Backup generators are the most overlooked emission source in facility environmental compliance. They sit on a concrete pad behind the building, start automatically during power outages, run for periodic testing, and rarely cross anyone's mind during permitting discussions. Then a state inspector asks for documentation of the engine's classification, maintenance records, and operating hours, and the facility manager discovers that the generator needed a permit five years ago.
The regulatory framework for stationary diesel and natural gas engines is more complex than most people expect. The EPA RICE NESHAP (Reciprocating Internal Combustion Engines National Emission Standards for Hazardous Air Pollutants) applies to virtually every stationary engine in the country, whether it powers a backup generator, a compressor, an irrigation pump, or a fire pump. The requirements depend on the engine's size, fuel type, age, and whether it is classified as emergency or non-emergency. Getting the classification wrong can mean the difference between minimal paperwork and a Title V permit. This guide walks through the RICE NESHAP requirements, explains the emergency vs non-emergency distinction, and covers the Tier emission standards that affect new engine purchases.
RICE NESHAP: What It Is and Who It Covers
The RICE NESHAP (40 CFR Part 63, Subpart ZZZZ) regulates hazardous air pollutant emissions from stationary reciprocating internal combustion engines. It covers compression ignition (diesel) engines, spark ignition (natural gas, gasoline, propane, landfill gas) engines, and dual-fuel engines. The rule applies to engines at major sources of HAPs and area sources of HAPs, which means it covers virtually every facility with a stationary engine, regardless of the facility's overall emission level.
The rule is organized by engine type, size, and location. Existing engines (manufactured before specific dates) and new engines have different requirements. Engines at major sources have more stringent requirements than engines at area sources. Engines above 500 HP have different requirements than engines below 500 HP. Emergency engines have far less stringent requirements than non-emergency engines. The combination of these factors determines which specific provisions apply to your engine.
For most facilities, the RICE NESHAP requirements for emergency engines are manageable: install a non-resettable hour meter, limit operation to emergencies and maintenance/testing (not to exceed 100 hours/year for non-emergency purposes), change oil and filters on schedule, inspect the engine and replace belts and hoses according to the manufacturer's instructions, and maintain records of operation and maintenance. No emission testing is required for emergency engines at area sources.
Non-emergency engines face significantly more stringent requirements, including emission limits that may require catalytic converters, diesel particulate filters, or other controls; periodic emission testing; and more detailed recordkeeping and reporting. The cost difference between emergency and non-emergency compliance can be $20,000-$100,000 or more in controls and testing, which is why getting the emergency classification right is critical.
Emergency CI engine at area source: Maintenance + 100-hr limit + records
Emergency CI engine at major source: Same + initial notification
Non-emergency CI engine (>500 HP, new): Emission limits + testing + controls
Emergency SI engine (gas/propane) at area source: Maintenance + records
Non-emergency SI engine (>500 HP, new): Emission limits + testing + controls
CI = compression ignition (diesel). SI = spark ignition (gas).
Emergency Generator Emissions Calculator
Calculate emissions from emergency and standby diesel generators. Check RICE NESHAP compliance with runtime hour tracking, non-emergency use limits, and annual emissions totals for permit applications.
Emergency vs Non-Emergency: The Classification That Changes Everything
The EPA defines an emergency engine as one that operates only during emergencies (loss of primary power, fire, flood) and for maintenance and testing purposes. The definition is specific and has important limitations. An engine qualifies as emergency only if it is not used for peak shaving (reducing demand charges by running the generator instead of drawing grid power), demand response programs (being paid by the utility to reduce grid load), or as a primary power source. Using a backup generator for any of these purposes, even occasionally, reclassifies it as a non-emergency engine.
The 100-hour rule is the key operational constraint. Emergency engines are allowed to operate up to 100 hours per year for maintenance and testing without losing their emergency classification. This includes monthly load-bank testing, weekly no-load exercising, and any other non-emergency operation. Actual emergency operation (power outages) does not count toward the 100-hour limit. If your generator runs 20 hours per year in actual outages and 80 hours for testing, your total non-emergency hours are 80, which is within the limit.
There is also a provision allowing up to 50 of the 100 hours to be used for non-emergency purposes other than testing, such as demand response to maintain grid reliability. However, this 50-hour allowance comes with significant restrictions and record-keeping requirements. Most facilities find it simpler to stay within the testing-only interpretation and avoid the complexity of the 50-hour provision.
