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Compare proposals on the same facility boundary, annual period, IT-load profile, weather assumptions, and measurement method. Require absolute annual energy and water totals in addition to PUE and WUE; separate potable water from other sources; document cooling, climate, renewable-energy accounting, and heat reuse. A low efficiency ratio is not evidence of low total resource demand unless those conditions match.
Start with a common comparison boundary
Most misleading comparisons begin with different definitions of what is being measured. Put a written data request in the tender and require every bidder to answer it for the same boundary and 12-month period.
Define the facility boundary
State whether the boundary includes the data halls, cooling plant, pumps, lighting, offices, shared building services, fuel used by generators, and losses between the utility meter and the IT equipment. The EU reporting method measures total data-center energy and IT energy at specified locations, so a bidder should identify each meter and any loads excluded from the calculation.
- Facility energy: all energy entering the data-center boundary, including electricity, fuels, and cooling-related energy. Ask for backup-generator energy separately rather than allowing it to disappear in an aggregate figure.
- IT energy: energy measured at the agreed IT-equipment boundary, with the meter location, interval, and method stated.
- Water boundary: all water entering the facility boundary, plus a separate potable-water total. Identify whether cooling-tower makeup, blowdown, humidification, domestic services, irrigation, and shared-building uses are included.
- Mixed-use buildings: require the provider to attribute only data-center-related equipment and operations to the data-center figures. Shared services must be allocated by a documented method.
Fix the annual period and load scenario
Ask for a complete operating year, not a single design-day estimate. Require the same reporting year or a normalized weather year for every bidder, and show results by month or quarter so seasonal peaks are visible. The request should specify expected IT load, rack density, utilization, growth, redundancy mode, and whether the result represents design capacity, contracted capacity, or expected operating load.
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Have bidders provide at least two cases when the workload is uncertain: the contracted IT load and a realistic ramp-up or utilization case. Label every number as measured, modeled, or estimated. A modeled annual figure based on 100% utilization is not directly comparable with a measured figure from a partially occupied facility.
Use PUE and WUE without mistaking them for demand
PUE: a relationship between two energy totals
Power Usage Effectiveness is calculated as:
PUE = total data-center energy (EDC) ÷ IT-equipment energy (EIT)
A lower PUE means less non-IT energy per unit of IT energy under that definition. It does not tell you the facility’s annual energy consumption. A large site with a low PUE can use more electricity than a smaller site with a higher PUE. Request both annual EDC and annual EIT, the resulting PUE, the measurement points, and the calculation interval.
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Water Usage Effectiveness divides water input by IT energy, but the result depends on the boundary. Under the EU method, WUE uses total water input (WIN) divided by IT energy in megawatt-hours, and the reporting category must be stated. The U.S. Department of Energy’s 2024 guide describes a site-based WUE in litres per kilowatt-hour: “The site-based WUE metric is defined as the ratio of the annual site water usage to the total energy drawn by all IT equipment.” Those units and boundaries are not interchangeable.
Ask for annual litres as well as the ratio. Two proposals can show the same WUE while having very different IT loads and therefore very different water withdrawals.
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Separate potable and non-potable water
Request annual potable-water input as its own field, with source and meter identified. Also list reclaimed, rain, surface, or other non-potable sources and any treatment losses. A proposal that advertises low WUE but relies on a constrained municipal potable supply may create a greater local risk than one using more water from an available non-potable source.
Do not mix site-based and source-based WUE
The DOE guide distinguishes site-based WUE from a source-based variant that also counts water consumed off site to produce the energy used on site. A source-based value will generally have a broader boundary. Never rank it directly against a site-only number; show the category, units, energy source assumptions, and included off-site processes beside every value.
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Use one spreadsheet or form for every proposal. The following fields make the answers auditable and comparable.
