Overhead Ratio and PUE
Total energy at the outer boundary divided by energy at the inner boundary. For a data center, M4 over M3, it equals power usage effectiveness (PUE). Lower is better, and it can never be below 1.
The formula
OR = E(M4) / E(M3)
Calculated from energy over a stated period, not from instantaneous power, following the definitions of ISO/IEC 30134-2:2026, published in January 2026.
Rules that keep it honest
Reference points
| Reference | Value | Source |
|---|---|---|
| Google fleet, trailing twelve months | 1.09 through 2025 and the first half of 2026 | Google data-center efficiency page |
| EU rating scheme, energy grade A | PUE of 1.15 or less | Latham & Watkins, 25 September 2026 |
| EU rating scheme, energy grade G | PUE above 1.9 | Latham & Watkins, 25 September 2026 |
Why the ledger matters here
A PUE says nothing about whether the IT energy did useful work. The Joule Ledger pairs the Overhead Ratio with Decision Yield at M3, so a facility cannot look efficient while running idle servers.
Questions
What is a good PUE?
Lower is better. Under the EU rating scheme adopted in September 2026, an A energy grade needs a PUE of 1.15 or less (Latham & Watkins, 25 September 2026).
Sources
Keep reading
Take the next step.
The JL-26 Overhead Ratio, facility energy over system energy: equal to PUE for data centers, calculated from energy over a period per ISO/IEC 30134-2:2026.