Misclassification is the most common RICE NESHAP violation. A facility that uses its backup generator for peak shaving during summer demand peaks, even for a few hours per month, has a non-emergency engine that requires a different set of controls, testing, and permitting. If an inspector reviews the engine's hour meter and finds it running 300+ hours per year with no corresponding record of power outages, the facility has a classification problem. The cost of coming into compliance with non-emergency requirements retroactively, plus potential penalties for the years of non-compliance, far exceeds the energy savings from peak shaving.
• Peak shaving (running to avoid demand charges)
• Demand response programs (paid load curtailment)
• Serving as primary power for any portion of the facility
• Running to supply power during voluntary shutdowns
• Exceeding 100 hours/year of non-emergency operation
Any of these activities triggers non-emergency classification and its full compliance requirements.
Tier Standards: What They Mean for New Engines
EPA Tier standards set emission limits for new non-road and stationary diesel engines. The standards have progressively tightened through four tiers, each requiring significant reductions in nitrogen oxides (NOx), particulate matter (PM), carbon monoxide (CO), and hydrocarbons (HC). Tier 4 Final, the current standard for new engines, requires reductions of approximately 90% in NOx and PM compared to unregulated engines.
For stationary emergency generators, the applicable Tier standard depends on the engine's manufacture date and horsepower. Engines manufactured today must meet Tier 4 Final emission standards, which typically require diesel particulate filters (DPF) and selective catalytic reduction (SCR) with diesel exhaust fluid (DEF, also called urea). These aftertreatment systems add $10,000-$30,000 to the cost of a generator compared to a Tier 2 or Tier 3 engine, and they require ongoing maintenance (DPF regeneration, DEF refills, catalyst replacement).
Some states allow the purchase of Tier 2 or Tier 3 engines for emergency generator applications through flexibility provisions or state-specific rules. California, however, generally requires the most stringent available standards (CARB certification) and has been tightening emergency engine provisions. Check your state's rules before purchasing a new generator, because the engine Tier determines both the purchase cost and the ongoing maintenance requirements.
For existing generators, the Tier standard that was in effect when the engine was manufactured continues to apply. A Tier 2 engine installed in 2010 does not need to be retrofitted to meet Tier 4 standards. However, as engines age and are replaced, the fleet transitions to newer, cleaner engines. Facilities planning generator replacements should factor in the higher cost of Tier 4 engines and their more complex aftertreatment systems when budgeting for capital equipment replacement.
Tier 1 (1996-2003): NOx 6.9 g/kWh, PM 0.40 g/kWh
Tier 2 (2001-2006): NOx 4.0 g/kWh, PM 0.20 g/kWh
Tier 3 (2006-2008): NOx 3.0 g/kWh, PM 0.20 g/kWh
Tier 4i (2008-2014): NOx 2.0 g/kWh, PM 0.02 g/kWh
Tier 4 Final (2015+): NOx 0.40 g/kWh, PM 0.02 g/kWh
Tier 4 engines typically require DPF + SCR aftertreatment.
Maintenance and Testing Requirements
The RICE NESHAP requires emergency engine owners to follow the manufacturer's recommended maintenance schedule or, at minimum, a set of EPA-specified maintenance practices. For diesel engines, this includes changing oil and filters every 500 hours or annually (whichever comes first), inspecting the air cleaner every 1,000 hours or annually, inspecting all hoses and belts every 500 hours or annually, and checking battery electrolyte levels and connections. Spark ignition engines have similar requirements plus spark plug inspection and replacement.
Maintenance and testing must be documented. The regulations require records of each maintenance action performed, the date, and the hours on the engine at the time. A non-resettable hour meter is required on every engine subject to the RICE NESHAP. If your engine does not have one, install one immediately. The hour meter is the primary tool inspectors use to verify that emergency engines are staying within the 100-hour non-emergency operation limit.
Testing should include both no-load exercising and load testing. Monthly no-load runs (starting the engine and letting it idle for 15-30 minutes) verify that the engine starts and runs. Annual load testing (operating the engine at 75-100% of rated capacity for 1-2 hours using a load bank or actual building load) verifies that the engine can carry its rated load and that the transfer switch functions properly. Load testing is critical because an engine that idles smoothly can still fail under load due to fuel system, cooling system, or exhaust system problems.
Track testing hours carefully. A monthly 30-minute no-load exercise for 12 months is 6 hours. An annual 2-hour load test adds 2 hours. Total: 8 hours of non-emergency operation. Well within the 100-hour limit. But if the generator is also used for outage simulations, emergency drills, commissioning tests for new equipment, or any other purpose, those hours count. Maintain a log that records every start event, the duration, the reason (test, maintenance, emergency), and the cumulative non-emergency hours for the rolling 12-month period.
