Making Large UPS Systems More Efficient

Overview

The opening comparison: two 1 MW UPS systems with an identical published 93% full-load efficiency, in 2N at 300 kW load, have 10-year loss costs of $293,165 versus $793,021. Field surveys indicate 2N data centres operate at 20-40% of their 2N capacity, and the paper assumes a typical 30% load.

UPS losses are split into no-load (fixed), proportional and square-law components. No-load losses can represent over 40% of all UPS losses, while square-law losses become significant (1-4%) at higher loads. Manufacturers often exclude input filters that reduce quoted efficiency by 0.5-1%; at 30% load a filter costing 3 points raises the annual loss cost of a 1 MW UPS from $32,481 to $42,781. Eco-mode bypass efficiency is typically 98% to 99% versus 94% to 97% for the base UPS.

Topology matters: double conversion UPSs draw 9% to 30% input THD at 0.9 to 0.8 power factor, while delta conversion draws less than 3% at unity power factor. A right-sized modular delta UPS (400 kW at 75% load, 96.9%) compared with an oversized non-scalable double conversion unit (88.7% at 30%) shows a 75% lower 10-year cost of inefficiency at 1N and 80% at 2N. The appendix gives the loss models as fractions of rating: delta y = 0.01081x2 + 0.00640x + 0.01333; double conversion y = 0.03455x2 + 0.00959x + 0.03234. Cooling adds 0.4 kW per kW of loss. The loss curves rest on third-party measurements, and the related standard is IEC 62040-3.

Key points

1. Two 1 MW UPSs with the same 93% full-load efficiency, 2N at 300 kW: 10-year loss $293,165 vs $793,021.
2. 2N data centres operate at 20-40% of 2N capacity; 30% is assumed typical.
3. No-load losses can exceed 40% of UPS losses; square-law losses reach 1-4% at higher loads.
4. Input filters cut efficiency 0.5-1%; at 30% load a 1 MW UPS annual loss cost rises from $32,481 to $42,781.
5. Eco-mode path 98-99% vs 94-97% for the base UPS.
6. Double conversion input THD 9-30% (PF 0.9-0.8); delta under 3% at unity PF.
7. Right-sized modular delta cuts 10-year inefficiency cost 75% at 1N and 80% at 2N.
8. Cooling adds 0.4 kW per kW of loss.

Using it in design

• Store UPS losses as a three-coefficient model (no-load, proportional, square-law) or a 0/25/50/75/100% curve, and record whether quoted efficiency includes input filters or transformers.
• Compute per-module loading from IT load, redundancy (N, N+1, 2N) and installed modules, then annual and lifetime loss cost with the cooling penalty (default 0.4 kW/kW).
• Flag operating points below about 30% load and suggest fewer or modular power modules.
• Show eco-mode efficiency as a separate operating mode, not the default figure.

Who should read it

Electrical designers specifying large UPSs and comparing operating cost.

Related standards

• IEC 62040-3

Publication details

Authors

Richard L. Sawyer

Publisher

Schneider Electric

Year

2012

Document number

WP 108

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