Title: Data Center Generators: Backup Power and Air Permits | FOUND Description: Why data centers keep diesel generators, how often federal rules let them run, how North Carolina permits them, and which emissions questions stay open. Canonical: https://lotsfound.com/insights/data-center-generators/ # Data center generators: backup power and air permits Data centers keep generators to carry the load when grid power fails. Federal rules let an emergency engine run without limit in an outage and up to 100 hours a year for testing and other use. North Carolina air permits cap the emissions, but exposure near homes comes from models, not monitors. [See how we check power](/what-we-check/power-and-substations/) In this article 1. [Why data centers have generators](#why-data-centers-have-generators) 2. [How often data center generators run](#how-often-data-center-generators-run) 3. [Bridge power: generators that run before the grid arrives](#bridge-power-generators-that-run-before-the-grid-arrives) 4. [How North Carolina permits data center generators](#how-north-carolina-permits-data-center-generators) 5. [What comes out of a diesel generator](#what-comes-out-of-a-diesel-generator) 6. [What emissions questions remain](#what-emissions-questions-remain) 7. [What it means for a site](#what-it-means-for-a-site) 8. [Key recap](#key-recap) 9. [Questions](#questions) 10. [References](#references) > **Quick summary:** This guide explains why data centers keep backup generators, how often the rules let them run, and how North Carolina permits them. It is for residents, landowners, and local officials who read a data-center proposal. The key point: a generator permit caps emissions on paper, and the exposure at nearby homes comes from modeling, not from monitors. ## Why data centers have generators A data center keeps generators so its computers keep running when the grid fails. A [federal design guide for data centers](https://www.energy.gov/sites/default/files/2024-07/best-practice-guide-data-center-design_0.pdf) lists a backup generator as a standard part of the power path. The same guide describes uninterruptible power supply systems, often battery banks, as a second layer of backup power. Diesel is the common fuel. A [Virginia legislative study](https://jlarc.virginia.gov/pdfs/reports/Rpt598.pdf) found that nearly all data centers in that state use diesel generators for backup power. It counted an average of 54 permitted generators per site, typically one to three megawatts each. Virginia had about 8,000 permitted data-center generators in total. Operators plan for failure in layers, and generators are the last layer. [Data center redundancy: what it means and what it asks of land](/insights/data-center-redundancy/) explains why. ## How often data center generators run Federal rules let an emergency engine run without a time limit during an emergency. Outside emergencies, the [federal standard for diesel engines](https://www.ecfr.gov/current/title-40/section-60.4211) and the [federal air-toxics rule for engines](https://www.ecfr.gov/current/title-40/section-63.6640) allow up to 100 hours a year for maintenance checks and readiness testing. Up to 50 of those hours can go to other non-emergency use. The rules bar that time from peak shaving or selling power, with narrow exceptions for local grid emergencies. An engine that runs outside these limits loses its emergency status and must meet the rules for non-emergency engines. Actual use in Virginia ran well under those limits. Operators told the Virginia study that testing usually means a monthly test of 10 to 30 minutes and one annual test of one to four hours. Most reported zero to two minor outages per site in two years, nearly all one to five hours long. In 2023, the industry's actual generator emissions in Virginia were 7 percent of what its permits allowed. These are Virginia figures. A North Carolina campus keeps its own run records under its own air permit. ## Bridge power: generators that run before the grid arrives Some projects run engines every day until the utility connection is ready. In August 2026, the [state air agency issued two permits](https://content.govdelivery.com/accounts/NCDEQ/bulletins/427509e) for one campus in Richmond County. - **Emergency engines.** The operator's permit allows 588 emergency backup engines. They run during outages and for limited periods of maintenance, testing, and activation. - **Bridge-power engines.