Running A UPS In ECO Mode: Efficiency Vs Reliability (White Paper)

A look at ECO mode and whether the energy savings on offer are worth the trade-off in reliability and protection.

With energy savings never far from the top of most data centre operators’ and facilities managers’ minds, UPS manufacturers continue to develop ever more efficient models.

Many modern uninterruptible power supplies can reach efficiency ratings of up to 97% even in double-conversion online mode. While most manufacturers now provide versions featuring ‘economy’ operating modes offering enhanced efficiencies.

This white paper explores the broader concept of UPS efficiency before outlining how these ECO operating modes work. It goes on to examine whether the energy savings they deliver are worth the corresponding trade-off in reliability and protection.

Explaining UPS Efficiency

Let’s start with the concept of UPS efficiency. What does it mean?

It relates to the ratio of power entering the UPS to the power exiting to supply the load. Whenever current passes through the components of the UPS, a certain amount of energy dissipates as heat and sound, resulting in energy losses.

So in practice, a UPS system with 95% efficiency rating will see 95% of the input powering the load and the remaining 5% ‘wasted’ running the UPS itself.

The efficiency ratings manufacturers publish tend to be based on running in online mode and depend on the peak of the efficiency curve, which can be anywhere from 30% through to 80% load.

As a general rule, as the load changes so too does the efficiency. Compared to UPS of the past, most modern transformerless models of today have a far flatter efficiency curve and are still capable of high efficiency (>95%) even at loads of just 20-25%.

Now over the course of a UPS’s 10-15 year lifespan, even a 1-2% improvement in operating efficiency can add up to substantial energy savings.

And as well as the financial benefits, higher UPS efficiency also has a positive environmental impact. It reduces CO2 emissions and overall power consumption.

How Does ECO Mode Work?

Pretty much every double-conversion online UPS system today offers users the choice of a dedicated economy or energy-saving mode.

ECO mode is also commonly referred to as ‘high efficiency mode’, ‘bypass mode’, ‘energy saving mode’ or ‘active standby’.

It operates in a similar way to an offline/standby UPS, where the inverter is switched off and on standby while the bypass line (i.e. raw mains supply) powers the load.

If there’s an issue with the mains, the load experiences a fractional break in supply while the automatic bypass transfers it back to the inverter.

The main benefit of running in ECO mode is a major boost to UPS efficiency, which means a reduction in electrical losses.

Typically, operating in ECO mode increases UPS efficiency to more than 99% compared to 93-97% for systems running in online mode. So that’s a difference of anywhere between 2-6%.

For energy intensive environments such as a data centre, ECO mode usage could theoretically lead to thousands of pounds worth of energy savings a year.

In addition, some of the internal components inside the UPS’s rectifier and inverter are less stressed during ECO mode operation, reducing natural wear and tear and increasing their lifespan.

ECO Mode Drawbacks

The major shortcoming with ECO mode is that the IT load is completely exposed to the raw mains utility without any of the power conditioning provided by double-conversion online UPS.

If there’s a clean and stable mains supply and low harmonic generating loads, ECO mode works with relatively little risk. But if there’s any issue with the quality of the mains, then it could cause big problems.

In ECO mode the UPS needs to continuously monitor the mains and quickly switch back to the inverter when it detects a problem.

However, no UPS comes with a crystal ball. Whenever there’s an issue with the raw supply, the UPS must go through the following process:

  • Detect the power problem
  • Determine whether and how to respond
  • Energise the inverter
  • Open the static bypass switch
  • Transfer the load onto the inverter output

All of this takes time. While in practice this process may only take between 1-16 milliseconds, that’s vital time during which the critical load is subjected to the power problem.

And while a typical server may be able to override even a 16ms loss of power, just an 8ms loss to a transformer can cause it to surge when voltage is restored to normal levels, leading to it tripping breakers.

Similarly, power loss for even a few milliseconds on a circuit feeding a PDU with a static switch will result in a state change on that switch. This can lead to a wider state change on the overall power system, including overloads or dropped loads.

In addition, there are many devices, such as switches, pumps, or fans, that aren’t as robust at handling short-term dips and sags as servers are.

Finally, a typical static bypass will not open instantaneously. So even if the UPS detects a mains overvoltage, in ECO mode it can’t protect the load.

As well as the loss of electrical protection, operating in ECO mode runs the risk of several other side effects. These include increased harmonics, thermal shock, reduced battery lifespan, and an impact on fault clearing.

How ECO mode behaves is linked to the site where it is used, the mains power quality, and the effect of the mains on other loads within the facility.

End-users are able to adjust the settings on their UPS to match their own specific circumstances. This requires significant testing to achieve the optimum balance.

If the sensitivity is too high, the UPS could overreact to small mains disturbances. On the flipside, if the setting is too low, the UPS could take too long to react to a potentially damaging power problem.

The Active ECO Alternative

In the last few years, advances in firmware control and electrical designs have led to the introduction of an additional operating mode: Active ECO (also known as Advanced ECO).

As with standard ECO mode, Active ECO sees the load powered through the bypass line i.e. the mains supply. The big difference is that the inverter remains on at all times and runs in parallel with the input, without actually carrying the load current.

Because the inverter is already ‘on’, it can take over the load far quicker than standard ECO mode in the event of a mains failure.

Active ECO offers another advantage over standard ECO mode, namely power filtering. As mentioned previously, standard ECO mode doesn’t offer any of the shielding from harmonics or power factor that you find in double-conversion online UPS mode.

But with Active ECO, the inverter is on and connected to the output, enabling it to correct the waveform and power factor.

However, there is a trade-off when using Active ECO. Efficiency is reduced slightly by between 0.5-1% because the inverter circuit is on.

Of course, this reduced efficiency is still higher than traditional online operating mode offers, so has come to be seen as something of a happy medium.

Weighing Up The Pros And Cons

ECO mode does have a role to play, but for mission-critical sites in particular, it should be used sparingly. A good example would be overnight or out of hours when a site’s critical loads are inactive.

Another alternative use could be in a N+X parallel redundant installation. One of the UPS would operate in online mode as the primary unit, while the remainder run in ECO mode until they’re required to actively support the load

While running in ECO mode undoubtedly delivers energy savings and reduced total cost of ownership, it does introduce a greater level of operational risk.

That’s why it’s not a fair comparison to liken its use to that of free cooling within a data centre, which some organisations have suggested.

And with the risks seemingly outweighing any perceived rewards, many data centre operators and plant owners are still reluctant to use it. However, the introduction of more sophisticated alternatives such as Active ECO, which mitigate the risks whilst still delivering higher efficiencies, offers something of a happy medium.

As well as downloading the whitepaper below, you can also read an article we’ve contributed to Data Centre Management (DCM) and Electrical Review magazines outlining the risks and rewards of running in ECO mode.