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Water, power and backup generators, explained
Power, water and backup generators are where most of the technical questions about a data center land. This guide explains what each one involves, gives the national figures published by federal sources, and points to the Iowa and federal rules that apply. It doesn't estimate the effects of any particular project, which depend on its size, design and location.
Power
How much electricity data centers use
According to a December 2024 report by Lawrence Berkeley National Laboratory for the U.S. Department of Energy, data centers used about 4.4 percent of U.S. electricity in 2023, or about 176 terawatt-hours, up from 58 terawatt-hours in 2014. The report projects 6.7 to 12 percent by 2028, or roughly 325 to 580 terawatt-hours, depending on how fast demand grows.
Units that come up
- Megawatt (MW) measures demand at a point in time. One megawatt is 1,000 kilowatts.
- Megawatt-hour (MWh) measures energy used over time. A 1 MW load running for one hour uses 1 MWh.
- Terawatt-hour (TWh) is one million megawatt-hours, used for national totals.
Efficiency: PUE
Power usage effectiveness (PUE) is the facility's total electricity use divided by the electricity used by the IT equipment itself. A PUE of 1.0 would mean every unit of power goes to computing. The amount above 1.0 is the share used for cooling, power conversion, lighting and other building systems.
On-site power plants
Iowa Code chapter 476A generally requires a certificate from the Iowa Utilities Commission before construction of an electric generating facility with a total capacity of 25 megawatts or more at a single site (§§ 476A.1 and 476A.2). The chapter has exceptions and definitions that matter. Whether it applies to a given project's equipment is a question for counsel and the commission.
Water
Why data centers use water
Most water use comes from cooling. Evaporative systems, such as cooling towers, remove heat by evaporating water. According to a January 2026 fact sheet by the Environmental Law Institute, roughly 80 percent of the water drawn by evaporative cooling is lost to evaporation and about 20 percent is discharged. The same fact sheet describes air cooling, direct-to-chip liquid cooling and immersion cooling as using little or no water on site.
How much water
Figures from the Lawrence Berkeley National Laboratory report, as summarized by the Environmental Law Institute:
- U.S. data centers directly consumed about 21.2 billion liters of water in 2014 (about 5.6 billion gallons).
- Direct consumption was about 66 billion liters in 2023 (about 17.4 billion gallons).
- Generating the electricity data centers used in 2023 consumed about 800 billion liters more (about 211 billion gallons). This is often called indirect water use.
Gallon figures are converted from liters at 3.785 liters per gallon.
Efficiency: WUE
Water usage effectiveness (WUE) is water consumed divided by the electricity used by IT equipment, in liters per kilowatt-hour. Site WUE counts water used at the facility. Source WUE also counts water used to generate the facility's electricity.
Iowa water use permits
Under Iowa Code § 455B.268, a permit from the Department of Natural Resources is required to divert, store or withdraw water from any surface or groundwater source, except for a nonregulated use. Section 455B.261 defines a nonregulated use as any beneficial use of less than 25,000 gallons per day. Whether a project withdraws water itself or buys it from a utility affects which permits are involved, so it's worth asking.
Backup generators
What they're for
Data centers typically keep generators on site to carry the load if utility power fails. Batteries in an uninterruptible power supply (UPS) cover the short gap until the generators start.
Federal limits on testing
For diesel (compression ignition) emergency engines covered by federal performance standards, 40 CFR 60.4211(f) allows operation for maintenance checks and readiness testing up to 100 hours per calendar year. Up to 50 of those hours may be used in certain non-emergency situations. Operation during an actual emergency isn't limited by those hours. Other operation outside what the rule allows means the engine is no longer treated as an emergency engine under that subpart.
Iowa air permits
Under 567 Iowa Administrative Code 22.1(1), no one may construct, install or alter equipment that emits air pollutants without first obtaining a permit from the Department of Natural Resources, unless an exemption applies. The exemption for small engines in 22.1(2)"r" covers internal combustion engines under 400 brake horsepower, with conditions. Engine size and count on the application tell you which rules to look at.
Questions worth asking
- What is the expected demand in megawatts at opening and at full build-out, and what is the design PUE?
- Has the utility confirmed it can serve the load, and who pays for new substations or lines?
- What cooling method is planned, and what is the expected water use, average and peak, in gallons per day?
- Where does the water come from, and which DNR permits are required?
- How many generators, of what size and fuel, and which air permits apply?
- When and how often will generators be tested, and will the hours be reported?
About these figures. National totals describe the industry as a whole and don't predict any single facility. Ask each applicant for project-specific figures, and check whether each one is a firm commitment or an estimate. This guide isn't legal advice. For permit questions, consult the relevant agency and your attorney.
Sources
- U.S. Department of Energy, "DOE Releases New Report Evaluating Increase in Electricity Demand from Data Centers," December 20, 2024: https://www.energy.gov/articles/doe-releases-new-report-evaluating-increase-electricity-demand-data-centers
- Shehabi, A., et al., 2024 United States Data Center Energy Usage Report, Lawrence Berkeley National Laboratory, December 2024: https://eta-publications.lbl.gov/sites/default/files/2024-12/lbnl-2024-united-states-data-center-energy-usage-report_1.pdf
- Environmental Law Institute, Data Centers and Water fact sheet, January 2026 (secondary source summarizing the LBNL report): https://www.eli.org/sites/default/files/files-pdf/Data%20Centers%20and%20Water%20Fact%20Sheet%20ELI%20January%202026%20(1).pdf
- 40 CFR 60.4211, What are my compliance requirements if I am an owner or operator of a stationary CI internal combustion engine?: https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-60/subpart-IIII/section-60.4211
- 567 Iowa Administrative Code chapter 22, Controlling Air Pollution: https://www.legis.iowa.gov/docs/iac/chapter/03-19-2025.567.22.pdf
- Iowa Code § 455B.261 (definitions, including nonregulated use): https://www.legis.iowa.gov/docs/code/455B.261.pdf
- Iowa Code § 455B.268, When permit required: https://www.legis.iowa.gov/docs/code/455B.268.pdf
- Iowa Code § 476A.1 (definitions, including facility): https://www.legis.iowa.gov/docs/code/476A.1.pdf
- Iowa Code § 476A.2 (certificate required): https://www.legis.iowa.gov/docs/code/476A.2.pdf