** A utility's separate permit allows 57 temporary non-emergency engines. They supply the campus until it connects to the grid. The utility must retire them within a year of starting operations. The agency states that the two sets of engines will not run at the same time under normal operation. A bridge-power arrangement still changes the question. Emergency engines sit idle most of the year. Bridge engines carry the load every day until the grid connection arrives. [Behind-the-meter data centers: on-site power and what it asks of land](/insights/behind-the-meter-data-center/) covers on-site power in more depth. ## How North Carolina permits data center generators The North Carolina Department of Environmental Quality's Division of Air Quality decides whether a facility needs an air permit. Its [exemption rules](https://www.deq.nc.gov/about/divisions/air-quality/air-quality-planning/air-quality-rules-regulations/permit-exemptions-registration) exempt a facility whose actual emissions stay under five tons a year for each listed pollutant and under 10 tons in total. The facility must also meet the other conditions in the rule. A facility with total actual emissions from 5 to under 25 tons a year can register instead of holding a permit. An exempt or registered facility must still meet every state and federal rule that applies to it. Large generator fleets go through a full permit. The Richmond County permits show the steps. 1. The agency held a joint public hearing and a 30-day comment period on the draft permits. 2. It analyzed both projects under worst-case operating scenarios and found that compliance with applicable standards can be achieved. 3. Its dispersion modeling found that toxic air pollutant emissions are not expected to violate health-based air quality levels. 4. After public comment, it added fuel-use monitoring, more initial engine testing, and more recordkeeping and reporting. The agency also states the limit of its role. Its air permit review does not cover siting, land use, or electricity supply. Other agencies and the local government decide those questions. > **The short version:** An emergency generator runs in outages plus no more than 100 hours a year for testing and limited other use. A bridge-power engine is not an emergency engine. Find out which kind a project proposes before you weigh its air permit. ## What comes out of a diesel generator Diesel generators emit nitrogen oxides, carbon monoxide, and particulate matter, according to the Virginia study. Nitrogen oxides are the largest share, and they can contribute to ground-level ozone. Federal standards set emission limits for engines, and a site permit can cap the total a site emits each year. The World Health Organization's cancer agency [classifies diesel engine exhaust as carcinogenic to humans](https://www.iarc.who.int/wp-content/uploads/2018/07/pr213_E.pdf), based on evidence of lung cancer. That is a hazard rating. Whether exhaust from a specific campus reaches nearby homes at a harmful level is an exposure question. [Are data centers dangerous to live near?](/insights/are-data-centers-dangerous/) separates the two for each concern. Engine type matters too. The Virginia study reports that most data centers there use "Tier 2" engines. It states that "Tier 4" engines are designed to emit significantly less nitrogen oxides and particulate matter. Add-on catalytic controls can also reduce nitrogen oxides. Ask which tier and which controls a proposal names. ## What emissions questions remain The open questions concern local exposure, not whether generators emit. - **Modeled, not measured.** The Richmond County finding rests on models of worst-case operation. A model predicts compliance. It does not measure the air at a nearby home. - **Clusters are new.** The Virginia study found that no other kind of development uses nearly as many generators on one site. It reported that intermittent use makes the effect hard to model and that regional monitors do not capture localized effects. Virginia's air agency started a three-year study to monitor data-center generator emissions directly. - **Long outages.** The Virginia study found that, in a long regional outage, campuses could reach their annual emission caps within a few days. - **Fuel storage.** A campus that stores diesel above ground can fall under [federal spill-prevention rules](https://www.ecfr.gov/current/title-40/section-112.1). Those rules exempt a site whose aboveground oil storage totals 1,320 gallons or less. > **Take action:** If a buyer asks about your land, or a campus is proposed next to it, read the generator count and engine type in the air permit before the hearing. Start with [Is your land a data-center site?](/who-we-serve/landowners/) ## What it means for a site For a site, generators are a distance question first. Generator yards make noise during tests and emit exhaust when they run. The distance from the generator yard to the nearest home shapes both. [Data-center noise, and why it now decides sites](/insights/data-center-noise/) covers sound, and [Are data centers bad? The objections, checked against the record](/insights/data-center-misconceptions/) places generators among the other objections. The public record shows the air permit, the permit review, the hearing record, and the permit conditions. It cannot show how many hours the engines will actually run or what a neighbor will breathe. The air agency, the permit's compliance reports, and the operator answer those questions. ## Key recap - Data centers keep generators, usually diesel, to run through grid outages. Battery systems add a second layer of backup power. - Federal rules allow