Sum of the wattage of all appliances you plan to run simultaneously.
Sum of the *additional* wattage required by appliances with motors (like refrigerators, pumps, AC units) when they start up.
0.8 (Typical for most appliances)
0.7
0.9
1.0 (Resistive loads only)
Represents the ratio of real power to apparent power. 0.8 is a common and safe assumption.
25%
30%
50%
10%
20%
A buffer to account for unexpected loads or generator aging. 25% is recommended.
Understanding Your Generator Needs
An emergency generator is a crucial investment for ensuring continuity during power outages. Properly sizing your generator is key to avoiding underperformance, damage to appliances, and unnecessary costs. This calculator helps you determine the minimum continuous and starting wattage your generator needs to provide.
Key Concepts:
Wattage (W): The unit of power. Appliances have two wattage ratings: running wattage and starting wattage.
Running Wattage: The continuous power an appliance requires to operate.
Starting Wattage (Surge Wattage): The momentary surge of power needed to start an appliance with a motor (e.g., refrigerators, air conditioners, pumps). This can be 2 to 3 times the running wattage.
Total Connected Wattage: The sum of the running wattages of all the appliances you intend to power simultaneously.
Total Starting Wattage: The sum of the *additional* starting wattage required by all motor-driven appliances that might start at the same time. The highest starting wattage appliance is the most critical factor here.
Power Factor: Represents the efficiency of electrical power usage. For generators, it's often factored into calculations, with 0.8 being a common value. It relates Watts (Real Power) to Volt-Amps (Apparent Power). Watts = Volts * Amps * Power Factor.
Safety Margin: A recommended buffer (e.g., 25%) added to the calculated load. This ensures the generator isn't constantly running at its maximum capacity, which extends its lifespan and handles unexpected fluctuations.
How the Calculator Works:
The calculator uses your input values to estimate the required generator size.
Calculate Total Load:
The calculator first sums the Total Connected Wattage (running watts) with the highest individual Total Starting Wattage required by any single appliance that might start simultaneously. This gives a realistic peak load estimate.
Note: For simplicity, this calculator uses the provided Total Starting Wattage figure. A more precise calculation would identify the single appliance with the highest starting surge. However, summing starting watts can be a conservative approach if multiple motor loads are expected to start together.
Apply Safety Margin:
The calculated peak load is then multiplied by the selected Safety Margin percentage.
The result is the minimum continuous wattage your generator should be rated for. Generators are often rated in both Watts (continuous) and Volt-Amps (VA). The power factor is used to relate these, but for simplicity, this calculator focuses on Wattage output.
The calculation implicitly accounts for the power factor by using the provided Wattage figures, which are typically the real power consumed.
Example Calculation:
Let's say you want to power:
A refrigerator with 800W running and a 1200W starting surge.
A well pump with 1000W running and a 2000W starting surge.
Lights and electronics totaling 500W running (negligible starting surge).
Total Starting Wattage: 1200W (fridge surge) + 2000W (pump surge) = 3200W (Assuming both could start around the same time). A more precise method would consider the highest surge (2000W) plus running watts of other items. For this calculator, we'll use the provided input.
Power Factor: 0.8 (Typical)
Safety Margin: 25% (Factor = 1.25)
Calculation Steps:
Calculate approximate peak load: The highest starting surge is from the pump (2000W). If the pump starts while the fridge is running, the load is 1000W (pump running) + 800W (fridge running) + 500W (lights) = 2300W. The surge adds 2000W to the pump's running load. So, the peak demand could be around 2300W (running total) + 2000W (pump surge) = 4300W.
Using the calculator's input method: Peak Load = 2300W (Total Running) + 3200W (Total Starting) = 5500W. This is a more conservative estimate if multiple motors might start concurrently. Let's proceed with the calculator's conservative input method for the example: Peak Load = 5500W.
Apply Safety Margin: 5500W * 1.25 = 6875W
Result: You would need a generator with at least 6875 running watts.
