What will run on the site, and how often?
Ask what engines or turbines will run, for how many hours, and under which permit.
Backup engines and power plants at a site near you can give off air pollutants. Look past the label “backup.” The fuel, the equipment, the pollution controls and the running hours matter more.
A backup generator can run for a test, a power cut or a grid emergency. The hour rules differ.
Test: the engine runs to show it is ready. The federal rule caps these hours each year.
Power cut: the grid supply fails and the engine keeps the site running. The federal rule sets no time limit for a real emergency.
In a grid emergency, 2026 federal orders had grid operators call on backup generators as a last resort. EPA said no hour limit applies then.
A state or local permit can set stricter hours. The site’s own records show the hours each engine really ran.
Sources: Electronic Code of Federal Regulations (eCFR), 40 CFR 60.4211(f), Standards of Performance for Stationary Compression Ignition Internal Combustion Engines, accessed November 11, 2026; US Environmental Protection Agency, Clean Air Act Resources for Data Centers, October 1, 2026.
What good looks like
- The equipment list, the allowed uses and the pollution controls are public.
- Where required, the air study looks at people nearby and other sources of pollution.
- Running sites report generator hours. They also report emissions or compliance results.
What to watch for
- Power made on a regular basis is called emergency backup.
- A new or larger set of equipment is missing from the records cited.
- A permit’s upper limit is reported as if it were actual emissions.
Where to look
- Air-agency permit and equipment inventory The air permit, and the list of equipment at the site.
- Air-quality analysis The study of the project’s effect on the air nearby.
- Operating logs, emissions reports and inspection findings Records of when equipment ran, reports of what it released, and what inspectors found.
The federal rule caps testing of an emergency engine at 100 hours a year. It sets no time limit for a real emergency.
One year has 8,760 hours (365 days x 24 hours)
Testing and upkeep allowed
Up to 100 hours
Running that is not an emergency
Up to 50 hours
These hours count as part of the 100. They are not added on top.
Running in a real emergency
No time limit in the rule
Adopted requirement
Source: Electronic Code of Federal Regulations (eCFR), 40 CFR 60.4211(f), Standards of Performance for Stationary Compression Ignition Internal Combustion Engines, accessed November 11, 2026.
Air permits set a ceiling. In Virginia in 2023, data centers emitted 7% of what their permits allowed.
What the permits allowed
The permit ceiling
What was emitted
7%
The report says most of these emissions came from maintenance testing.
Measured result
Source: Virginia Joint Legislative Audit and Review Commission (JLARC), Data Centers in Virginia (Report 598), December 9, 2024.
The trade-off
Backup equipment helps keep a site running during power outages. Its local effects still depend on how it is run and controlled.
Is a final air permit public for the equipment you are checking?
- Yes
- You have the issued permit, and it matches the named equipment and project phase. It does not prove zero emissions. It does not prove the rules are being followed.
- No
- The agency’s record directly shows one of three things. The relevant permit has not been issued, has been denied, or is no longer valid.
- Not known
- You have an application, a press statement, a record you could not open, or equipment not matched to a permit.
Two real projects
Lighthouse — Port Washington
Port Washington, Wisconsin
Is a final air permit for this project public in the listed records?
YesAdopted requirement
The Wisconsin Department of Natural Resources (DNR) lists permits for 45 diesel emergency generators. They are final construction permit 25-MIN-094 and operation permit 24618528A-F01. Both were issued on June 12, 2026. These are air permits, which set the pollution rules a site must follow. Having them does not mean zero emissions.
Source: Wisconsin DNR, Port Washington proposal and issued permits, June 12, 2026.
Microsoft San José Data Center 04
San José, California
Is a final air permit for this project public in the listed records?
Not knownOfficial record
The CEC page describes proposed diesel backup equipment. A state small power plant exemption would not replace the air district’s permit. The air district is the local agency that manages air quality. Microsoft’s July 2025 filing says each engine may run no more than 50 hours a year for testing and maintenance. It says this limit comes from the air district’s permit to build (its “Authority to Construct”) and a California Air Resources Board rule. The filing does not include an Authority to Construct or say that one was issued. The listed records do not include a final air permit. An air permit is the document that sets a site’s air pollution rules.
Sources: California Energy Commission, San José Data Center 04 project record, accessed November 11, 2026; Microsoft applicant representative / CEC docket, Fourth revised project description, TN 264764, July 14, 2025.