The relevant measure of energy efficiency of a data center is the PUE (power usage effectiveness). It indicates how much of the energy used by a data center is actually “delivered” to the servers and storage systems – or, as Dirk Kabelitz, head of the data center in Biere, puts it precisely: “the ratio of total amount of energy used by a data center to the IT stream of the equipment used.” Therefore, the lower this number, the better. The ideal value is 1. In reality, of course, this ideal is unattainable, due merely to the physics involved. Highly experimental structures at highly suitable locations have achieved values between 1.1 and 1.2.
In comparison: the average PUE of German data centers is around 1.8, according to experts. And that’s a good figure in international comparison. A server with a connected load of 1 kW, for example, that runs around the clock, will incur energy costs of roughly € 880 in one year. Calculated using an average commercial electricity price of 20 cents/kWh. In an average data center with a PUE of 1.8, an air conditioning load of 400 watts will add another € 700 to the actual server operating expenses. Total costs: around € 1,600.
Optimal data centers on dry land can achieve a PUE of 1.3. This is only a number at first, but a rough calculation shows the economic effects (economics is also a domain of numbers, after all). In a data center with a PUE of 1.3, only 150 watts are needed for air conditioning, incurring annual costs of a little over € 262, or € 1,150 in total. That’s a difference of over € 400. Per year. Per server. That means around 30 percent lower costs – for total server operations.