Home Generator Size Calculator

Use this calculator to estimate the running and starting wattage requirements for a home backup generator. Select the appliances you wish to power and specify the quantity for each. The calculator will sum the running watts and identify the largest single starting wattage to determine your total generator needs.

1. Select Appliances and Quantities

Enter the quantity for each appliance you plan to power simultaneously. Typical wattages are provided.

Appliance	Running Watts (Approx.)	Starting Watts (Approx.)	Quantity
Refrigerator (Energy Star)	600	1800	<input type="number" id="qtyRefrigerator" value="0" min="0" oninput="if(this.value
Freezer	500	1500	<input type="number" id="qtyFreezer" value="0" min="0" oninput="if(this.value
Central AC (3-ton)	3500	7000	<input type="number" id="qtyCentralAC" value="0" min="0" oninput="if(this.value
Window AC (10,000 BTU)	1200	2400	<input type="number" id="qtyWindowAC" value="0" min="0" oninput="if(this.value
Microwave (1000W)	1000	1500	<input type="number" id="qtyMicrowave" value="0" min="0" oninput="if(this.value
Electric Oven (per element)	1500	0	<input type="number" id="qtyElectricOven" value="0" min="0" oninput="if(this.value
Well Pump (1/2 HP)	1000	2000	<input type="number" id="qtyWellPump" value="0" min="0" oninput="if(this.value
Sump Pump (1/3 HP)	800	1600	<input type="number" id="qtySumpPump" value="0" min="0" oninput="if(this.value
Lights (LED/CFL, per 10 bulbs)	100	0	<input type="number" id="qtyLights" value="0" min="0" oninput="if(this.value
Television (LCD/LED)	200	0	<input type="number" id="qtyTV" value="0" min="0" oninput="if(this.value
Desktop Computer + Monitor	300	0	<input type="number" id="qtyComputer" value="0" min="0" oninput="if(this.value
Hair Dryer	1500	0	<input type="number" id="qtyHairDryer" value="0" min="0" oninput="if(this.value
Coffee Maker	1000	0	<input type="number" id="qtyCoffeeMaker" value="0" min="0" oninput="if(this.value
Garage Door Opener	500	1000	<input type="number" id="qtyGarageDoorOpener" value="0" min="0" oninput="if(this.value

2. Custom Appliance (Optional)

Add any other appliance not listed above.

Appliance Name:

Running Watts: <input type="number" id="customRunningWatts" value="0" min="0" oninput="if(this.value

Starting Watts: <input type="number" id="customStartingWatts" value="0" min="0" oninput="if(this.value

Quantity: <input type="number" id="customQty" value="0" min="0" oninput="if(this.value

3. Safety Margin

Add a Safety Margin (%): <input type="number" id="safetyMargin" value="20" min="0" oninput="if(this.value

Generator Size Recommendation

Based on your selections, here are the estimated wattage requirements for your home generator:

Recommended Running Watts: 0 Watts

Recommended Starting Watts: 0 Watts

Always consult with a qualified electrician or generator professional to confirm your specific needs and ensure proper installation.

