PUE and energy efficiency: reading a data center before moving in

Two data centers with the same rent can cost you very differently, for a reason invisible on the quote: the site's energy efficiency. The PUE sums it up in one number, and that number ends up on your bill.

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PUE in one sentence #

PUE (Power Usage Effectiveness) divides the site's total energy by the energy actually delivered to IT equipment. A PUE of 1.5 means that for every kWh your servers use, the site draws 1.5 from the grid: the extra half goes to cooling, electrical losses and the rest of the building.

Reading the values #

Around 1.2, the site is very efficient, often thanks to outside-air cooling. Around 1.5, it is a decent site of the current generation. Near 2.0 and beyond, the supporting infrastructure consumes as much energy as your servers themselves, doubling the total: the sign of dated installations. Mind the measurement: a measured yearly average PUE is worth more than an advertised "design" PUE.

Why it ends up on your bill #

If energy is billed on actual use, many sites apply the PUE as a multiplier: your metered kWh, times the site's PUE, at the kWh price. If energy is flat-rate, the PUE is already in the price. Either way, an inefficient site gets paid for; only the path differs.

The levers of a good PUE #

Cooling first: outside air (free cooling), aisle containment, adiabatic systems, liquid cooling for high densities. Then the electrical chain: modern high-efficiency UPS units. Climate helps, a Nordic site starts ahead, but design matters more than latitude.

Beyond PUE #

PUE does not tell everything. Water use (summarized by WUE) becomes sensitive with evaporative systems. Heat reuse, warming neighbouring buildings, and ISO 50001 energy-management certification complete the picture of a serious site. And the electricity's origin weighs in your own carbon footprint.

The deciding calculation, and the questions to ask #

The calculation is simple: your average power, times the PUE, times the hours in a year, times the kWh price. On a 5 kW rack, the gap between a PUE of 1.3 and 1.8 runs to thousands of euros a year. Hence the questions: what PUE measured over twelve months, how is energy billed, at what kWh price, and with what indexation.

FAQ #

Can a PUE be contractually guaranteed?

Some sites commit to a maximum PUE used for energy billing. Failing that, demand the measured PUE of the last twelve months rather than a design value, and the method used to compute the energy share.

Does a small data center necessarily have a bad PUE?

No, but pooling helps large sites. A recent, well-designed small site beats an old large one. The measured value decides, not the size.

Does PUE matter if my energy is flat-rate?

Yes: the flat rate was computed with the site's PUE. An efficient site can sell the same package cheaper, or hold prices at the next electricity increase.