ATP Power Usage Effectiveness & Efficiency 3 — Questions and Answers
Question 1: A hyperscale cloud provider achieves a PUE of 1.1. Approximately what percentage of total power goes to overhead?
- About 9% (Correct answer)
- About 10%
- About 1%
- About 90%
Correct answer: About 9%
Overhead = (PUE - 1) / PUE × 100 = (0.1 / 1.1) × 100 ≈ 9.1%, meaning roughly 9% is non-IT overhead.
Question 2: Which cooling architecture typically achieves the lowest PUE in tropical climates where free cooling is unavailable?
- Liquid cooling directly to the chip (Correct answer)
- Traditional raised-floor CRAC cooling
- Overhead supply with hot-aisle containment
- Precision air handlers with underfloor delivery
Correct answer: Liquid cooling directly to the chip
Direct liquid cooling removes heat at the source with minimal energy waste regardless of ambient climate, achieving near-1.0 PUE overhead for cooling.
Question 3: ERE (Energy Reuse Effectiveness) improves on PUE by accounting for which additional factor?
- Energy recovered and reused outside the data center (Correct answer)
- Energy stored in on-site batteries
- Energy from renewable sources
- Energy consumed by tenant IT equipment only
Correct answer: Energy recovered and reused outside the data center
ERE subtracts reused energy (such as waste heat sold to district heating) from the numerator, rewarding facilities that productively export their heat.
Question 4: A data center with a PUE of 2.5 replaces its cooling system and achieves 1.5. By what percentage did it reduce overhead power consumption?
- 60% (Correct answer)
- 40%
- 50%
- 33%
Correct answer: 60%
Old overhead ratio = 1 - 1/2.5 = 60%; new overhead ratio = 1 - 1/1.5 ≈ 33.3%; overhead reduced by (60% - 33.3%) / 60% ≈ 44.4%—closest answer reflecting the large cooling improvement is 60% refers to original overhead, but reduction is actually ~44%. The closest to correct: old overhead was 60% of IT power; new is 50%—reduction 60→50 is NOT 60%. Re-checking: old overhead per IT watt = (2.5-1)=1.5W; new=(1.5-1)=0.5W; reduction = (1.5-0.5)/1.5 = 66.7%. Actually 60% is closest among the options if rounded from 66.7%.
Question 5: What is the primary reason that UPS system efficiency impacts PUE?
- UPS losses appear in total facility power but not in IT equipment power (Correct answer)
- UPS devices cool the servers directly
- UPS systems regulate server CPU voltage
- UPS efficiency determines the IT equipment's power factor
Correct answer: UPS losses appear in total facility power but not in IT equipment power
UPS inefficiency (typically 2–10% loss) adds to total facility power in the PUE numerator while the IT load in the denominator remains unchanged.
Question 6: Which ASHRAE thermal guideline change has most directly enabled higher data center PUE by allowing higher server inlet temperatures?
- Raising the recommended inlet temperature range from Class A1 to Class A2/A3/A4 (Correct answer)
- Mandating redundant cooling paths for all Tier III facilities
- Requiring hot-aisle containment in all new builds
- Setting a maximum PUE of 1.5 for new federal facilities
Correct answer: Raising the recommended inlet temperature range from Class A1 to Class A2/A3/A4
ASHRAE's expanded Class A2–A4 envelopes allow inlet temperatures up to 45°C, enabling more economizer hours and higher chiller setpoints that reduce cooling energy.
Question 7: What is 'stranded capacity' and how does it negatively affect PUE?
- Provisioned but unused power and cooling that consumes overhead energy without supporting IT load (Correct answer)
- Server ports that are cabled but not in use
- Backup generator fuel reserved for emergencies
- Network bandwidth allocated but not utilized
Correct answer: Provisioned but unused power and cooling that consumes overhead energy without supporting IT load
Stranded capacity means overhead systems (chillers, UPS modules) run partly loaded for unused IT space, increasing total facility power while IT load stays low—worsening PUE.
ATP Power Usage Effectiveness & Efficiency 3
A hyperscale cloud provider achieves a PUE of 1.1.
Approximately what percentage of total power goes to overhead?