Power usage effectiveness (PUE) is a ratio that measures how efficiently a data center delivers energy to its IT equipment. It is calculated by dividing the total energy used by the facility by the energy used by the servers, storage and network equipment. A PUE of 1.0 would mean every unit of energy goes to IT equipment; anything above that is overhead for cooling, power conversion, lighting and other support systems. The metric was popularized by the industry group The Green Grid and has since been published as an international standard (ISO/IEC 30134-2).
At a glance
- PUE = total facility energy ÷ IT equipment energy; lower is better and 1.0 is the theoretical minimum.
- It measures overhead, mainly cooling and power distribution losses, not the efficiency of the IT equipment itself.
- Results depend on climate, cooling design, redundancy, how full the facility is and how and where energy is measured.
- For colocation buyers, PUE matters most when it affects power pricing or sustainability reporting.
What problem it solves
Data centers use a great deal of energy, and much of it never reaches a server. Cooling systems, uninterruptible power supplies, transformers, lighting and offices all draw power. Before PUE, there was no simple, widely accepted way to compare how much of a facility’s energy went to overhead.
PUE gave operators a single figure to track improvements over time and gave buyers a starting point for comparing facilities. For businesses reporting on emissions or energy use, it also helps estimate the share of a facility’s energy attributable to their equipment.
How it works
The formula. PUE is total facility energy divided by IT equipment energy, measured over the same period. A facility drawing 1.5 megawatt-hours for every 1 megawatt-hour delivered to IT equipment has a PUE of 1.5: half a unit of overhead for each unit of IT load.
Where energy is measured. Total energy is usually measured at the utility meter. IT energy can be measured at the UPS output, at the power distribution units or at the server inputs. Measuring further downstream captures more losses as overhead and gives a more accurate but higher figure. The standard defines categories for these measurement points.
Over what period. Annual figures smooth out seasonal swings in cooling demand. A figure taken on a cool day or as a design target can look better than the facility’s actual year-round performance.
What moves it. Free cooling with outside air or water, warmer allowed inlet temperatures, efficient UPS systems and good airflow management all lower PUE. A lightly loaded facility often has a worse PUE because fixed overhead is spread over less IT load. More redundant designs, such as those built to higher tier classifications, may carry more overhead from equipment running at partial load.
Variants. Operators sometimes report partial PUE for one part of a facility or design PUE for a planned building. These are useful but are not the same as a measured annual figure for the whole site.
For help comparing facilities, including their power pricing and efficiency, see our Colocation solution page.
When it matters for buyers
- Comparing colocation quotes. If overhead is passed through in power pricing, a facility’s PUE can change your monthly bill.
- Sustainability reporting. Your share of a data center’s energy, and its emissions, depends partly on its PUE.
- High-density or AI workloads. Cooling approach, and with it efficiency, can vary sharply for very dense racks.
- Moving out of an on-premises server room. Office server rooms are often far less efficient than purpose-built facilities, which can offset some colocation costs.
Questions to ask vendors
- What is the facility’s measured annual PUE, and for which period?
- Where is IT energy measured: UPS output, power distribution units or another point?
- Is the figure for the whole site, one hall, or a design target?
- How is cooling and other overhead reflected in our power pricing?
- What is the energy source, and can you provide data for our emissions reporting?
- What water use and cooling methods does the facility rely on?
How it differs from data center tier classification
PUE measures efficiency: how much energy a facility spends on overhead. Tier classification measures resilience: how well the power and cooling design tolerates maintenance and failures. The two can pull against each other, because spare equipment and redundant paths add overhead, though well-designed modern facilities can score well on both. A buyer usually needs to weigh both rather than choose a facility on one number.