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border-radius: 5px; font-size: 1em; min-width: 100px; max-width: 250px; } .appliance-table { width: 100%; border-collapse: collapse; margin-top: 15px; margin-bottom: 20px; } .appliance-table th, .appliance-table td { border: 1px solid #ddd; padding: 10px; text-align: left; color: #333; } .appliance-table th { background-color: #f2f2f2; font-weight: bold; } .appliance-table input[type="number"] { width: 70px; padding: 8px; border: 1px solid #ccc; border-radius: 4px; font-size: 0.95em; } button { display: block; width: auto; padding: 12px 25px; background-color: #007bff; color: white; border: none; border-radius: 5px; font-size: 1.1em; cursor: pointer; margin: 20px auto 0 auto; transition: background-color 0.3s ease; } button:hover { background-color: #0056b3; } .calc-output-group { background-color: #eaf6ff; padding: 20px; border-radius: 8px; border: 1px solid #b3d9ff; margin-top: 20px; } .calc-output-group p { font-size: 1.1em; color: #333; } .calc-output-group strong { color: #0056b3; } .calc-output-group span { font-weight: bold; color: #007bff; } .calc-output-group .note { font-size: 0.9em; color: #777; margin-top: 15px; border-top: 1px dashed #cce0ff; padding-top: 10px; } @media (max-width: 600px) { .form-row { flex-direction: column; align-items: flex-start; } .form-row label { margin-bottom: 5px; min-width: unset; } .form-row input[type="number"], .form-row input[type="text"] { width: 100%; max-width: 100%; } .appliance-table th, .appliance-table td { padding: 8px 5px; font-size: 0.9em; } .appliance-table input[type="number"] { width: 50px; } } function calculateGeneratorSize() { var totalRunningWatts = 0; var maxStartingWattForSingleAppliance = 0; // Define appliance wattages var appliances = { "Refrigerator": { running: 600, starting: 1800 }, "Freezer": { running: 500, starting: 1500 }, "CentralAC": { running: 3500, starting: 7000 }, "WindowAC": { running: 1200, starting: 2400 }, "Microwave": { running: 1000, starting: 1500 }, "ElectricOven": { running: 1500, starting: 0 }, "WellPump": { running: 1000, starting: 2000 }, "SumpPump": { running: 800, starting: 1600 }, "Lights": { running: 100, starting: 0 }, // per 10 bulbs "TV": { running: 200, starting: 0 }, "Computer": { running: 300, starting: 0 }, "HairDryer": { running: 1500, starting: 0 }, "CoffeeMaker": { running: 1000, starting: 0 }, "GarageDoorOpener": { running: 500, starting: 1000 } }; for (var key in appliances) { if (appliances.hasOwnProperty(key)) { var qtyId = "qty" + key; var qtyInput = document.getElementById(qtyId); var quantity = parseFloat(qtyInput.value); if (!isNaN(quantity) && quantity > 0) { totalRunningWatts += quantity * appliances[key].running; // Only consider the starting watts if the appliance is selected (quantity > 0) maxStartingWattForSingleAppliance = Math.max(maxStartingWattForSingleAppliance, appliances[key].starting); } } } // Custom Appliance var customQty = parseFloat(document.getElementById('customQty').value); var customRunning = parseFloat(document.getElementById('customRunningWatts').value); var customStarting = parseFloat(document.getElementById('customStartingWatts').value); if (!isNaN(customQty) && customQty > 0 && !isNaN(customRunning) && customRunning >= 0) { totalRunningWatts += customQty * customRunning; if (!isNaN(customStarting) && customStarting >= 0) { maxStartingWattForSingleAppliance = Math.max(maxStartingWattForSingleAppliance, customStarting); } } var requiredGeneratorRunningWatts = totalRunningWatts; var requiredGeneratorStartingWatts = totalRunningWatts + maxStartingWattForSingleAppliance; // Safety Margin var safetyMarginInput = document.getElementById('safetyMargin'); var safetyMarginPercent = parseFloat(safetyMarginInput.value); if (isNaN(safetyMarginPercent) || safetyMarginPercent < 0) { safetyMarginPercent = 0; // Default to no margin if invalid } var safetyFactor = 1 + (safetyMarginPercent / 100); var finalRunningWatts = requiredGeneratorRunningWatts * safetyFactor; var finalStartingWatts = requiredGeneratorStartingWatts * safetyFactor; // Display results document.getElementById('outputRunningWatts').innerText = Math.ceil(finalRunningWatts).toLocaleString(); document.getElementById('outputStartingWatts').innerText = Math.ceil(finalStartingWatts).toLocaleString(); } // Initial calculation on page load for default values document.addEventListener('DOMContentLoaded', calculateGeneratorSize);

Understanding Home Generator Sizing

Choosing the right size generator for your home is crucial for ensuring you have reliable power during outages without overspending or underpowering your essential appliances. This guide will help you understand the key concepts and how to use the calculator effectively.

Why Generator Sizing Matters

An undersized generator won't be able to power all the appliances you need, potentially leading to tripped breakers or damage to the generator itself. An oversized generator, while capable, will cost more upfront, consume more fuel, and might not run as efficiently when operating at a low load.

Running Watts vs. Starting Watts

These are the two most important terms when sizing a generator:

Running Watts (Rated Watts): This is the continuous power an appliance needs to operate once it's turned on. When you sum up the running watts of all the appliances you want to power simultaneously, you get your total running wattage requirement.
Starting Watts (Surge Watts): Many motor-driven appliances (like refrigerators, air conditioners, well pumps, etc.) require a brief burst of extra power to start up. This surge can be 2-3 times their running wattage. A generator must be able to handle this momentary surge. However, you typically only need to account for the *single largest* starting wattage among your selected appliances, as it's unlikely all motor-driven appliances will start at the exact same moment.

How the Calculator Works

Our Home Generator Size Calculator simplifies this process:

Sums Running Watts: It adds up the running watts for all the appliances you've selected and specified a quantity for. This gives you the total continuous power your generator needs to supply.
Identifies Largest Starting Wattage: It then finds the highest individual starting wattage among all your chosen appliances.
Calculates Total Starting Watts: The total starting watts required by the generator is the sum of your total running watts plus that single largest starting wattage. This ensures the generator can handle the initial surge of the most demanding appliance while still powering everything else.
Applies Safety Margin: A safety margin (typically 10-20%) is added to both the running and starting wattage totals. This provides a buffer for unexpected loads, future appliance additions, or variations in appliance performance, helping to prevent overloading your generator.

Tips for Accurate Sizing

Prioritize Appliances: Decide which appliances are absolutely essential during an outage (e.g., refrigerator, lights, well pump) and which are optional (e.g., hair dryer, secondary TV).
Check Appliance Labels: While our calculator provides typical wattages, the most accurate numbers will be found on the appliance's data plate or owner's manual. Look for "rated watts," "running watts," or "maximum power consumption."
Consider Simultaneous Use: Think realistically about which appliances will run at the same time. You might not need to power your microwave and coffee maker simultaneously if you only use them for short periods.
Consult an Electrician: For whole-home standby generators or complex setups, always consult a licensed electrician. They can perform a detailed load calculation and ensure your generator is installed safely and correctly.

By using this calculator and understanding these principles, you can make an informed decision about the right generator size for your home, ensuring comfort and safety during power interruptions.