Factors affecting fuel consumption and power output in generators

This article explores factors influencing fuel consumption and power output in air-cool home standby generator systems fueled by natural gas and liquid propane. Key considerations include fuel variability, load variation, altitude, temperature, fuel system integrity, and the importance of proactive maintenance for efficient and reliable operation. 
 

Environment

This article applies Generac air-cool home standby generators. 
 

Factors in fuel consumption

• Fuel Variability—The type and quality of fuel (natural gas, liquid propane, gasoline) can impact how much fuel a generator consumes. Higher-quality or more consistent fuel can lead to more efficient burning and less fuel consumption.
• Load variation—Generators consume more fuel when operating at higher loads, particularly near their maximum capacity. Efficient fuel consumption is typically observed at moderate steady loads. 
• Fuel system integrity—A well-maintained fuel system allows efficient fuel delivery to the engine, minimizing leaks or blockages that could lead to increased fuel consumption. 

 

Factors in power output

Fuel variability - The energy content of the fuel directly influences the amount of power a generator can produce. For instance, liquid propane is more energy-dense than natural gas. As a result, liquid propane burns hotter and cleaner, leading to more efficient combustion and a higher conversion of fuel to usable power. 

• For example - A 26kW air-cooled home standby generator has a maximum continuous power output of 26,000 watts while running on liquid propane, but only 22,500 watts while running on natural gas. 

 

Altitude—The air becomes thinner at higher altitudes, leaving less oxygen for combustion. Poor combustion can decrease efficiency and lower power output. 

• Maximum power decreases about 3.5% for each 1,000 feet (304.8 meters) above sea level.

Temperature—Fuel vaporization significantly affects the rate at which fuel vaporizes. In colder environments, fuels like gasoline and propane vaporize less readily. At higher temperatures, the reduction in fuel and engine efficiency leads to a reduction in power output. 

• A decrease of about 1% for each 6° C (10° F) above 16° C (60° F).

 
 

Understanding generator fuel consumption at constant wattage

Fuel consumption in generators can vary based on multiple factors described above. While it is important to remember that no system is 100% efficient and various external conditions can impact fuel usage, understanding the basic principles of calculating fuel consumption can help estimate your generator's operational time and costs. 

 

Note: The following fuel consumption estimates are for informational purposes only and may not be fully accurate, as actual results depend on additional complex factors not included in the calculation. 
 

Basic calculation of fuel consumption

To calculate the fuel consumption of a generator running at a constant wattage, you can follow these steps: 

1. Identify the fuel's energy content:

Different fuels have different energy contents, usually measured in British Thermal Units (BTUs) per gallon or cubic foot. For example 

• Liquid Propane: ~ 91,500 BTU/gallon 
• Natural Gas: ~ 1,037 BTU/ cubic foot

2. Calculate the electric energy required:

To determine the energy your generator needs to produce, multiply the power consumption (in kilowatts, kW) by the time of operation (in hours, hr).

Energy required (kWhr) = Power Consumption (kW) x Time (hr).

 

3. Convert electric energy to fuel energy: 

Convert the required electrical energy to the amount of fuel energy needed. Fuel Energy Required (BTU) = Energy Required (kWhr) x 3412.14 BTU/kW. Here, 3412.14 BTU is the amount of energy equivalent to 1kWh of electricity. 

4. Calculate the fuel consumption: 

Finally, the fuel energy required is converted into the volume of fuel consumed. 

 

For more information on fuel consumption, see the following articles 

• How Do I Determine the Fuel Cost To Run My Home Standby Generator?
• How Much Fuel Does My Home Standby Generator Use?