Choosing a Generator:
Always round up to the nearest available generator size. It's better to have a slightly oversized generator than one that is too small. Consider which appliances are essential (lights, fridge, medical equipment) versus optional (large appliances, entertainment systems) when estimating your needs.
function calculateGeneratorSize() {
var totalWatts = parseFloat(document.getElementById("totalWatts").value);
var startingWatts = parseFloat(document.getElementById("startingWatts").value);
var powerFactor = parseFloat(document.getElementById("powerFactor").value);
var safetyMargin = parseFloat(document.getElementById("safetyMargin").value);
var resultElement = document.getElementById("result");
resultElement.innerHTML = ""; // Clear previous results
// Input validation
if (isNaN(totalWatts) || totalWatts <= 0) {
resultElement.innerHTML = "Please enter a valid Total Connected Wattage (must be greater than 0).";
return;
}
if (isNaN(startingWatts) || startingWatts < 0) {
// Allow 0 starting watts if no motor appliances
resultElement.innerHTML = "Please enter a valid Total Starting Wattage (must be 0 or greater).";
return;
}
// Simplified calculation logic:
// 1. Calculate the theoretical peak load. A common approach is to take the total running watts
// and add the highest starting surge wattage from a single appliance.
// However, the user provides "Total Starting Wattage", which can be interpreted as the sum of surges.
// For a conservative estimate, we can sum them, but a more accurate method would be:
// Peak Load = Total Connected Wattage + (Highest Single Starting Wattage – Its Running Wattage)
// Or, if "Total Starting Wattage" refers to the sum of *additional* watts needed for starting:
// Peak Load = Total Connected Wattage + Total Starting Wattage (this is a very conservative approach)
// Let's use a balanced approach: Assume the higher of (Total Watts) or (Total Watts + Highest individual start surge).
// Since we don't have individual surges, we'll use the provided totalWatts and startingWatts.
// A common simplified method is to add the highest starting wattage component to the total running wattage.
// Let's interpret 'startingWatts' as the sum of surge requirements for *all* motor appliances.
// A pragmatic approach is often: Max(Total Watts, Total Watts + (Highest Single Surge Wattage – its Running Wattage))
// Given the input structure, let's calculate based on the total running watts PLUS the total surge watts provided,
// recognizing this is a conservative estimate.
// peakLoadEstimate = totalWatts + startingWatts; // Very conservative
// Alternative pragmatic calculation: total running watts + the highest starting surge needed.
// If 'startingWatts' represents the sum of ALL starting surges, it might be an overestimation.
// A better approach is often Total Watts + Max(Individual Appliance Surge – Appliance Running Watts).
// Let's refine: assume 'startingWatts' is the sum of the *additional* power needed when motors start.
// A simplified but effective method:
var requiredContinuousWatts = totalWatts * safetyMargin;
var requiredStartingWatts = (totalWatts + startingWatts) * safetyMargin; // A more conservative peak
// Consider the actual peak demand: It's the total running watts plus the largest *additional* surge needed.
// If the user enters 'startingWatts' as the sum of surge wattages, we can use it to estimate peak.
// A very common recommendation: Total Running Watts + (Largest Motor Starting Wattage – Largest Motor Running Wattage)
// Lacking individual appliances, let's use a blend: the total running watts scaled up by the safety margin,
// and also consider the surge requirement.
// Method 1: Focus on Continuous Load + Safety Margin
var calculatedWatts1 = totalWatts * safetyMargin;
// Method 2: Consider Peak Load (Running + Starting Surge) + Safety Margin
// This assumes 'startingWatts' is the sum of surge requirements for *all* motor appliances.
// A more realistic peak load would be: totalWatts + highest_single_surge
// Let's calculate a peak wattage needed:
var peakDemand = totalWatts; // Start with running watts
// If startingWatts represents the sum of surge needs, adding it to totalWatts is very conservative.
// A more standard calculation considers the highest individual starting surge.
// Let's use a conservative approach based on inputs:
var totalPeakLoadEstimate = totalWatts + startingWatts; // This is highly conservative if startingWatts is sum of surges
var calculatedWatts2 = totalPeakLoadEstimate * safetyMargin;
// Use the higher of the two estimates for a safer bet.
var finalRequiredWatts = Math.max(calculatedWatts1, calculatedWatts2);
// The power factor is implicitly handled if the inputs (Watts) are real power.
// Generators are rated in Watts (real power) and VA (apparent power).
// Watts = VA * Power Factor. So VA = Watts / Power Factor.
// If the calculation resulted in Watts, and the user needs VA, then VA = finalRequiredWatts / powerFactor.
// However, most generators are sold by their Watt rating.
resultElement.innerHTML = "Your estimated required generator size is: " + finalRequiredWatts.toFixed(0) + " Watts";
resultElement.innerHTML += "(Based on continuous load, starting surge, and a " + ((safetyMargin – 1) * 100) + "% safety margin)";
}