| Area | Require from every bidder | Why it matters |
|---|---|---|
| Facility energy | Annual total energy at the stated boundary; electricity and fuels by source; cooling energy; backup-generator energy separately | Prevents exclusions and makes fuel use visible. |
| IT energy and PUE | Annual IT energy, meter location, interval, method, excluded loads, and PUE calculation | Allows the ratio to be reproduced. |
| Water | Annual total water, potable water, other sources, units, meters, treatment losses, and WUE category | Prevents a broad water boundary being compared with a narrow one. |
| Cooling | System type, tower or evaporative processes, makeup and blowdown volumes, setpoints, economizer operation, and seasonal assumptions | Cooling choices drive both energy and water. |
| Local conditions | Site, climate data set, cooling-degree or equivalent weather basis, water restrictions, and utility conditions | Weather and local availability change the result and its risk. |
| Energy sourcing | Annual renewable energy, on-site generation, contractual instruments, and accounting method | Separates physical consumption from procurement claims. |
| Heat reuse | Quantity delivered beyond the facility boundary, recipient, temperature, operating months, and substituted energy | Shows whether reuse is real and operational rather than a design intention. |
| Evidence quality | Meter diagrams, reporting year, estimates, missing loads, calibration, independent verification, and data-completeness statement | Indicates how much confidence to place in the figures. |
Examine the cooling design and operating assumptions
Air management and setpoints
Ask for supply-air temperature, humidity-control range, chilled-water temperature, rack inlet limits, and control tolerances. Hot-aisle and cold-aisle separation, blanking panels, containment, and reduced mixing can permit higher chilled-water temperatures and lower airflow. The DOE guide cites a potential 20% reduction in chiller energy for the combination of effective air management and higher chilled-water temperatures. Treat that as a context-specific opportunity, not a guaranteed saving for a particular proposal; request the modeled conditions and measurement plan behind any claimed reduction.
Raising temperature setpoints or widening humidity limits can increase air-side economizer hours, but savings depend on the actual outdoor temperature and humidity and must remain within the equipment manufacturer’s allowable envelope. Require bidders to state the control range used in their annual model rather than simply describing the design as “free cooling.”
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Air-side and water-side economizing
Air-side economizing uses suitable outside air instead of mechanical cooling. Water-side economizing can reduce chiller energy, but its water impact depends on the configuration and on whether cooling towers operate during those hours. Towers consume water through evaporation and blowdown. Ask for tower cycles of concentration, annual makeup water, blowdown, drift controls, and the weather conditions that trigger tower operation.
Challenge “water-free” language
No cooling approach should be called water-free without a stated boundary. A system may avoid on-site tower water while embedding water consumption in electricity production, using water for humidification, or relying on a neighboring shared plant. Require both site water and, where offered, a clearly labeled source-based assessment.
Normalize climate, geography, and seasonality
Cooling technology and climate affect water use as well as energy use. Obtain the facility location, weather data set and years, design temperatures, humidity assumptions, and expected economizer hours. If bidders model different weather years, have them rerun the same normalized weather basis or show a sensitivity range.
Local water availability is a separate decision from water efficiency. Record municipal restrictions, drought status, permitted abstraction, reclaimed-water availability, discharge limits, and the consequences of a supply interruption. A proposal with a favorable WUE can still be unsuitable where its source is potable water in a stressed basin.
Request monthly energy and water profiles. Annual averages can hide summer cooling-tower demand, winter humidification, or a short high-load season that determines utility capacity and water-storage requirements.
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Keep renewable energy and heat reuse as separate dimensions
Renewable-energy claims
Ask for physical electricity consumption and renewable-energy accounting in separate columns. Identify on-site generation, utility supply, power-purchase agreements, guarantees of origin or similar contractual instruments, and the period matched to the load. The EU framework distinguishes renewable energy from these procurement instruments and on-site sources. A renewable claim does not reduce the facility’s physical electricity demand, so it cannot substitute for disclosure of EDC, EIT, or PUE.
Heat reuse
If a bidder claims heat reuse, request the delivered quantity outside the facility boundary, temperature and quality, recipient, operating months, and the external energy use it replaces. The European Commission’s assessment uses Energy Reuse Factor (ERF) and Renewable Energy Factor (REF) alongside PUE and WUE. These indicators add context, but neither replaces absolute energy and water totals.
Calculate and compare proposals consistently
- Lock the boundary: attach a one-page diagram showing utility meters, generators, cooling equipment, IT meters, water meters, shared services, and the treatment of mixed-use loads.
- Lock the scenario: specify IT load by month or operating case, rack density, utilization, redundancy, growth, and operating hours.
- Collect raw totals: obtain annual facility energy, IT energy, total water, potable water, and other-source water before accepting any ratios.
- Recalculate ratios: calculate PUE from the submitted totals and calculate WUE only after confirming category, units, and water boundary.
- Normalize weather: use one agreed weather basis or show sensitivity for each site’s actual climate and economizer operation.
- Separate procurement effects: list renewable contracts and on-site generation separately from physical energy use; list heat reuse separately from facility efficiency.
- Test peak conditions: review monthly maxima, drought operation, generator testing, and a high-utilization IT case, not only the annual mean.
When converting units, keep the original values and conversion basis in the workbook. In particular, do not silently convert a WUE reported in litres per kilowatt-hour to a value based on megawatt-hours while changing the water boundary.
