In this guide
The formula and a worked example
The Green Grid defines PUE using the energy consumed by the data centre and its ICT equipment. For this fictional electricity-only example, assume matching meters and a consistent interval:
| Input | Fictional measured energy |
|---|---|
| Total facility energy | 750,000 kWh |
| IT-equipment energy | 500,000 kWh |
| PUE | 750,000 ÷ 500,000 = 1.50 |
The difference is 250,000 kWh. That is 50% of IT energy and one-third of total facility energy. Those denominators answer different questions; calling both percentages “overhead” without defining them creates confusion.
Try the calculation
The starting numbers are fictional. Use matching periods, units and boundaries.
A simple arithmetic check. It does not validate your source records or method.
Match the period, units and meter boundary
Record the start and end time for both datasets. Do not divide a whole month's facility energy by one hour's IT power reading. kWh measures energy; kW measures power. A consistently measured instantaneous power ratio should not be presented as an annual energy-based PUE.
Identify the facility boundary and IT measurement point. Retain information about shared buildings, cooling systems, power-distribution losses and non-electric energy where relevant to the selected methodology. A whole-office utility bill divided by a server-room meter does not automatically measure a standalone data centre.
The GreenRE data-centre guide illustrates that a named assessment scheme can specify measurement points and operating-load conditions. Follow the actual scheme when making a scheme-specific claim. This worksheet is not a certification tool.
Combine periods using energy totals
Suppose a fictional facility has these two periods:
| Period | Facility energy | IT energy | Period PUE |
|---|---|---|---|
| A | 160,000 kWh | 100,000 kWh | 1.60 |
| B | 390,000 kWh | 300,000 kWh | 1.30 |
| Combined | 550,000 kWh | 400,000 kWh | 1.375 |
The combined ratio is 550,000 ÷ 400,000, or 1.375, usually displayed as 1.38 at two decimals. The simple mean of 1.60 and 1.30 is 1.45 and gives the lower-energy period too much weight. Preserve unrounded energy totals and round the displayed result at the end.
Do not aggregate periods with different measurement boundaries without first assessing and documenting the change. A meter upgrade may improve measurement rather than physical efficiency.
Review surprising results
If IT energy is zero or missing, the ratio is unavailable. Do not return zero PUE. If the stated total facility energy is less than IT energy, investigate the boundary, units, timestamps, missing records and meter treatment before using the result.
A lower PUE can result from changing IT load as well as infrastructure changes. Report the numerator, denominator and operating context when comparing periods. Do not infer better computing efficiency solely from a lower facility-overhead ratio.
Keep any estimates visible. A missing interval filled using a documented method should remain identifiable in the calculation record. It should not become indistinguishable from a meter reading after the data is summarised.
What to retain with the result
Keep both meter exports, the meter-boundary diagram, interval-selection rule, energy-unit conversions, missing-data treatment and reviewed calculation. Record who prepared the result and who checked the source selection.
If the number is later revised, retain the old value and reason. The audit-trail guide explains the same discipline for figures used in climate reporting.
Questions you might have
Does renewable electricity improve PUE?
Changing the electricity procurement attribute does not, by itself, change the facility-to-IT energy ratio. Carbon and renewable-energy claims require their own accounting basis and evidence.
Does a PUE of 1.0 mean the computing is efficient?
It would mean no additional facility energy relative to IT energy under the stated boundary. It would not establish that the IT workload itself is useful or efficiently delivered.
Does Auditably calculate or monitor PUE?
No. The calculator on this page is an educational arithmetic aid. Auditably's paid product concerns evidence and review around prepared climate-reporting figures; it is not a data-centre monitoring platform. Check the coverage matrix when evaluating fit.
Prepared with AI assistance using the linked sources and original fictional examples. No customer results, field measurements or independent specialist review are claimed.