(This is our final blog in our PUE series)
When measuring efficiency in the data centre PUE gives us a starting point but, as with most standards, does not necessarily meet all the needs of the industry. Although we’ve touched on some of the limitations of PUE in our previous blog, it’s important that data centre specifiers and managers are aware of the challenges of using the PUE metric as an indicator of overall efficiency and are not mislead by some PUE claims.
Calculating the PUE metric takes time and effort and although the guidelines are very specific, it can be challenging. As we close off our series on PUE here’s our top five list of points to note when looking at PUE figures:
- Collection of Data
The time of day, the points at which the monitoring takes place, and the frequency that data is collected all influence the PUE results and make it very difficult to have an accurate comparable PUE indicator. There will be spikes in loads at different times in the day and even different times in the year. A simple example is that on the hottest day of the year more cooling is going to be needed than on the coldest day of the year. (See the full list of requirements on our first blog - What is PUE and Why is it Useful?)
While yearly results can help at an overview level, the ideal is to have continual daily monitoring to get a genuine impression of efficiency in a data centre. This can be quite an undertaking for a data centre manager and is more suitable to some environments than others.
- Partial PUE
Different data centre settings need different metrics. PUE is based on the premise that the data centre facility is only used as a data centre and does not factor in that other departments might also share space in the facility. It does not work well with the business model of data centre services or colocation providers either when they are not operating at maximum design capacity.
This has resulted in the Partial PUE metric that allows the active data centre zone to be measured instead of areas not being actively used for data centre services. This means that the partial PUE will indicate how efficient a data centre is at part or low load which is imperative to data centre services companies, (see our blog When is PUE not PUE).
- Miscalculating and Misreporting
Some managers find it difficult to understand the PUE metric and may report a PUE value of less than 1.0. This is impossible to achieve because all energy will have an output of at least 1.0 and therefore anything lower is not achievable.
Not all reporting is accurate because some of the aspects of the data centre have been excluded for example, calculations have been based only on the cooling system. Or in the case of modular designs only the electricity supply within the modular environment has been measured rather than including the electricity points actually supplying the electricity to the modular unit.
- Non Comparable
Although PUE can provide an overview of how a data centre is performing over time, the complexity of the calculations and different data centre environments means that comparing the PUE across data centres is not very meaningful. To make useful comparisons, we need to look at the PUE calculations and reporting in detail.
- Limited scope
Efficient sourcing of energy is not included in the metric so energy-saving generation measures such as cogeneration, waste-heat re-use and local power generation are not reflected in the PUE value.
In conclusion, while PUE is a useful indicator, it is not a case of one-size fits all and analysis needs to be tailored so that it is relevant to the individual data centre environment. Correct monitoring and management, that are able to drive actionable insight are required to deliver real efficiency cost savings. You can find out more about our monitoring solutions here