The Carbon-Footprint of Diesel Generators
Diesel-fueled generators have long been the foundation of emergency backup power. According to economic and public policy consulting group, M.Cubed (“New Study Shows a Rapid Increase of Diesel-Fueled Backup Generators Across California” Business Wire, October 2021) the number of diesel generators increases as the need for backup power increases. This is a national trend, influenced by growing grid instability, and climate change. As we search for sustainable ways to operate our buildings, diesel generators keep us dependent on fossil fuels, however it’s a difficult technology to replace. So, it begs the question, what is the environmental impact of a typical diesel generator?
The EPA has long regulated stationary diesel generators like the ones used for backup power. Diesel generators produce particulate matter (PM), volatile organic compounds (VOCs), nitrous oxide (NOx) among other harmful pollutants that create smog and exacerbate respiratory conditions. EPA regulations limit diesel generator pollutants and, technologies like Selective Catalytic Reduction (SCRs) and Diesel Particulate Filters (DPFs), help improve their emissions. Absent from the EPA regulations are greenhouse gas (GHG) emissions however, corporate sustainability initiatives take matters into their own hands with plans curtail if not eliminate GHG emissions over the next 10 to 15 years.
Calculating Diesel Generator GHG Emissions
A typical diesel generator meets or exceeds EPA standards, but what is its impact on a corporate sustainability initiative? How much GHG does it produce? What does it take to offset the GHG produced by a diesel generator? To answer these questions, FEA analyzed a typical diesel generator system with the following characteristics:
Facility load = 2 Megawatts
Generator Redundancy = 2N
Generator Unit Rating = 2 Megawatts
Number of Generators Running = 2 Generators
Generator Running Capacity = 4 Megawatts
Generator Load Factor = 50% (each 2MW Generator will carry 1 Megawatt of load)
Annual Generator Runtime = 100 hours (EPA limit for testing and maintenance)
Annual Generator Energy Production = 200 Megawatt-Hours
With the generator load factor (50%) and the annual generator runtime (100 hours) we can use the engine manufacturer’s published fuel consumption rate of 78 Gallons/Hour at 50% load  to determine the annual diesel fuel consumption.
Annual Diesel Fuel Consumption = 78 Gallons/Hour * 100 Hours/Year * 2 Generators
Annual Diesel Fuel Consumption = 15,600 Gallons / Year
The EPA/Department of Transportation (Federal Register 2010) common conversion factor to convert gallons of diesel fuel to metric tons of CO2 emissions  is:
10.180 x 10-3 Metric Tons of CO2 / Gallon of Diesel Fuel
Multiplying the annual diesel fuel consumption by the conversion factor determines the annual CO2 emissions for the diesel generators:
Annual CO2 Emissions = 15,600 Gallons/Year * 10.180 x 10-3 Metric Tons of CO2/Gallon
Annual CO2 Emissions = 159 Metric Tons of CO2/Year
To sum it up, this facility’s diesel generators emit 159 Metric Tons of CO2 annually.
Comparing GHG Emissions of Diesel Generators to Grid Energy
Based on the above example we now know that these diesel generators emit 159 Metric Tons of CO2 to produce 200 Megawatt-Hours of energy. Put another way, this diesel generator emits 0.79 Metric Tons of CO2 for every Megawatt-hour of energy produced. How does this compare to the energy we consume from the grid?
To answer this question, we use two, readily available values for the United State energy sector:
Total Annual Grid Energy Produced: 4,007 million MWh 
Total Annual Grid CO2 Emissions: 1,482 million Metric Tons of CO2 
These values are based on the U.S. Energy Information Administration and EPA national datasets for 2020, the last year with complete data. Dividing the emissions by energy, the U.S. grid emits 0.37 Metric Tons of CO2 for every Megawatt-Hour of energy produced. Compare this to the diesel generator emissions found above, a typical diesel generator emits 2.15 times more CO2 than the U.S. grid.
It comes as no surprise that diesel generators emit more GHGs because the grid energy includes renewables and nuclear, two sources that do not emit GHGs. In fact, depending on your location, grid power may be even cleaner if your energy mix uses less coal or more renewables than the national average.
Offsetting Diesel Generator GHG Emissions
In the example above we determined that our facility’s diesel generators emit 159 Metric Tons of CO2 annually to produce 200 Megawatts-hours of energy. If, instead, this energy was sourced from the grid, only 74 Metric Tons of CO2 would have been emitted. So, our facility emitted 85 Metric Tons of CO2 more by running on diesel generator. What would it take to offset this additional carbon?
To answer this question, we must calculate how much grid energy will emit 85 Metric Tons of CO2. This can be determined by dividing the 85 Metric Tons of CO2 by the 0.37 Metric Tons of CO2 per Megawatt-Hour of grid energy determined above:
Grid Megawatt-Hour Equivalent = 85 Metric Tons of CO2 / 0.37 Metric Tons of CO2 per Megawatt-Hour
Grid Megawatt-Hour Equivalent = 229 Megawatt-Hours
To offset the additional GHG’s emitted by the diesel generators, this facility must consume 229 Megawatt-Hours less of grid energy. This can either be achieved through energy efficiency, simply consuming less energy, or the facility could produce its own green energy like from a PV array. Here in the northeast, a typical 200kW DC rated PV array will produce approximately 250 Megawatt-Hours of energy per year . Your own system size, energy production and requirements will depend on your generator system and specific location but can be easily determined.
Diesel-fueled generators provide reliable emergency power but have a negative environmental impact. In addition to harmful pollutants, they also produce greenhouses gases. A typical diesel generator produces more than twice the amount of GHGs then grid power. Depending on your location the difference could be even bigger. Diesel generators have been and continue to be the most common option for backup power, even as low-carbon and no-carbon alternatives emerge. It’s fair to say diesel generators are here to stay for the foreseeable future and its likely our reliance on them will only increase as grid instability grows. In this article we showed one way to offset the GHG emissions of a typical installation. Other ways include energy storage systems (ESS), natural gas generators, wind turbines, and hydroelectric. Companies will need to consider all options as they work to meet their carbon reduction goals and offset the environmental impact of diesel generators.
Caterpillar Inc. - Standby 2000 ekW 2500 kVA 60 Hz 1800 rpm 12 470 Volts Diesel Engine-Generator Technical Data
United State Environmental Protection Agency - 2022. Greenhouse Gases Equivalencies Calculator - Calculations and References.
U.S. Energy Information Administration, Monthly Energy Review, Table 7.2a, January 2022 and Electric Power Monthly, February 2022
United State Environmental Protection Agency - 2022. Greenhouse Gas Inventory Data Explorer - Electricity generation, entire sector, all gases, by category.
National Energy Renewable Laboratory: The PVWatts (registered trademark) Model