What is a good PUE for a liquid-cooled data center?

Well-run liquid-cooled facilities typically reach 1.05-1.2, versus roughly 1.4-1.6 for legacy air-cooled sites. PUE is total facility energy divided by IT energy, so liquid cooling improves the score by shrinking the cooling overhead — it does not reduce IT power itself.

Why can PUE comparisons be misleading?

Because PUE depends on climate and boundary definitions as much as on technology. The same design scores better in a cold, dry climate than in the tropics (free-cooling hours track wet-bulb temperature), and a single cabinet in a shared room has no well-defined facility boundary at all. Compare like climates and stated boundaries, not headline numbers.

Formula · 2 min read

PUE math — and what it hides

PUE = total facility energy ÷ IT energy: 100 kW of IT with 15 kW of overhead scores 1.15. Liquid cooling reaches 1.05-1.2 where air needs 1.4-1.6 — but the number assumes facility-scale infrastructure and honest climate, which single-cabinet deployments don't have.

Formula

PUE = (P_IT + P_cooling + P_{power loss} + P_misc) / P_IT

Worked example

(100 + 10 + 4 + 1) / 100 = 1.15

A facility moving 100 kW of IT heat with 10 kW of cooling energy (efficient free-cooling or liquid), 4 kW of UPS/conversion losses and 1 kW of lighting and controls runs at PUE 1.15. The same site on legacy air with 45 kW of cooling energy runs 1.50.

Try it

IT load (kW)Cooling energy (kW)Power losses + misc (kW)

1.15

✅ 1.1-1.2 — good; typical for well-run liquid-cooled sites

What it hides

Climate is invisible. The same hardware scores differently in Munich and Singapore — free cooling hours depend on wet-bulb (wet-bulb physics). Cross-climate PUE comparisons are marketing, not engineering.
Liquid cooling doesn't shrink IT power. It shrinks the cooling term. A rack's IT draw is the same; what changes is how cheaply the heat leaves the building.
A cabinet in a room has no honest PUE. If the "facility" is office AC shared with humans, the boundary is undefined. Use the 1.1× room load rule instead for micro deployments.

Room AC load · Wet-bulb & free cooling · Liquid vs air

Sources

Definition: The Green Grid / ISO/IEC 30134-2. Band context: Uptime Institute global surveys (air-cooled averages ≈ 1.5; leading liquid-cooled facilities 1.1 or below).