Record for each engine:
• Equipment ID and location
• Engine make, model, HP, Tier, and manufacture date
• Non-resettable hour meter reading (monthly)
• Date and type of each operation (test, maintenance, emergency)
• Duration of each operation
• Rolling 12-month non-emergency hours total
• Oil change dates and hours
• Filter replacement dates and hours
• Annual inspection date and findings
Permitting: What Your State Actually Requires
Air permit requirements for emergency generators vary significantly by state. In some states, emergency generators below a certain size (typically 200-600 HP) are exempt from permitting by regulation or registered under a general permit that requires minimal paperwork. In other states, every stationary engine above a de minimis horsepower or emission rate needs an individual air permit, even if it is classified as emergency.
The potential to emit (PTE) calculation for a generator typically assumes the engine could operate 8,760 hours per year at full load, producing emissions based on AP-42 or manufacturer emission factors. A 500 kW diesel generator can have a PTE of 20-30 tons/year of NOx and 1-2 tons/year of PM if calculated at maximum hours. This PTE contributes to the facility's total PTE for major source threshold calculations, even though actual emergency operation may be only 10-50 hours per year.
Facilities can limit their generator PTE by accepting federally enforceable hour limits in their permit. An emergency generator limited to 500 hours/year (including both emergency and non-emergency operation) has a PTE roughly one-sixteenth of an unrestricted engine. This can keep the facility below major source thresholds. Most states allow hour-limited permits for emergency generators, and the recordkeeping to demonstrate compliance is straightforward (the non-resettable hour meter).
Notification and registration requirements exist even for exempt engines. Under the RICE NESHAP, owners of engines at area sources are generally not required to submit notifications, but owners of engines at major sources must submit an initial notification within specified timeframes. Many states have their own registration requirements that apply regardless of the federal notification provisions. Check with your state air quality agency to determine whether your emergency generator requires a permit, a registration, a notification, or just recordkeeping. The answer depends on your state, the engine's size and fuel type, and your facility's overall emission status.
Fuel Combustion Emissions Calculator
Calculate CO2, NOx, SOx, and PM emissions from fuel combustion using EPA AP-42 emission factors. Supports natural gas, propane, diesel, fuel oil, and coal with annual emissions totals and cost-per-ton estimates.
Diesel Fuel and Sulfur: The Other Emission Factor
Sulfur content in diesel fuel directly affects SO2 emissions and can impact aftertreatment system performance. Ultra-low sulfur diesel (ULSD, 15 ppm sulfur) is required for on-road vehicles and is now the standard fuel available at most suppliers. However, some stationary engines, particularly older ones, may still have tanks containing higher-sulfur fuel. Tier 4 engines with SCR and DPF aftertreatment systems require ULSD; higher-sulfur fuel will damage the catalysts and void the engine warranty.
SO2 emissions scale directly with fuel sulfur content. ULSD at 15 ppm produces negligible SO2. Older low-sulfur diesel (500 ppm, no longer commonly available) produces about 0.001 lbs SO2 per gallon. High-sulfur diesel (5,000 ppm, still found in some heating oil applications) produces about 0.01 lbs SO2 per gallon. For facilities tracking SO2 emissions for permit compliance, the fuel sulfur content is a critical input to the emission calculation.
Particulate matter emissions also depend on fuel quality and engine maintenance. Black smoke from a diesel generator indicates incomplete combustion caused by overloading, restricted air intake, worn injectors, or improper fuel timing. Visible emissions are both a regulatory violation (most permits prohibit visible emissions beyond brief startup periods) and a sign that the engine needs maintenance. A well-maintained diesel engine running at proper load on ULSD should produce no visible exhaust under steady-state operation.
Fuel storage and handling matter for compliance. Diesel fuel degrades over time, especially in warm climates. Water accumulation in tanks promotes microbial growth that clogs filters and injectors. Fuel that has been sitting in a tank for more than 12 months should be tested for quality before relying on it for emergency power. A generator that fails to start during an actual emergency because of degraded fuel defeats the purpose of having backup power, and the resulting sewage overflow, production loss, or safety hazard is a far greater cost than the fuel maintenance program that would have prevented it.
• Use ULSD (15 ppm sulfur) for all engines, especially Tier 4
• Test stored fuel annually for water, microbial contamination, and degradation
• Treat fuel with biocide and stabilizer if storage exceeds 6 months
• Drain water separators monthly
• Replace fuel filters annually regardless of hours
• Rotate fuel stock by using oldest fuel first during testing
Diesel Fuel Sulfur & Emissions Calculator
Calculate SOx emissions from diesel fuel based on sulfur content. Compare ULSD, off-road, and heating oil grades. See annual sulfur dioxide emissions and fleet-level environmental impact.