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Generator Emissions Calculator - RICE NESHAP Compliance, 100-Hour Rule & Tier Emissions

Calculate emergency and non-emergency generator emissions with EPA tier standards and RICE NESHAP hour limits

Calculate air emissions from stationary diesel and natural gas generators with EPA RICE NESHAP compliance tracking. Enter generator size, fuel type, EPA tier rating, and annual operating hours to get emissions of NOx, CO, PM, VOC, SOx, and CO2 in tons per year. Includes emergency vs. non-emergency classification, the 100-hour maintenance and testing rule, 50-hour demand response limit, and emission tier factor comparison from Tier 1 through Tier 4. Flags when generator usage approaches limits that would reclassify the unit from emergency to non-emergency status.

Pro Tip: The 100-hour rule for emergency generators is widely misunderstood. You get 100 hours per year for maintenance and testing, plus unlimited hours during actual emergencies (grid outages, natural disasters). But if you run the generator for non-emergency purposes like demand response or peak shaving beyond 50 hours, the unit is reclassified as non-emergency and must meet much stricter emission standards and add-on controls. One extra hour of demand response can trigger $50,000+ in control requirements.

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Emergency Generator Emissions Calculator
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How It Works

  1. Enter Generator Specifications

    Input generator rated output in kW or HP, fuel type (diesel or natural gas), and EPA emission tier (Tier 1 through Tier 4). Tier rating determines the applicable emission factors for NOx, CO, PM, and VOC.

  2. Classify Emergency vs. Non-Emergency

    Select whether the generator operates as emergency-only (100-hour testing limit) or non-emergency (unlimited hours but stricter standards). The calculator applies the correct RICE NESHAP requirements for each classification.

  3. Enter Operating Hours

    Input annual hours for maintenance/testing, demand response, and emergency operation separately. The calculator tracks each category against RICE NESHAP limits and flags potential reclassification triggers.

  4. Add Fuel Details

    Specify diesel sulfur content (ULSD 15 ppm is required for Tier 4 engines) and fuel consumption rate in gallons per hour. If unknown, the calculator estimates fuel use from rated output and load factor.

  5. Review Emissions and Compliance

    See annual emissions by pollutant in tons/year, RICE NESHAP compliance status, hour tracking with warning indicators, and comparison of your tier vs. the current regulatory baseline.

  6. Evaluate Tier Upgrade Impact

    Compare emissions from your current tier engine against a Tier 4 replacement to see emission reductions and determine if an upgrade changes your major source status.

Built For

  • Facility managers tracking emergency generator runtime against the RICE NESHAP 100-hour limit
  • Environmental staff including generator emissions in annual emission inventories
  • Building engineers evaluating whether backup generators trigger major source permit requirements
  • Data center operators managing multiple generators for Tier V or Title V compliance
  • Hospital facility managers documenting generator compliance for Joint Commission and EPA
  • Industrial plants evaluating the emission cost of demand response participation
  • Consultants preparing RICE NESHAP compliance documentation for client facilities

Frequently Asked Questions

RICE NESHAP (Reciprocating Internal Combustion Engine National Emission Standards for Hazardous Air Pollutants) is the EPA regulation under 40 CFR Part 63, Subpart ZZZZ that sets emission standards for stationary diesel and gas engines. It covers both major and area sources and establishes different requirements based on engine type (compression ignition vs. spark ignition), size, use (emergency vs. non-emergency), and whether the source is major or area for HAPs.
Emergency generators may operate up to 100 hours per year for maintenance and testing without being reclassified as non-emergency units. Emergency operation during actual power outages does not count toward this limit. An additional 50 hours per year is allowed for demand response if the unit meets certain conditions. Exceeding these limits reclassifies the engine as non-emergency, triggering stricter emission standards including possible add-on controls.
EPA established increasingly stringent emission standards in tiers: Tier 1 (1996-2003), Tier 2 (2001-2006), Tier 3 (2006-2008), and Tier 4 (2008-2015, depending on engine size). Tier 4 requires 90%+ reduction in PM and NOx compared to Tier 1, typically requiring diesel particulate filters and SCR or EGR aftertreatment. Older pre-Tier 1 engines have no emission controls and produce significantly more pollution per kW.
Emergency generators' potential to emit (PTE) is calculated at 500 hours/year per EPA guidance (not 8760 hours). A 2000 kW Tier 2 diesel generator at 500 hours may contribute 5-15 tons/year of NOx to your facility total. Facilities with multiple large generators can approach or exceed the 100 tpy major source threshold for NOx from generators alone, especially if other combustion sources are present.
Yes. ULSD (15 ppm sulfur) is required for Tier 4 engines because high sulfur poisons the catalytic aftertreatment systems (DPF and SCR) that Tier 4 engines use to meet stringent PM and NOx limits. Using higher-sulfur fuel in a Tier 4 engine will damage the aftertreatment system and cause the engine to exceed emission limits. Even for older-tier engines, ULSD reduces SO2 emissions by 97% compared to 500 ppm diesel.
RICE NESHAP requires maintaining records of: all hours of operation (with reason codes for emergency vs. maintenance/testing vs. demand response), maintenance activities performed, fuel sulfur content documentation, and any emission-related malfunctions. Records must be kept for 5 years and made available to the EPA or state agency upon request. Many facilities use automatic hour meters and runtime logs to ensure accurate tracking.
Peak shaving (running a generator to reduce utility demand charges) reclassifies the engine as non-emergency regardless of hours. Even one hour of peak shaving triggers non-emergency requirements. Demand response through a utility-operated program is allowed up to 50 hours/year under specific conditions, but voluntary peak shaving for cost savings is not considered demand response by the EPA.
Disclaimer: This calculator provides emission estimates based on EPA emission tier factors and RICE NESHAP requirements. Actual emissions depend on engine condition, load factor, fuel quality, altitude, and ambient temperature. RICE NESHAP compliance requirements vary by engine type, size, location, and use classification. Always verify compliance obligations with your state air quality agency or a qualified environmental professional. ToolGrit does not provide regulatory compliance advice.

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