A Systematic Review of Energy Efficiency Metrics for Optimizing Cloud Data Center Operations and Management
Abstract
1. Introduction
1.1. Background and Statistics
1.2. Systematic Review Methodology
1.3. Literature Review
1.4. Existing Gaps and Contribution
- The complete analytics of the energy efficiency metrics of CDCs;
- Presenting the energy-consuming components of a CDC;
- Describing different centralized and decentralized setups of Uninterruptible Power Supplies (UPSs) and Power Distribution Units (PDUs) in CDCs;
- Providing the challenges, limitations, and the associated potential research directions for each metric.
1.5. Paper Structure
2. Energy Management in Cloud Data Centers
2.1. Servers and Racks
2.2. Uninterruptible Power Supply (UPS)
2.3. Power Distribution Unit (PDU)
- Centralized (Figure 5): Electricity from a single UPS is distributed to multiple PDUs, which then channel the power to server racks. To avoid any delay in switching to UPS power, CDCs are equipped with double conversion UPS systems.
- Distributed (Figure 6 and Figure 7): Instead of a centralized UPS, a battery cabinet serves every multiple rack, supporting the servers (Figure 6). This approach eliminates double conversion by modifying server power supply units to accept both AC power from the grid and DC power from the battery cabinet. The battery cabinet distributes DC power directly to the servers. As another form of distributed UPS, a battery is integrated into each server following the UPS. This configuration eliminates the AC/DC/AC double conversion, enhancing energy efficiency during normal operation, and it positions AC distribution closer to the IT load before conversion (Figure 7).
2.4. Information Technology (IT) Rooms and Equipment
2.5. Heating, Ventilation, and Air Conditioning (HVAC)
2.6. Facility Security, Lighting, and Offices
3. Energy Efficiency Metrics in Cloud Data Centers
- IT-related metrics (Table 2 and Table 3): Measurements that evaluate the energy performance of computing and networking components. These metrics assess how effectively IT resources utilize energy to perform computational tasks, focusing on the ratio of computational output to energy consumed, the degree of resource utilization, and the adaptability of power consumption to workload variations.
- Non-IT-related metrics (Table 4 and Table 5): These are used in the evaluation of supporting infrastructure such as power distribution, cooling systems, and building facilities. These metrics measure the proportion of energy used by non-IT systems relative to total energy consumption, with an emphasis on minimizing overhead and improving the efficiency of physical infrastructure and environmental controls.
4. Real-World Examples
5. Challenges and Future Works
5.1. Challenges
5.1.1. IT-Related Metrics
5.1.2. Non-IT-Related Metrics
5.2. Future Works
5.2.1. IT-Related Metrics
5.2.2. Non-IT-Related Metrics
6. Conclusions
Supplementary Materials
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
Abbreviations
AC | Alternating Current |
AI | Artificial Intelligence |
APC | Average Power Consumption |
CADE | Corporate Average Data center Efficiency |
CAGR | Compound Annual Growth Rate |
CDC | Cloud Data Center |
CER | Cooling Effectiveness Ratio |
CPE | Compute Power Efficiency |
CPU | Centeral Processing Units |
CRAC | Computer Room Air Conditioning |
CRAH | Computer Room Air Handler |
CUE | Carbon Usage Effectiveness |
DC | Direct Current |
DC-FVER | Data Center Fixed to Variable Energy Ratio |
DCA | Data Center Availability |
DCeP | Data Center energy Productivity |
DCgE | Data Center green Efficiency |
DCiE | Data Center infrastructure Efficiency |
DCPD | Data Center Power Density |
DCPE | Data Center Performance Efficiency |
DH-UE | Data Hall Utilization Efficiency |
DH-UR | Data Hall Utilization Rate |
DT | Digital Twin |
DWPE | Data Center Workload Power Efficiency |
EBS | Energy Baseline Score |
ESG | Environmental, Social, and Governance |
EWR | Energy Wastage Ratio |
GEC | Green Energy Coefficient |
GPU | Graphical Processing Unit |
H-POM | Hardware Power Overhead Multiplier |
HVAC | Heating, Ventilation, and Air Conditioning |
IEA | International Energy Agency |
IoT | Internet of Things |
