Tip: Most portable generators use ~0.5 gallons per hour at 50% load.
Estimated Total Run Time:
How to Calculate Generator Run Time
Knowing how long your generator will run on a single tank of fuel is critical for emergency preparedness and site planning. The formula for generator run time is straightforward: Total Run Time = Tank Capacity / Consumption Rate.
However, the "Consumption Rate" is the variable that changes most. Most manufacturers provide fuel consumption data for 25%, 50%, and 100% load capacities. As you plug in more appliances, the engine works harder and burns fuel faster.
Key Factors Influencing Run Time
Electrical Load: A generator running a refrigerator and a few lights (low load) will last significantly longer than one running a central A/C unit and a water heater (high load).
Fuel Type: Diesel generally offers the highest energy density and longest run times, followed by gasoline, and then Propane (LPG), which typically has the lowest energy density per unit.
Engine Efficiency: Inverter generators are much more efficient than standard portable generators because they can throttle their engine speed down when demand is low.
Altitude and Temperature: High altitudes and extreme heat can reduce engine efficiency, slightly increasing fuel consumption.
Typical Fuel Consumption Examples (Approximate)
Generator Size
Load Level
Avg. Consumption (Gasoline)
2,000 Watt (Inverter)
25% Load
0.10 – 0.15 Gallons/Hr
5,000 Watt (Portable)
50% Load
0.45 – 0.60 Gallons/Hr
10,000 Watt (Large)
50% Load
0.80 – 1.10 Gallons/Hr
Safety Tips for Long Run Times
If you anticipate running your generator for extended periods, remember to check the oil level every 8–24 hours of operation. Most small engines require an oil change every 50–100 hours. Always shut down the generator and let it cool for at least 5–10 minutes before refueling to prevent fires from spilled fuel on hot engine parts.
function calculateGeneratorRuntime() {
var tankSize = parseFloat(document.getElementById('tankSize').value);
var consumptionRate = parseFloat(document.getElementById('consumptionRate').value);
var resultDiv = document.getElementById('runtime-result');
var finalTimeDisplay = document.getElementById('finalTime');
var detailsDisplay = document.getElementById('resultDetails');
if (isNaN(tankSize) || tankSize <= 0) {
alert("Please enter a valid fuel tank capacity.");
return;
}
if (isNaN(consumptionRate) || consumptionRate 0) {
resultString += "and " + minutes + " Minutes";
}
finalTimeDisplay.innerHTML = resultString;
detailsDisplay.innerHTML = "Based on a " + tankSize + " unit tank burning " + consumptionRate + " units per hour.";
resultDiv.style.display = 'block';
}