PUE Is Not Necessarily a Measure of Sustainability
PUE - Power Utilization Effectiveness - is a common measure of energy efficiency for ICT facilities. We have often cited it in our articles, but we are mindful that efficiency does not necessarily lead to sustainability. Here are some areas where PUE can fall short.
Reducing GHG is a critical aspect of Green ICT, so the energy source is important in assessing sustainability. Electricity from natural gas can emit half the GHG/KWh of coal generation. A coal-based data center with a PUE of 1.5 still creates 50% more GHG from electricity generation than a same-size gas-based one with 2.0 PUE.
Electricity from renewable sources - wind, solar, geothermal or hydro - typically emit GHG at 10% or less compared to natural gas. One cannot imagine a renewables-based data center with a PUE high enough to create more GHG than a same-size gas-based one with 1.5 PUE.
Another way in which PUE can mask sustainability issues in its definition of efficiency as the ratio of ICT gear's consumption to the facility's total energy consumption. This rewards reducing the energy consumption of non-equipment elements like cooling. A facility that first reduces the energy consumption associated with cooling will see its PUE improve. When it improves equipment energy consumption as the second initiative, it will see its PUE worsen, even as its total energy consumption continues to drop.
1MW data center
PUE of 2.0
Green ICT phase 1:
Reduce electricity required for cooling and other facility elements by 50%
Consumption improves to 0.75 MW
PUE improves to 1.5
Green ICT phase 2:
Reduce electricity required to power gear by 50%
Consumption further improves to 0.5 MW
PUE now worsens to 2.0
Facilities tempted to game their PUE can ignore equipment energy use to do so.
The above is an example of a perverse incentive. Another was reported by the New York Times about Microsoft's Quincy, WA, data center in late 2011. "In an attempt to erase a $210,000 penalty the utility said the company owed for overestimating its power use, Microsoft proceeded to simply waste millions of watts of electricity, records show. Then it threatened to continue burning power in what it acknowledged was an “unnecessarily wasteful” way until the fine was substantially cut, according to documents obtained by The New York Times." Nice PUE stats can fall by the wayside when big money is at stake.
Finally, PUE does not address the energy consumption or GHG emissions embodied in a facility's equipment's lifecycles.
This is all a good reminder that meaningful Green ICT efforts require comprehensive understanding and multifaceted action.
We found reinforcement from a WSP/NRDC study shortly after we wrote the above. "While PUE remains a key opportunity for efficiency improvements in server rooms, the potential emissions savings from improving PUE may be less than what can be achieved from increasing server utilization or using cleaner electricity." Consistent with the above analysis, increasing server utilization through tactics like server consolidation could worsen PUE while lowering overall energy consumption and GHG emissions.