How to Calculate Generator Size

Generator Size Calculator

Use this calculator to estimate the running and surge wattage required for your generator. Enter the quantity of each appliance you plan to power, and adjust the typical running and starting watts if you know your specific appliance's ratings.

Appliance Typical Running Watts Typical Starting Watts Quantity
Refrigerator (Energy Star)
Freezer (Chest/Upright)
Well Pump (1/2 HP)
Window AC (10,000 BTU)
Microwave (1000W)
Lights (LED/CFL, 100W equivalent)
Television (LCD/LED)
Desktop Computer + Monitor
Sump Pump (1/2 HP)
Electric Heater (Small, 1500W)
Coffee Maker
Hair Dryer
Garage Door Opener (1/2 HP)
Electric Water Heater (Small, 2000W)
function calculateGeneratorSize() { var totalRunningWatts = 0; var maxIndividualStartingWatts = 0; var runningWattsOfMaxIndividualStartingAppliance = 0; var numAppliances = 14; // Update this if you add/remove rows for (var i = 0; i < numAppliances; i++) { var runningW = parseFloat(document.getElementById('runningWatts' + i).value) || 0; var startingW = parseFloat(document.getElementById('startingWatts' + i).value) || 0; var qty = parseFloat(document.getElementById('quantity' + i).value) || 0; if (qty < 0) qty = 0; // Ensure quantity is not negative if (runningW < 0) runningW = 0; if (startingW maxIndividualStartingWatts) { maxIndividualStartingWatts = startingW; runningWattsOfMaxIndividualStartingAppliance = runningW; } } // Calculate the required continuous running watts var requiredContinuousWatts = totalRunningWatts; // Calculate the required surge watts // This is the total running watts of all appliances MINUS the running watts of the largest starting appliance // PLUS the starting watts of that largest starting appliance. var requiredSurgeWatts = totalRunningWatts – runningWattsOfMaxIndividualStartingAppliance + maxIndividualStartingWatts; // Apply a safety margin (e.g., 25%) var safetyMarginFactor = 1.25; var recommendedRunningWatts = requiredContinuousWatts * safetyMarginFactor; var recommendedSurgeWatts = requiredSurgeWatts * safetyMarginFactor; // Ensure recommended running watts can handle the surge if the generator's surge factor is low // A common rule of thumb is that a generator's surge capacity is 1.5x to 2x its running capacity. // If the calculated surge is very high, the running watts might need to be higher to support it. // For simplicity, we'll recommend based on the higher of the two primary calculations. // The generator's running watts should be able to handle the continuous load. // The generator's surge watts should be able to handle the peak load. var resultDiv = document.getElementById('result'); if (totalRunningWatts === 0 && maxIndividualStartingWatts === 0) { resultDiv.innerHTML = "Please enter quantities and wattages for the appliances you wish to power."; } else { resultDiv.innerHTML = "

Recommended Generator Size:

" + "Total Continuous Running Watts Required: " + Math.ceil(requiredContinuousWatts) + " Watts" + "Peak Surge Watts Required: " + Math.ceil(requiredSurgeWatts) + " Watts" + "(These are the raw calculated values without safety margin)" + "
" + "Recommended Generator Running Watts (with 25% safety margin): " + Math.ceil(recommendedRunningWatts) + " Watts" + "Recommended Generator Surge Watts (with 25% safety margin): " + Math.ceil(recommendedSurgeWatts) + " Watts" + "When purchasing a generator, look for one with a 'Running Watts' rating at or above the recommended Running Watts, and a 'Surge Watts' rating at or above the recommended Surge Watts."; } } .generator-size-calculator { font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif; max-width: 800px; margin: 20px auto; padding: 25px; border: 1px solid #e0e0e0; border-radius: 10px; background-color: #f9f9f9; box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08); color: #333; } .generator-size-calculator h2 { text-align: center; color: #2c3e50; margin-bottom: 20px; font-size: 1.8em; } .generator-size-calculator p { margin-bottom: 15px; line-height: 1.6; color: #555; } .calculator-container { background-color: #ffffff; padding: 20px; border-radius: 8px; border: 1px solid #e9e9e9; } .appliance-table { width: 100%; border-collapse: collapse; margin-bottom: 25px; } .appliance-table th, .appliance-table td { border: 1px solid #ddd; padding: 10px 12px; text-align: left; vertical-align: middle; } .appliance-table th { background-color: #f2f2f2; font-weight: bold; color: #333; } .appliance-table td input[type="number"] { width: 80px; padding: 8px; border: 1px solid #ccc; border-radius: 4px; box-sizing: border-box; font-size: 0.95em; } .appliance-table td:first-child { font-weight: 500; color: #444; } .generator-size-calculator button { display: block; width: 100%; padding: 12px 20px; background-color: #007bff; color: white; border: none; border-radius: 5px; font-size: 1.1em; cursor: pointer; transition: background-color 0.3s ease; margin-top: 20px; } .generator-size-calculator button:hover { background-color: #0056b3; } .calculator-result { margin-top: 30px; padding: 20px; border: 1px solid #d4edda; background-color: #eaf7ed; border-radius: 8px; color: #155724; font-size: 1.1em; line-height: 1.7; } .calculator-result h3 { color: #28a745; margin-top: 0; font-size: 1.4em; margin-bottom: 15px; } .calculator-result p { margin-bottom: 8px; color: #155724; } .calculator-result strong { color: #0f3d1a; } .calculator-result hr { border: 0; border-top: 1px solid #c3e6cb; margin: 15px 0; } /* Responsive adjustments */ @media (max-width: 600px) { .generator-size-calculator { padding: 15px; } .appliance-table th, .appliance-table td { padding: 8px; font-size: 0.9em; } .appliance-table td input[type="number"] { width: 60px; padding: 6px; } .generator-size-calculator button { font-size: 1em; padding: 10px 15px; } .calculator-result { font-size: 1em; padding: 15px; } }