an emergency engine unlimited emergency use and up to 100 hours a year for testing and limited other use. - Bridge-power engines run before the grid connection and are not emergency engines. One North Carolina permit requires their retirement within a year of starting operations. - North Carolina's air agency exempts, registers, or permits a facility by its actual emissions. Large fleets go through a permit with public comment. - Permit findings rest on models. We found no measured exposure data near a North Carolina campus in the public record. ## Questions ### How long can a data center run its generators? A data center can run an emergency generator without a time limit during an emergency. Outside emergencies, federal rules cap an emergency engine at 100 hours a year for maintenance, testing, and limited other use. A site's air permit can set tighter limits. ### Are data center generators diesel? Most are. A Virginia legislative study found that nearly all data centers in that state use diesel generators for backup power. A proposal's air permit application names the engines and the fuel. ### Does an air permit mean the generators are safe? An air permit means the agency found that modeled emissions can meet the applicable standards, and it sets conditions. It does not measure the air at a nearby home. Ask the air agency what monitoring and reporting the permit requires. ## References Primary sources cited on this page, in APA style. - 40 C.F.R. § 112.1 (2026). [https://www.ecfr.gov/current/title-40/section-112.1](https://www.ecfr.gov/current/title-40/section-112.1) - 40 C.F.R. § 60.4211 (2026). [https://www.ecfr.gov/current/title-40/section-60.4211](https://www.ecfr.gov/current/title-40/section-60.4211) - 40 C.F.R. § 63.6640 (2026). [https://www.ecfr.gov/current/title-40/section-63.6640](https://www.ecfr.gov/current/title-40/section-63.6640) - International Agency for Research on Cancer. (2012, June 12). *IARC: Diesel engine exhaust carcinogenic* (Press release No. 213) [Press release]. [https://www.iarc.who.int/wp-content/uploads/2018/07/pr213_E.pdf](https://www.iarc.who.int/wp-content/uploads/2018/07/pr213_E.pdf) - Joint Legislative Audit and Review Commission. (2024, December 9). *Data centers in Virginia* (Report 598). Commonwealth of Virginia. [https://jlarc.virginia.gov/pdfs/reports/Rpt598.pdf](https://jlarc.virginia.gov/pdfs/reports/Rpt598.pdf) - North Carolina Department of Environmental Quality. (2026, August 28). *NCDEQ's Division of Air Quality requires additional monitoring, testing in final air quality permits for Amazon data center and Duke Energy projects* [Press release]. [https://content.govdelivery.com/accounts/NCDEQ/bulletins/427509e](https://content.govdelivery.com/accounts/NCDEQ/bulletins/427509e) - North Carolina Department of Environmental Quality. (n.d.). *Permit exemptions and registration*. Retrieved October 2, 2026, from [https://www.deq.nc.gov/about/divisions/air-quality/air-quality-planning/air-quality-rules-regulations/permit-exemptions-registration](https://www.deq.nc.gov/about/divisions/air-quality/air-quality-planning/air-quality-rules-regulations/permit-exemptions-registration) - Van Geet, O., & Sickinger, D. (2024, July). *Best practices guide for energy-efficient data center design* (DOE/GO-102024-6283). U.S. Department of Energy, Federal Energy Management Program. [https://www.energy.gov/sites/default/files/2024-07/best-practice-guide-data-center-design_0.pdf](https://www.energy.gov/sites/default/files/2024-07/best-practice-guide-data-center-design_0.pdf) ## Power for data-center sites - [Power decides the site. The map only starts the question.](/what-we-check/power-and-substations/) - [A substation next door is not power](/insights/power-proximity-is-not-capacity/) - [Data center power requirements: what a campus needs from the grid](/insights/data-center-power-requirements/) - [Electric cooperative vs investor-owned utility: what changes for a site](/insights/electric-cooperative-vs-investor-owned-utility/) - [What a will-serve letter is, and what it is worth](/insights/what-is-a-will-serve-letter/) - [What a substation is and what it tells you about land](/insights/what-is-a-substation/) - [Utility easements: what they allow and how they shrink usable land](/insights/utility-easement/) - [Data center redundancy: what it means and what it asks of land](/insights/data-center-redundancy/) - [Large load interconnection: how a very large customer gets power](/insights/large-load-interconnection/) - [Behind-the-meter data centers: on-site power and what it asks of land](/insights/behind-the-meter-data-center/) - [How much energy does AI use?](/insights/how-much-energy-does-ai-use/) - [How much electricity do data centers use?](/insights/data-center-energy-consumption/) - [Do data centers raise electric bills in North Carolina?](/insights/do-data-centers-raise-electric-bills/) - [Why are so many data centers being built?](/insights/why-are-so-many-data-centers-being-built/) Send the requirements. 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