Data centres use roughly one to two percent of the world's electricity, and the figure is growing as more computing moves to them. Operators work on this in several ways: more efficient cooling, using outside air, buying renewable power, and reusing waste heat for nearby buildings.
Water is another cost. Some cooling systems evaporate large amounts of it, which matters in dry regions. When a provider claims to be green, look for specifics: the share of renewable energy, how it is verified, the efficiency figure, and what happens with waste heat. The cleanest server is also the one you do not need, so trimming a bloated site is a small contribution that costs nothing.
Where the energy goes
Nearly all the electricity entering a data centre ends up as heat. The servers are the useful part of the load; everything else is overhead, mostly cooling, plus losses in the UPS and power distribution, and lighting. The standard measure of that overhead is PUE, power usage effectiveness: total facility energy divided by the energy used by the IT equipment. A PUE of 1.0 would mean no overhead at all, which is not reachable. Older halls often run around 1.6 to 2.0, and modern efficient sites can get near 1.1 to 1.3, depending on climate and load.
Cooling and water
Cooling is the biggest overhead item, and the design choices are mostly about climate. In cool regions, "free cooling" brings in outside air or uses cold water from the environment for much of the year, so compressors run rarely. In hot places, chillers do more work. Evaporative systems cool well while using little electricity, but they consume water as it evaporates, which is the trade-off the opening paragraph hinted at.
Water is measured as WUE, water usage effectiveness, in litres per kilowatt-hour of IT energy. A low-electricity design can have a high WUE and vice versa, so neither figure alone settles whether a site is "green". The answer depends on whether the water is potable, reclaimed or taken from a stressed source.
Renewable power: reading the claim
"Powered by renewable energy" can mean several different things. The strongest version is a direct contract with a wind or solar generator, matched to consumption closely in time. A common weaker one is buying certificates that cover annual consumption on paper, while the electrons at a given hour may come from the ordinary grid. Both are legitimate, but they are not the same.
| Question to ask | A good answer sounds like |
|---|---|
| What share is renewable? | A percentage, for a named year |
| How is it verified? | Certificates or contracts, with the scheme named |
| What is the PUE? | A measured annual average |
| Where does waste heat go? | A district heating link or a plain "nowhere yet" |
Heat reuse and hardware lifespan
A hall full of servers is a modest power station in reverse. Some operators pipe warm water to neighbouring buildings or greenhouses, which turns a waste stream into a product. It works best where a heat network already exists, since it needs a customer next door.
Then there is the hardware itself. Manufacturing a server carries a carbon cost of its own, often called embodied carbon, so running machines for longer, repairing them, and recycling them properly can matter as much as the electricity bill. Virtualisation helps here by packing more sites onto fewer, busier machines.
What a site owner can do
You cannot choose the cooling plant, but you can choose a provider that publishes numbers, and you can shrink your own footprint. Every request that is cached, compressed or never made saves a small amount of CPU somewhere. Compress and resize images, remove unused plugins, cache pages, and delete old staging copies that sit on disk and in backups. Our page weight tool shows what a typical page costs in bytes.
Checking it yourself
Look on the provider's site for a named PUE, a renewable percentage with a year, and any independent certification such as an environmental management standard. If the page offers only adjectives, ask for the figures in an email and see how long the answer takes. On your own side, curl -sI -H 'Accept-Encoding: gzip' https://example.com/ | grep -i content-encoding tells you whether compression is on.
What else people want to know
Is a low PUE enough?
It shows efficient overhead, but says nothing about where the electricity comes from or how much water was used.
Are bigger data centres better?
Often they are more efficient per server, thanks to scale. They also concentrate demand on local grids and water supplies.