Understanding Generator Sizing for Your Home

Choosing the right size generator is crucial for ensuring your essential appliances run smoothly during a power outage without overloading the unit. An undersized generator won't power your needs, while an oversized one can be an unnecessary expense and less fuel-efficient.

Running Watts vs. Starting Watts (Surge Watts)

The most important concept in generator sizing is the difference between running watts and starting watts:

  • Running Watts (Continuous Watts): This is the power an appliance needs to operate continuously once it's turned on. For example, a refrigerator might need 150-200 running watts.
  • Starting Watts (Surge Watts): Many motor-driven appliances (like refrigerators, well pumps, air conditioners, and sump pumps) require a brief burst of extra power to start their motors. This surge can be 2 to 6 times their running wattage. Once the motor starts, the power requirement drops back to the running watts.

Your generator must be able to handle both the total running watts of all appliances you want to operate simultaneously AND the highest single starting watt requirement of any appliance you plan to start while others are already running.

How to Calculate Your Generator Needs

  1. List Essential Appliances: Identify all the appliances you absolutely need to power during an outage. Think about lights, refrigeration, heating/cooling, communication, and water supply.
  2. Determine Running Watts: Find the running wattage for each appliance. This information is usually on a label on the appliance itself, in the owner's manual, or you can use typical values provided in the calculator.
  3. Determine Starting Watts: For motor-driven appliances, find their starting wattage. If not listed, you can often estimate it as 2-3 times the running wattage for smaller motors, and up to 4-6 times for larger motors like central AC units. The calculator provides typical values.
  4. Sum Running Watts: Add up the running watts of all appliances you expect to run at the same time.
  5. Identify the Largest Starting Load: Look at your list and find the appliance with the highest starting wattage. This is the critical surge your generator must be able to handle.
  6. Calculate Peak Load: Subtract the running watts of your largest starting load from your total running watts, then add the starting watts of that largest starting load. This gives you the maximum instantaneous power your generator will need to supply.
  7. Add a Safety Margin: It's always wise to add a 20-25% safety margin to your calculated running and surge wattages. This accounts for potential inefficiencies, future appliance additions, or unexpected power draws, preventing your generator from running at its absolute maximum capacity, which can extend its lifespan.

Example Scenario:

Let's say you want to power a refrigerator (150 running, 600 starting), 5 LED lights (15 running each, 15 starting each), and a TV (100 running, 100 starting).

  • Total Running Watts: (150W Ref) + (5 * 15W Lights) + (100W TV) = 150 + 75 + 100 = 325 Watts
  • Largest Starting Load: Refrigerator at 600 Watts (its running watts are 150W)
  • Peak Surge Calculation: (Total Running Watts – Running Watts of Refrigerator) + Starting Watts of Refrigerator = (325W – 150W) + 600W = 175W + 600W = 775 Watts
  • With 25% Safety Margin:
    • Recommended Running Watts: 325W * 1.25 = 406.25 Watts
    • Recommended Surge Watts: 775W * 1.25 = 968.75 Watts

In this example, you would look for a generator with at least 400-500 running watts and 900-1000 surge watts.

Tips for Efficient Generator Use:

  • Prioritize: Don't try to power everything at once. Cycle high-wattage appliances (e.g., run the microwave, then turn it off before starting the well pump).
  • Energy-Efficient Appliances: If possible, use energy-efficient appliances, especially for refrigeration and lighting.
  • Soft Start Kits: For large motor loads like central AC, consider a "soft start" kit, which can significantly reduce the starting wattage requirement.
  • Fuel Management: Always have enough fuel on hand, and store it safely.
  • Ventilation: Generators produce carbon monoxide; always operate them outdoors in a well-ventilated area, far from windows and doors.

By carefully calculating your power needs, you can select a generator that provides reliable backup power without overspending or underperforming.

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