IT | Information Technology |
ITEE | IT Equipment Energy Efficiency |
ITEU | IT Equipment Utilization |
LED | Light-Emitting Diode |
LIB | Lithium-Ion Battery |
NN | Neural Network |
NSERC | Natural Sciences and Engineering Research Council of Canada |
OSWE | Operational System Workload Efficiency |
PDE | Power Delivery Efficiency |
PDU | Power Distribution Unit |
PEsavings | Power Efficiency Savings |
PPA | Power Purchase Agreement |
PpW | Performance per Watt |
PRISMA | Preferred Reporting Items for Systematic reviews and Meta-Analyses |
PUE | Power Usage Effectiveness |
PUEreliability | PUE Adjusted for Reliability |
RAM | Random Accessible Memory |
RCI | Rack Cooling Index |
RES | Renewable Energy Source |
ScE | Server compute Efficiency |
SI-POM | System Infrastructure Power Optimization Metric |
SPUE | Server Power Usage Effectiveness |
SWaP | Space, Wattage, and Performance |
TUE | Total Utilization Efficiency |
UPS | Uninterruptible Power Supply |
VM | Virtual Machine |
WUE | Water Usage Effectiveness |
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Cooling Strategy | Methodology | Advantages | Disadvantages | Examples |
---|---|---|---|---|
Air-Based Cooling | Uses CRAC or CRAH units to circulate air around servers. | Simple and widely adopted; low initial cost. | High energy consumption; inefficient for high-density workloads. | [109,110,111,112,113,114,115,116] |
Cold/Hot Aisle Containment | Separates hot and cold air paths, using containment structures. | Improves energy efficiency and reduces hot spots. | Requires proper planning and layout; retrofitting is difficult. | [110,117,118,119,120] |
Direct-to-Chip Liquid Cooling | Delivers coolant directly to components, such as CPUs, via cold plates. | High cooling efficiency; ideal for high-performance systems. | Higher cost; risk of leaks. | [121,122,123,124,125,126] |
Immersion Cooling | Submerges components in dielectric fluid for heat transfer. | Excellent thermal performance; quiet operation. | Expensive setup; fluid compatibility and maintenance complexity. | [127,128,129,130,131,132,133,134,135,136] |
Rear Door Heat Exchangers | Cooled water absorbs heat via coils mounted at the back of racks. | Scalable and effective for dense racks. | Adds rack weight; complex plumbing. | [137,138,139,140,141,142,143,144,145] |
In-Row Cooling | Places cooling units between server racks for localized cooling. | Targeted cooling; reduces airflow inefficiencies. | High cost; depends on data center layout. | [146,147,148,149,150,151,152,153,154,155,156] |
Evaporative Cooling | Uses evaporating water to pre-cool intake air. | Very energy-efficient in dry climates. | Ineffective in humid climates; water treatment needed. | [109,157,158,159,160,161,162,163,164,165] |
Chilled Water Cooling | Chiller cools water that circulates through air handlers. | Suitable for large-scale operations; reliable. | Expensive infrastructure; needs regular maintenance. | [166,167,168,169,170] |
Economizers | Uses outside air or water for cooling when ambient conditions allow. | Major energy savings; eco-friendly. | Weather-dependent; air filtration often required. | [171,172,173,174,175,176,177,178,179,180] |
Hybrid Systems | Combines multiple cooling methods (such as air + liquid, or free cooling). | Flexible and efficient under varying loads. | High complexity and initial setup cost. | [123,140,180,181,182,183,184,185,186,187,188,189,190] |
Metric | Definition | Primary Use | Examples |
---|---|---|---|
APC | Average power usage of IT equipment over time. | Monitors IT power trends. | [199,200,201] |
CPE | Computes output per unit of IT power. | Measures IT computational efficiency. | [202,203,204,205,206] |
DWPE | Workload processed per unit of IT power. | Workload efficiency at IT level. | [51,207] |
EWR | Energy wasted per unit of computational work. | Energy cost of work. | [208,209] |
ITEE | Efficiency of IT hardware. | Evaluates hardware efficiency. | [210,211,212] |
OSWE | System workload output per total energy. | Measures system-level workload efficiency. | [51] |
PpW | Performance per unit of power. | Measures IT hardware efficiency. | [213,214,215,216] |
