How UPS Eco-modes Save Energy and Reduce Data Center Emissions

Written by Art Salazar on Thursday, February 22nd 2018 — Categories: Colocation, Data Center Design, Green Data Center

Data centers are invariably focused on 100% availability, which comes down to reliability of power and various mechanical and electrical components throughout the facility. But energy efficiency is a major priority as well, even for data centers that don’t call themselves “green” or “sustainable”.

With electricity providing a bulk of the operating expense, any gains in efficiency can go a long way towards minimizing OpEx. Many data center efficiency measures focus on containment, cooling, and other measures within the white space, but critical power infrastructure can be a good target for efficiency gains as well.

Major UPS manufacturers often include an “ecomode,” or in the case of our Cheyenne data center, Eaton’s Energy Saver System (ESS). These modes can lead to efficiency gains of several percentage points, which sounds low, but in practice can lead to thousands of dollars of savings and carbon emission reductions in the hundreds or thousands of pounds.


Load Sharing and Typical UPS Efficiency

In an N+1 or greater facility, there are multiple Uninterruptible Power Supplies to cover a single maximum IT load. They are all powered on, so while a single one of the UPS could take on 100% of the load, the majority of facilities will share the load between them, often about 40-50% between each. This places less stress on each UPS and extends its lifespan while reducing the overall risk of failure.

At a 40% load level, a modern UPS can operate at around 95% efficiency. But where does that 5% of the electricity go? Typically the UPS will be running in double conversion mode. The incoming electricity from the utility provider is converted from AC to DC by the rectifier, which sends a charge to the batteries and passes on the current to the inverter. The inverter then converts electricity from DC back to AC for the IT load to use.

This helps keep any sudden changes in the electrical load from the utility, like sudden spikes or sags, from reaching the IT load, while also maintaining the battery levels. It also reduces the switching time from utility feed to battery power should there be a power outage, as the utility feed does not go directly to the IT load.

The switches within the rectifier and inverter lead to additional resistance, which leads to the generation of heat and a loss of energy from the electrical current. That can reduce the efficiency of the double conversion UPS by 5% on average.

How much energy can 5% be?

An efficiency rating of 95% seems pretty good, especially for the increased reliability a double conversion UPS offers. For our 5MW of power in Cheyenne, however, that adds up to over 2,500,000 kWh of electricity a year (at maximum load). Meanwhile, the additional heat generated by the switches also adds to the overall heat within the UPS and/or battery rooms in the data center, which require their own cooling systems – an additional expense.

According to the EPA, that lost electricity is equivalent to driving an average vehicle 4,560,163 miles, burning 2,035,604 pounds of coal, or providing electricity to 279 homes for a year. In other words, that measly 5% adds up to a very significant amount of energy in a midsize data center.

By operating our UPS in Energy Saver System mode, the efficiency rate is closer to 99%, allowing us to reduce our carbon footprint at maximum 5 MW load by approximately 1,800 metric tons (again according to EPA equivalencies). At this maximum load, we would also be saving between $125,000 and $250,000 per month.

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Why doesn’t everyone run in eco-mode?

With that kind of energy savings, you would think that everyone would run their UPS in eco or ESS mode. Just think about how much that takes off the power bill for a 10 or 20 MW facility!

These modes do come with a slight additional risk to the power supply, however. As mentioned above, a double conversion UPS provides safety by regulating the power supply so the utility feed does not go directly to the IT systems. It also virtually eliminates the time to switch to batteries, which further reduces the risk of downtime.

The UPS includes a Static Bypass, which has a static switch for rapid change between power sources, but nothing else between the utility and IT loads. This mode is often used during maintenance.

An eco or ESS mode will essentially place the UPS in bypass mode, while also keeping the regular path of rectifier –> battery –> inverter in a state of “extraordinary readiness,” with their control circuitry powered up and ready to use at a moments notice. The semiconductors and switches are powered off, receiving no electricity so they do not generate heat and energy loss.

This sounds risky, because if a power spike or sag comes down the line form the utility, it seems as though it could easily impact the IT load. The static switch and control circuits for the rectifier/inverter are monitoring the load at all times even with ESS or eco settings turned on, reducing the risk significantly. The guaranteed switching time between ESS and double conversion is 2 milliseconds, but it is often even less, so when a power hiccup is detected, the system automatically and rapidly changes back to double conversion mode, or in the case of total power loss, on-battery mode.


Green House Data has found the use of energy-saving modes within our double-conversion Uninterruptible Power Supplies to be a valuable way to reduce energy consumption and our overall environmental footprint. While there was some fine-tuning involved, the automated transition between ESS mode and double conversion falls well within federal and international standards for IT and information processing, so the power transition itself remains invisible to critical IT infrastructure on the downstream side of the UPS. In other words, your IT equipment receives the same level of power protection as a traditional UPS even while eco mode is enabled.