Electrical Efficiency Modeling for Data Centers

Overview

Three reasons are given: component efficiency falls at partial load, equipment runs far below its rating, and the heat from power and cooling equipment must itself be cooled. Conventional nameplate models underestimate electrical losses by a factor of two or more. The UPS example is striking: a UPS described as 91% efficient at full load is only 60% efficient at 10% load.

The proposed model expresses every infrastructure component as a no-load loss plus proportional and square-law terms, with loading derived from derating, redundancy and load-diversity oversizing. Typical losses as a fraction of rating: UPS 4% + 5% = 9%; PDU 3%; CRAC 9%; chiller plant 6% + 26% = 32%; lighting 1%; wiring 1%; switchgear 0.5%; generator 0.3%. Derating of 10-20% is recommended for highly available facilities, and PDU oversizing for load diversity is typically 30% to 100%.

In a typical dual-path, N+1 CRAC data centre at 30% load the power flow is IT 47%, chiller 23%, CRAC/CRAH 15%, UPS 6%, humidifier 3%, PDU 3%, lighting/aux 2%, switchgear/generator 1%, giving PUE = 2.13. A 1 MW facility costs $600,000 to $1,700,000 per year at $0.10/kWh, over $500,000 even with no IT load, and at 30% utilisation over 70% of electricity cost is infrastructure inefficiency. Multi-mode devices such as economiser cooling are handled by state-space averaging: per-mode loss curves weighted by the time fraction in each mode. As efficiency entitlements, reducing oversizing via modular architecture cuts losses 50%, improving cooling efficiency 30%, and reducing no-load losses 10%. The related standard is ISO/IEC 30134-2.

Key points

1. Nameplate models underestimate electrical losses by a factor of two or more.
2. A UPS 91% efficient at full load is 60% efficient at 10% load.
3. Typical facility at 30% load: IT 47%, chiller 23%, CRAC/CRAH 15%, UPS 6%; PUE 2.13.
4. Typical losses: UPS 9%, PDU 3%, CRAC 9%, chiller plant 32%, lighting and wiring 1% each.
5. Derating 10-20%; PDU diversity oversizing 30-100%.
6. A 1 MW facility costs $600,000-$1,700,000 per year at $0.10/kWh, over $500,000 with no IT load.
7. At 30% utilisation over 70% of electricity cost is infrastructure inefficiency.
8. Entitlements: modular architecture 50% loss reduction, cooling 30%, no-load losses 10%.

Using it in design

• Model each infrastructure block with no-load, proportional and square-law coefficients (Table 2 as defaults) and compute PUE as a curve over 0-100% IT utilisation.
• Derive component loading from redundancy (N+1/2N), derating margin and diversity oversizing.
• Take cooling load as IT plus the losses of in-room power and cooling equipment, and model multi-mode cooling by hours-per-mode weighting.
• Show a PUE at day-1 load beside design PUE, and warn when utilisation stays below about 30% for years and suggest phased capacity.

Who should read it

Design engineers building PUE, oversizing and energy-cost models.

Related standards

• ISO/IEC 30134-2

Publication details

Authors

Neil Rasmussen

Publisher

Schneider Electric

Year

2011

Document number

WP 113

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation