maxSingleStart;
}
// Recommended size is the higher of total running or total surge
var recommendedSize = Math.max(totalRunningWatts, totalSurgeWatts);
// Add a buffer (e.g., 10-20%) for safety and future needs
recommendedSize = recommendedSize * 1.15; // 15% buffer
// Round up to nearest standard generator size (e.g., 500W increments)
recommendedSize = Math.ceil(recommendedSize / 500) * 500;
// Display the result
var resultElement = document.getElementById("calculatedSize");
var noteElement = document.getElementById("note");
if (isNaN(recommendedSize) || recommendedSize <= 0) {
resultElement.textContent = "Invalid Inputs";
noteElement.textContent = "Please ensure all input values are valid numbers.";
} else if (recommendedSize < 5000) {
resultElement.textContent = recommendedSize.toLocaleString();
noteElement.textContent = "This size is typically for basic essential circuits. Consider a larger unit for more comfort.";
} else if (recommendedSize < 10000) {
resultElement.textContent = recommendedSize.toLocaleString();
noteElement.textContent = "Suitable for most homes needing essential circuits plus a few major appliances.";
} else if (recommendedSize < 20000) {
resultElement.textContent = recommendedSize.toLocaleString();
noteElement.textContent = "Good for whole-house power, including HVAC and multiple major appliances.";
} else {
resultElement.textContent = recommendedSize.toLocaleString();
noteElement.textContent = "This is a large capacity generator, suitable for extensive power needs or large homes.";
}
}