Power Usage Effectiveness (PUE) is the single most useful number in data centre efficiency, and also one of the most misunderstood. Ask most IT buyers what PUE means and you’ll get a vague answer about “green data centres.” Ask what PUE actually does to their monthly colocation bill, and most won’t be able to tell you. That’s a problem, because PUE isn’t a marketing metric — it’s a direct multiplier on how much you pay for power, every single month, for as long as your kit sits in that facility.
What PUE actually measures
PUE is calculated as:
PUE = Total Facility Energy ÷ IT Equipment Energy
“IT Equipment Energy” is the power that actually reaches your servers and does useful computing work. “Total Facility Energy” is everything the building draws to make that possible — cooling, lighting, UPS losses, power distribution losses, and building overhead. A PUE of 1.0 would mean a perfectly efficient facility where every watt drawn from the grid reaches IT equipment, with zero overhead. That’s a theoretical limit, not a real-world outcome — but it sets the scale you’re measuring against.
According to the Uptime Institute’s annual global survey, the industry average PUE has hovered around 1.5-1.6 for several years, having plateaued after a decade of steady improvement. That means for every 1kW of actual computing power a typical facility delivers to servers, it draws roughly 1.5-1.6kW from the grid — with the extra 0.5-0.6kW spent almost entirely on cooling and power conversion losses.
Why PUE is your problem, not just the data centre’s
Most UK colocation contracts bill power in one of two ways: metered consumption (you pay for exactly what your equipment draws, often with a facility overhead uplift baked into the rate) or a fixed power allocation (you pay for an amp or kW allowance whether you use it or not). Either way, the facility’s PUE determines the true cost of delivering power to your rack, and that cost gets passed through — directly in the electricity rate, or indirectly in the base colocation fee.
A facility running at PUE 1.6 is spending 60% more electricity than the theoretical minimum to keep your servers running. Someone pays for that overhead, and with UK commercial electricity prices where they’ve been over the past few years, the gap between a well-run facility and an inefficient one compounds fast across a multi-year colocation contract.
What actually drives PUE down
- Free/fresh-air and evaporative cooling. The UK’s climate is genuinely an asset here — ambient temperatures are cool enough for large parts of the year to reduce or eliminate mechanical chiller use, which is the single biggest PUE lever available.
- Water-based cooling systems. Rainwater harvesting for evaporative cooling reduces both energy draw and mains water consumption compared with traditional chilled water systems.
- Hot aisle / cold aisle containment. Physically separating hot exhaust air from cold intake air stops the two from mixing, which lets cooling systems run less aggressively for the same server inlet temperature.
- High-efficiency UPS and power distribution. Modern UPS systems in eco or ECO-mode operate above 97% efficiency versus older double-conversion units that waste considerably more as heat.
- On-site renewable generation. Solar generation offsets grid draw directly, and depending on how it’s accounted for, can improve the effective PUE and carbon figures reported to customers.
Our own Cambridgeshire data centre combines rainwater cooling and solar generation to run at a measured PUE of 1.07 — well below the industry average, and close to the practical ceiling for a facility that still needs conventional backup cooling for the hottest days of the year.
Working out what PUE means for your bill
Take a rack drawing a steady 3kW of IT load, running 24/7/365 — a reasonable assumption for a dense colocation deployment. That’s roughly 26,280 kWh of IT-equipment energy per year. At an industry-average PUE of 1.6, the facility needs to draw about 42,048 kWh from the grid to deliver that. At a PUE of 1.07, it needs about 28,120 kWh. That’s nearly 14,000 kWh of difference, every year, for a single 3kW rack — multiply that across a full cabinet deployment or a multi-rack estate and the gap between a PUE-1.6 facility and a PUE-1.07 facility becomes a genuinely material line item, independent of the headline power rate you’re quoted.
Questions worth asking any UK colocation provider
- What is your measured (not designed) annualised PUE, and how is it measured — at the facility level or per-suite?
- Is power billed metered or as a fixed allocation, and how does facility overhead factor into that rate?
- What cooling technology is in use, and how much of the year does it rely on mechanical chillers versus free cooling?
- Is any of the facility’s power drawn from on-site renewable generation?
PUE is also directly relevant to ISO/IEC 27001 and ESG reporting obligations many UK businesses now face from their own customers and investors — a low-PUE, renewably-powered facility makes those disclosures considerably easier to write. If you want the actual measured PUE and power billing structure for your workload before you commit to a contract, talk to our team — we’ll give you real numbers, not a marketing average.
