Data centre power monitoring and PUE inputs
Use ZEM-65 meters and local Edge to track facility and IT supplies, reconcile the power chain and prepare aligned energy inputs for PUE and measured load evidence for capacity reviews.
- Measure
- Facility, cooling, UPS and IT distribution supplies
- Calculate
- PUE monitoring needs complete facility and IT energy over the same period The Green Grid
- Sensors
- ZEM-65, meters; optional ZMB-31 for documented equipment registers
- On site
- ZGW-20 Gateway, Edge keeps local readings; a specialist platform is optional
Choose the electrical boundary before the dashboard
Start with the current single-line diagram. Follow the facility incomer through UPS input, UPS output and power distribution units (PDUs) to IT equipment, with cooling and auxiliaries on their own branches. A parent meter already includes its downstream children: never add both to the facility total. Record every A and B feed, bypass and alternative supply, and the operating states in which each meter still covers its named boundary.
| Operator question | Required evidence | Decision it supports |
|---|---|---|
| Where is the electrical load? | kW and per-phase current on identified supplies, with the observation period | Locate heavily loaded paths and investigate trends with the responsible electrical engineer |
| What are the PUE inputs? | Complete facility energy and complete IT energy at one declared metering level over identical start and end times | Calculate a period ratio whose denominator, coverage and boundary are stated |
| Where does the power chain lose energy? | Aligned upstream and downstream energy, included loads, uncertainty and battery charge state | Investigate an energy difference; a residual alone does not identify a fault or quantify conversion loss |
Hardware for data centre electrical metering
Count physical metering boundaries, then check existing UPS and PDU interfaces before specifying additional acquisition hardware.
Measures voltage, current, power and energy at each selected facility, UPS, cooling or distribution boundary
- Choose CTs for each supply: 120 A split-core, or 300 A, 1 kA and 3 kA Rogowski coils
Class 0.5S to IEC 62053-22 for the meter and calibrated CTs together, 110 to 480 V line to line
Collects the named electrical readings, retains local histories and runs configured monitoring rules
If the equipment exposes documented Modbus RTU registers and the segment permits this master
Reads documented energy, load or status registers from a compatible existing UPS, PDU or meter
Modbus RTU master over RS-485, mains powered
Data centre energy monitoring starts at named boundaries
Give every point a parent, physical location, unit and role in the calculation. Keep the facility total, IT denominator and diagnostic child meters distinct.
System architecture
From electrical boundaries to local power-chain records
Separate meters identify the energy boundaries. An optional read-only Modbus path adds equipment measurements or operating context where the exact model supports them.
| Position | What it tells you | Reference | Sensor |
|---|---|---|---|
| Complete facility energy | Energy supplying the data centre, including IT, cooling, power delivery and supporting loads | An office-building incomer is not a data-centre total. Shared cooling and other energy sources need an explicit allocation and accounting method DOE and LBNL | ZEM-65 (on this page) |
| UPS input and output | Two levels in the power chain and their aligned energy difference | UPS input includes conversion and storage effects. The basic UPS-output IT proxy also includes downstream distribution losses; record any non-IT loads DOE and LBNL | ZEM-65 (on this page) |
| PDU output and IT input | A declared IT denominator covering every supply path to compute, storage and network equipment | PDU output is an intermediate boundary; IT-equipment input is more detailed. Use one consistent level, include all A and B feeds, and do not mix parent and child readings The Green Grid | ZEM-65 (on this page) |
| Cooling and support loads | Where non-IT energy is used within the measured facility total | Cooling, fans, pumps and lighting explain the overhead. Facility minus IT is not cooling energy alone DOE and LBNL | ZEM-65 (on this page) |
| Existing equipment data | Mapped cumulative energy, load or bypass and battery status when the exact UPS or PDU exposes them | Verify the model, firmware, register map, units and timestamps. SNMP-only or Modbus TCP-only equipment does not connect through this ZMB-31 RS-485 path ZMB-3X datasheet (PDF, opens in a new tab) | ZMB-31 (on this page), optional |
A partial branch total is not whole-site PUE. If one required IT feed, cooling service or alternative energy input is absent, retain the available readings as named branch data. Do not calculate PUE from incomplete branches or label an estimated allocation as a fully metered result.
A residual needs an explanation. Upstream minus downstream energy can include unmetered auxiliaries, distribution loss, meter uncertainty, time misalignment and changes in stored battery energy. Check bypass state and charging before calling the difference UPS loss.
Load evidence informs a capacity review. Trend kW and per-phase current against limits approved by the site's electrical engineer, with the sampling interval and observed peaks recorded. kW is not kVA, and a low average does not prove spare capacity. Cable and breaker ratings, derating, phase balance, UPS limits and the required A and B failover state remain engineering inputs.
How to calculate PUE from aligned energy inputs
For the declared period, divide complete facility energy by complete IT energy at the chosen metering level. State that level and period beside the result. Use the ratio of energy totals, not an unweighted average of interval ratios; a daily result is not an annual PUE claim.
PUE for the declared period = Efacility ÷ EIT
non-IT energy at that boundary = Efacility − EIT- Efacility
- complete energy for the declared data-centre facility boundary and period, kWh on one agreed energy-accounting basis
- EIT
- complete energy across non-overlapping IT feeds at the declared metering level, over the same period, kWh; the denominator must be greater than zero
Worked example
Illustrative 24-hour period: the dedicated facility incomer records 1,500 kWh and the complete PDU-output IT boundary records 1,000 kWh over the same start and end times. The period ratio is 1,500 ÷ 1,000 = 1.50, with 500 kWh outside that IT boundary. If the separately metered cooling feeder uses 350 kWh, the remaining 150 kWh covers other overhead and losses; it is not additional facility energy. Report the result as a 24-hour PDU-output-boundary ratio: downstream cable losses remain inside its IT denominator. This is illustrative arithmetic, not a measured customer PUE, an annual result or a certified performance claim.