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Set project-specific thresholds after considering workload, facility type, geography, climate, water availability, and contractual scope. The available guidance does not establish a universal PUE or WUE cutoff that is valid for every project.
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- Resource demand: score annual facility energy, annual IT energy, annual total water, and annual potable water.
- Comparability: score boundary clarity, meter coverage, reporting period, assumptions, and reproducibility of calculations.
- Resilience: score alternate water sources, drought operation, storage, discharge capacity, and the effect of cooling-system failure.
- Operational credibility: score commissioning, controls, setpoint governance, maintenance, and the provider’s history of measured reporting.
- Carbon and circularity context: score renewable-energy accounting and heat reuse only after the physical energy and water fields are complete.
Use weighted scoring only after hard exclusions are applied. For example, a proposal that omits potable-water data or uses an unexplained shared-building allocation should be returned for clarification rather than receiving a favorable score for a low headline WUE.
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Questions to ask each data-center provider
- What exactly is inside your facility-energy, IT-energy, and water boundaries?
- Where are the energy and water meters located, what intervals do they record, and how are they calibrated?
- Are the figures measured, modeled, or estimated, and which loads remain missing or allocated?
- What annual IT load, utilization, rack density, redundancy mode, and growth path produced the proposal?
- What are the annual totals for facility energy, IT energy, total water, potable water, and each non-potable source?
- Which WUE definition and units are you using, and does it count only site water or off-site water associated with energy production?
- How many economizer hours are assumed, under what temperature and humidity limits, and what happens outside those limits?
- For cooling towers, what are annual makeup and blowdown volumes, cycles of concentration, and drought operating rules?
- How much energy and water are attributed to backup-generator testing and emergency operation?
- What climate data set and water-availability assumptions support the annual model?
- Which renewable-energy claims represent physical supply, on-site generation, or contractual instruments, and how are they matched to consumption?
- If heat reuse is claimed, who receives it, during which months, and what external energy use does it replace?
- Will you contractually guarantee measured performance using the same boundaries, reporting period, and verification method?
Verify evidence before award and after commissioning
Check the proposal package
- Reconcile the PUE calculation to the submitted EDC and EIT totals.
- Reconcile water meters, invoices, treatment records, and tower controls to the WUE calculation.
- Identify estimated values, gaps, meter changes, temporary construction loads, and shared-building allocations.
- Ask for an independent review or an auditable calculation file when the result is material to the award.
- Require the bidder to state data completeness and the reporting year’s operating conditions.
Write measurement into the contract
Specify the meter points, formulas, units, reporting frequency, weather-normalization method, treatment of generator fuel and shared loads, and the remedy if data are missing. Require monthly reporting with an annual reconciliation. Define performance against the agreed IT-load bands rather than against an undefined “typical” load.
Commission the controls
During commissioning, verify that temperature and humidity limits, economizer changeover, tower cycles, leak detection, and water meters operate as modeled. Establish a baseline year and document changes to IT density, controls, or cooling equipment before comparing later results.
Common comparison errors and their fixes
| Error | Why it misleads | Fix |
|---|---|---|
| Choosing the lowest PUE | PUE is a ratio and hides absolute facility size and load. | Compare annual EDC, EIT, and PUE together. |
| Comparing WUE values with no category | Site-based and source-based water boundaries differ. | Require category, units, included sources, and annual litres. |
| Calling a design water-free | Off-site or shared water may be excluded. | Disclose site and source-based water, including cooling and treatment. |
| Using different weather years | Economizer hours and tower operation change with weather. | Use one normalized weather basis or publish sensitivity results. |
| Accepting a renewable percentage as efficiency | Contracts can change accounting without changing physical demand. | Report renewable procurement separately from energy totals. |
| Ignoring potable-water demand | A low total may still rely on a constrained drinking-water supply. | Require potable and non-potable volumes and sources separately. |
| Counting heat-reuse potential as delivered savings | Unused or seasonal heat does not replace external energy. | Require measured delivery, recipient, operating months, and substituted use. |
| Relying on one annual average | Peaks determine utility, storage, and drought risk. | Review monthly profiles, design days, and high-load cases. |
What a defensible award recommendation contains
Your final comparison should let a reviewer reproduce every material number. Include a boundary diagram; annual and monthly energy and water totals; PUE and the stated WUE category; potable-water demand; cooling and climate assumptions; renewable-energy and heat-reuse disclosures; meter and verification evidence; and a contract plan for post-award measurement.
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