ScE | Server compute output per energy used. | Assesses server-level efficiency. | [217,218,219] |
SPUE | PUE-like metric for servers. | Server-specific efficiency. | [220] |
SWaP | Composite of performance vs. space and power. | Space–power performance balance. | [221,222,223] |
Metric | Formulation | Units | Best Value | Correlation with Other Metrics |
---|---|---|---|---|
APC | W | Lower is better | ↑ (ITEE), ↓ (PpW, CPE, ScE) | |
CPE | Ops/W | ↑ (PpW, ScE); ↓ (APC) | ||
DWPE | Tasks/W | ↑ (ITEE, CPE); ↓ (APC) | ||
EWR | kWh/Task | 0 | ↓ (CPE, PpW, ScE); ↑ (APC) | |
ITEE | % | 1 | ↑ (PpW, CPE); ↓ (EWR) | |
OSWE | % | 1 | ↑ (DCeP, DCPE); ↓ (PUE) | |
PpW | Perf/W | ↑ (CPE, ScE, ITEE); ↓ (EWR, APC) | ||
ScE | Ops/W | ↑ (PpW, CPE, ITEE); ↓ (EWR) | ||
SPUE | % | 1 | ↑ (PUE); ↓ (PpW, ScE) | |
SWaP | Ops/m2W | ↑ (DCPD); ↓ (EWR) |
Metric | Definition | Primary Use | Examples |
---|---|---|---|
CADE | Corporate-level efficiency | Corporate energy assessment | [51,224,225] |
DCA | Data Center Availability | Measures uptime reliability | [226,227] |
DCeP | Energy productivity | Productivity benchmark | [228] |
DCgE | Green energy efficiency | Green energy impact | [51,221,228,229,230] |
DCPD | Power density | Space optimization | [231] |
DCPE | Performance per energy | Performance efficiency measure | [51,232] |
DC-FVER | Fixed to variable energy | Cost structure insight | [233,234] |
DH-UE | IT floor space utilization | Space optimization | [235] |
DH-UR | Rack utilization rate | Rack deployment insight | [236,237,238,239] |
EBS | Baseline comparison score | Tracks savings | [240,241,242,243] |
H-POM | Non-computational (overhead) power consumed by hardware components | Includes everything consumed by the hardware | [244,245] |
PDE | Power delivery efficiency | Power loss assessment | [51,246] |
PEsavings | Energy efficiency savings | Savings tracking | [199,247] |
SI-POM | Infrastructure optimization | Infrastructure focus | [51] |
TUE | Holistic utilization | Total resource usage | [248] |
PUE | Total vs IT energy | Efficiency benchmark | [36,249,250,251,252,253,254,255] |
DCiE | Infra efficiency | Infrastructure energy ratio | [206,256] |
CUE | Carbon emissions per IT energy | Carbon footprint metric | [257] |
Metric | Formulation | Best Value | Units | Correlation with Other Metrics |
---|---|---|---|---|
CADE | 1 | Ratio | ↑ (ITEU, DCiE); ↓ (PUE) | |
DCA | 1 | Ratio | ↓ PUEreliability | |
DCeP | 1 | Output/W | ↑ (DCPE, OSWE); ↓ (PUE) | |
DCgE | 1 | Ratio | ↑ (GEC, DCiE); ↓ (CUE) | |
DCPD | Higher varies | W/m2 | ↑ (SWaP); ↓ (RCI) | |
DCPE | 1 | Output/W | ↑ (DCeP, OSWE); ↓ (PUE) | |
DC-FVER | Lower is better | Ratio | ↓ (CER) | |
DH-UE | 1 | Ratio | ↑ (DH-UR, TUE) | |
DH-UR | 1 | Ratio | ↑ (DH-UE, TUE) | |
EBS | 1 | Ratio | ↑ (PUE); ↓ (PEsavings) | |
H-POM | Custom Formula | 1 | Ratio | ↑ (ITEU, GEC, DCiE); ↓ (PUE) |
PDE | 1 | Ratio | ↑ (DCiE); ↓ (PUE) | |
PEsavings | 1 | Ratio | ↑ (DCiE); ↓ (PUE, EBS) | |
PUEreliability | 1 | Ratio | ↑ (PUE); ↓ (DCA) | |
SI-POM | Custom formula | 1 | Ratio | ↑ (CER, PDE); ↓ (PUE) |
TUE | 1 | Ratio | ↑ (ITEU, DH-UR, DH-UE); ↓ (PUE) | |
PUE | 1 | Ratio | ↓ (DCiE, PDE); ↑ (CUE) | |
DCiE | 1 | Ratio | ↑ (PDE); ↓ (PUE) | |
CUE | 0 | KgCO2/kWh | ↑ (PUE) |
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Safari, A.; Sorouri, H.; Rahimi, A.; Oshnoei, A. A Systematic Review of Energy Efficiency Metrics for Optimizing Cloud Data Center Operations and Management. Electronics 2025, 14, 2214. https://doi.org/10.3390/electronics14112214
Safari A, Sorouri H, Rahimi A, Oshnoei A. A Systematic Review of Energy Efficiency Metrics for Optimizing Cloud Data Center Operations and Management. Electronics. 2025; 14(11):2214. https://doi.org/10.3390/electronics14112214
Chicago/Turabian StyleSafari, Ashkan, Hoda Sorouri, Afshin Rahimi, and Arman Oshnoei. 2025. "A Systematic Review of Energy Efficiency Metrics for Optimizing Cloud Data Center Operations and Management" Electronics 14, no. 11: 2214. https://doi.org/10.3390/electronics14112214
APA StyleSafari, A., Sorouri, H., Rahimi, A., & Oshnoei, A. (2025). A Systematic Review of Energy Efficiency Metrics for Optimizing Cloud Data Center Operations and Management. Electronics, 14(11), 2214. https://doi.org/10.3390/electronics14112214