What Size Generator Do I Need?
Use this calculator to determine the appropriate running (continuous) and starting (surge) wattage your generator needs to provide power to your essential appliances during an outage or for off-grid use. Understanding the difference between running and starting watts is crucial for selecting the right generator.
Understanding Generator Sizing
Choosing the right generator size is critical for ensuring your essential appliances run smoothly without overloading the unit. A generator that's too small won't be able to power your devices, while one that's too large might be an unnecessary expense and consume more fuel than needed.
Running Watts vs. 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 700 running watts to keep its compressor going.
- Starting Watts (Surge Watts): Many appliances, especially those with electric motors (like refrigerators, air conditioners, sump pumps, and power tools), require a brief burst of extra power to start up. This initial surge can be 2 to 3 times their running wattage. Once the motor starts, the power requirement drops back to the running wattage.
The challenge in generator sizing comes from these starting watts. Your generator must be able to handle the highest starting wattage of any single appliance that might kick on while other appliances are already running. It's not about summing all starting watts, but rather identifying the largest single surge requirement on top of the total running load.
How the Calculator Works
Our calculator takes into account the running and starting watts for each appliance you list. It performs the following steps:
- Sums Total Running Watts: It adds up the continuous power needed for all appliances you want to run simultaneously.
- Identifies Peak Starting Load: It finds the appliance with the highest starting wattage. It then calculates the total power needed if all other appliances are running, and this largest motor-driven appliance suddenly starts. This is the critical "surge" requirement.
- Applies a Safety Factor: A safety factor (typically 10-20%) is added to both the running and starting wattage totals. This buffer helps prevent overloading, accounts for potential future needs, and ensures the generator isn't constantly running at its absolute maximum capacity, which can extend its lifespan.
Tips for Accurate Sizing
- Prioritize Appliances: Decide which appliances are truly essential during an outage. You might not need to power everything at once.
- Check Appliance Labels: Most appliances have a label or owner's manual that lists their running and starting wattage. If only amperage (amps) and voltage (volts) are listed, you can estimate wattage using the formula:
Watts = Amps × Volts. For starting watts, if not listed, assume 2-3 times the running watts for motor-driven appliances.
- Consider Simultaneous Use: Think about which appliances might realistically start at the same time. For instance, a refrigerator and a sump pump might both cycle on independently.
- Fuel Type and Run Time: Beyond wattage, consider the generator's fuel type (gasoline, propane, natural gas, diesel) and its run time on a full tank at a given load.
- Portable vs. Standby: Portable generators are great for temporary power, while standby generators automatically kick in when utility power fails and can power more of your home.
By using this calculator and understanding these principles, you can confidently select a generator that meets your specific power needs.
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function calculateGeneratorSize() {
var totalRunningWatts = 0;
var maxIndividualStartingWatts = 0;
var runningWattsOfMaxStartingAppliance = 0;
var applianceData = [];
for (var i = 1; i <= 8; i++) {
var runningWattsInput = document.getElementById('runningWatts' + i);
var startingWattsInput = document.getElementById('startingWatts' + i);
var quantityInput = document.getElementById('quantity' + i);
var currentRunningWatts = parseFloat(runningWattsInput.value);
var currentStartingWatts = parseFloat(startingWattsInput.value);
var currentQuantity = parseInt(quantityInput.value);
if (isNaN(currentRunningWatts) || currentRunningWatts < 0) currentRunningWatts = 0;
if (isNaN(currentStartingWatts) || currentStartingWatts < 0) currentStartingWatts = 0;
if (isNaN(currentQuantity) || currentQuantity 0 && currentQuantity > 0) {
totalRunningWatts += currentRunningWatts * currentQuantity;
applianceData.push({
running: currentRunningWatts,
starting: currentStartingWatts,
quantity: currentQuantity
});
}
}
// Find the highest individual starting wattage and its corresponding running wattage
for (var j = 0; j maxIndividualStartingWatts) {
maxIndividualStartingWatts = app.starting;
runningWattsOfMaxStartingAppliance = app.running;
}
}
var requiredContinuousWatts = totalRunningWatts;
// The surge calculation: total running watts MINUS the running watts of the appliance that causes the largest surge, PLUS that appliance's surge watts.
// This assumes only ONE motor-driven appliance will start at its peak surge while others are running.
var requiredSurgeWatts = totalRunningWatts – runningWattsOfMaxStartingAppliance + maxIndividualStartingWatts;
// Handle edge case where no motor-driven appliances are entered, so maxIndividualStartingWatts might be 0 or equal to runningWattsOfMaxStartingAppliance
// In this case, requiredSurgeWatts should just be totalRunningWatts
if (maxIndividualStartingWatts <= runningWattsOfMaxStartingAppliance) {
requiredSurgeWatts = totalRunningWatts;
}
var safetyFactor = parseFloat(document.getElementById('safetyFactor').value);
if (isNaN(safetyFactor) || safetyFactor 100) safetyFactor = 100; // Cap safety factor at 100%
var factorMultiplier = 1 + (safetyFactor / 100);
var finalContinuousWatts = requiredContinuousWatts * factorMultiplier;
var finalSurgeWatts = requiredSurgeWatts * factorMultiplier;
var resultDiv = document.getElementById('result');
if (totalRunningWatts === 0 && maxIndividualStartingWatts === 0) {
resultDiv.innerHTML = '
Generator Size Recommendation
Please enter at least one appliance with valid wattage to calculate.';
} else {
resultDiv.innerHTML =
'
Generator Size Recommendation
' +
'Minimum Continuous (Running) Watts Needed:
' + Math.ceil(finalContinuousWatts) + ' Watts' +
'Minimum Starting (Surge) Watts Needed:
' + Math.ceil(finalSurgeWatts) + ' Watts' +
'These values include a ' + safetyFactor + '% safety factor.';
}
}