In Edge
Edge retains local readings for a commissioned calculation or an optional receiving platform. Derive interval kWh from valid cumulative-register differences with the same timestamps, accounting for resets and rollover; do not sum instantaneous kW readings as kWh. Record clock source, time zone, interval and data age. Suppress the ratio when a required input is stale, incomplete, misaligned or has a zero denominator, and test that behaviour; a missing feed is not zero load. Keep recoverable cumulative-period totals distinct from unknown within-period peaks. A configured no-data rule must reach the agreed operator. This guide does not supply a prebuilt PUE dashboard or an automatic capacity model.
Commissioning checks
Commission under the site's approved change procedure, with the facilities and IT owners agreeing the same electrical map and reporting boundary.
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Account for every operating path
Mark incomers, UPS, bypass, PDUs, A and B feeds, cooling and auxiliaries on the single-line diagram. Identify shared services, generator operation, storage and every load outside the selected boundary.
Pass when the numerator and denominator have complete, documented coverage for each reported operating state, with no physical load counted twice.
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Verify the electrical channels
A competent electrical installer checks suitability, isolation and the installation procedure, then verifies CT rating, orientation, voltage references and phase association against a suitable reference measurement.
Pass when each named meter responds to its intended circuit with credible per-phase values, direction and energy increments; no unapproved interruption to critical supplies is needed.
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Qualify existing UPS and PDU registers
Check model, firmware, address, data type, byte order, scale, units and update rate against the equipment display. Establish bus ownership before adding a Modbus RTU master and use read-only acquisition.
Pass when the mapped points match the documented equipment values, no existing master is disrupted and no control writes are configured.
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Reproduce one complete energy period
Use recorded register values at identical start and end timestamps. Check resets, time-zone conversion and gaps, then reconcile the facility meter with non-overlapping branches using a tolerance based on the installed measurement uncertainty.
Pass when the raw readings reproduce the ratio, the residual is explained and the selected UPS-output, PDU-output or IT-input boundary appears beside the result.
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Prove invalid inputs stay invalid
Replay missing, stale, misaligned and zero-denominator inputs in an isolated calculation test. Use an approved communications test for the no-data rule without switching off a critical load.
Pass when invalid input suppresses the ratio, the operator receives the no-data condition and restoration preserves the gap instead of backfilling zero load.
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Agree the load-review limits
Record the engineer-approved per-phase and UPS limits, logging resolution and normal and failover configurations. Check that any configured threshold rule names the affected path and accountable recipient.
Pass when the operational trend is labelled with its observation period and resolution, and any decision to add IT load goes through the site's capacity review.
What power-chain records cannot establish
The system supplies local measurement evidence. Critical infrastructure decisions need the site's electrical, IT and operating procedures as well.
- This monitoring scope does not replace DCIM, an electrical power management system, protective relays, UPS controls or the site's incident response, and it does not guarantee uptime.
- PUE describes infrastructure energy overhead; it does not measure useful computing work, server utilisation, carbon intensity or overall business efficiency.
- Different IT metering levels include different distribution losses. Do not compare results as equivalent when their boundaries, periods, weather, IT load or data coverage differ.
- An electrical incomer alone is insufficient when shared cooling, fuel, on-site generation or another energy source contributes to the facility. A separately reviewed energy-accounting method is required; this five-meter example does not provide it.
- Interval trends are not protection-grade transient recording, breaker coordination, power-quality certification or proof of redundancy. They cannot authorise additional IT load or a failover test.
- Rack inlet temperature and humidity need their own sensors, placement and escalation plan. Electrical load does not establish thermal safety.
Sources and related guides
Sources
- Data Center Metering and Resource Guide (2017) (opens in a new tab) U.S. Department of Energy and Lawrence Berkeley National Laboratory. Facility and IT energy boundaries, UPS-output metering, shared services, end-use metering and commissioning. Used for measurement principles, not historical policy or market statistics.
- PUE: A Comprehensive Examination of the Metric, White Paper 49 (2012) (opens in a new tab) The Green Grid, hosted by Lawrence Berkeley National Laboratory. PUE numerator and denominator, UPS-output/PDU-output/IT-input measurement levels and disclosure of measurement frequency and reporting period; not a claim of certification or current regulatory compliance.
- ZEM-63 Wireless 3-Phase Electricity Monitor datasheet (opens in a new tab) EpiSensor. Specifications, ranges and ordering codes.
Related
- Data-centre rack temperature and humidity monitoring Application guide Plan data-centre rack temperature points and a separate humidity instrument, with local Edge trends, stale-data checks and a practical commissioning record.
- Transformer and distribution board monitoring Application guide Meter the incomer first, then the feeders that explain a decision. How to pick CTs, keep phase detail, and reconcile the board against its parent.
- Chiller performance monitoring Application guide Measure chiller electrical input and delivered cooling over the same interval, calculate COP and kW per ton in Edge, and prove the numbers before using them.
- Meter integration for a BMS or SCADA system Application guide What Edge reads, what it serves onward, and the settings both ends have to agree on before a register becomes a point in your BMS.
- Equipment performance monitoring boundaries Knowledge Base Define input, useful output, operating state and time boundaries for pumps, compressors, boilers, chillers and heat pumps.
Reviewed by EpiSensor Engineering on . Revision 2.
Send the single-line diagram and the reporting boundary
Include the UPS and PDU models, available register maps, A and B feeds, bypass paths, cooling supplies, existing meters and the operating decision you need to support. We can then scope the missing measurements and a commissioning plan.