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May 24, 2013

Uninterruptible Power Systems

Executive Summary

An uninterruptible power supply (UPS) is an electricity storage system that is used to reduce or avoid negative effects and costs associated with electrical service outages and/or poor power quality (PQ). During outages, the stored energy is used to maintain power to specific end-user’s electricity using equipment (aka: load). UPSs also filter and otherwise offset power quality anomalies, continuously. 

The UPS value proposition is familiar and well established. Consider that there is significant ongoing demand and that use of UPSs is fairly common in residential, commercial and industrial settings. UPS power ratings range from a few hundred Watts for a computer to a few MegaWatts (MW) for entire facilities. 

The bases for this value proposition are related to providing stable, reliable power with the power quality characteristics needed to serve loads. More specifically, UPSs provide stable and reliable power to: a) serve critical safety-related loads such as emergency lighting and medical equipment, b) reduce or avoid lost productivity, c) reduce or avoid production damage (i.e., for manufactures), d) reduce or avoid product or facilities damage (e.g., food spoilage) and e) reduce or avoid electricity-using equipment damage.


The UPS business is somewhat mature – there are millions of small under-desk units and many larger units in service. As such, the UPS value proposition is established and well understood by a broad range of stakeholders. 

To the extent that the benefit can be assessed, it is related to reduced or avoided costs that would be incurred (without the UPS) due to electrical service outages or poor power quality. Of particular note are: a) medical, safety and emergency response related loads, b) damaged inventory on-hand, c) damaged product being manufactured, d) process cleanup and re-start costs such as those for injection molding operations and other manufacturing lines, d) damage to electricity-using equipment and e) lost sales and revenues. 

For “must-run” operations, much of the benefit is not readily quantifiable. For example, hospitals, police and fire agencies, and some other government agencies need reliable, high quality power to function. In that case, the “benefit” may be substantially qualitative.

UPS for Electrical Service Outages

When electrical service is disrupted (i.e., an electrical service outage), the UPS activates and “picks up” the load connected to it. The UPS may provide power for durations ranging from 10 to 20 seconds to several hours. Shorter duration UPSs are designed to: 1) carry the load during the start-up of back-up electrical generators, typically diesel engine driven generators, and 2) enable a smooth transition to the generator as the power source. In many cases, the UPS is designed to provide power for five to 20 or perhaps 30 minutes. The purpose is to enable an orderly shutdown of operations – thereby avoiding an abrupt shutdown which would otherwise cause equipment damage, product/work losses or a security/safety hazard. The under-desk UPS for PCs is an example.

UPSs with enough energy to provide power for several hours are somewhat rare. A key reason is that, in most situations, it is less expensive to store energy in the form of diesel fuel (for generators) if power is needed for several to many hours. Grid-connected storage may be preferable if noise or air emissions are an issue or if diesel fuel cost is high.

UPS for Power Quality

Some UPSs are designed to offset or filter out electricity with poor power quality. Poor power quality is related to electricity that does not have the necessary characteristics – such as voltage and frequency stability – that is needed for electricity using equipment to function correctly. The results can be similar to those for complete electrical service outages including electricity-using equipment shut-down and damage, and lost productivity or inventory, and interruptions to must-run operations.

Poor power quality can take several forms and have several characteristics. Some common ones include the following:

  • Spike – A high, short duration (milliseconds or microseconds) voltage increase. Most spikes are caused by lightning or when electrical service is restored after an outage. Spikes are especially damaging to electronic equipment.
  • Swell or Surge – Electrical service voltage exceeds the nominal voltage for one second to one minute. A common cause is high power equipment shutting off. Common examples of such high power equipment include large pump motors, air conditioning equipment and manufacturing processes.
  • Sag or Undervoltage – Electrical service voltage exceeds the nominal voltage for one second to one minute. A common cause is high power equipment turning on. If the sag lasts long enough and/or is severe enough then electricity using equipment will shut off and may be damaged.
  • Noise – A disturbance of the smooth flow of electricity in the form of electro-magnetic interference (EMI) or radio frequency interference (RFI). Common sources of noise are motors and electronic devices. Noise can affect performance of some types of electricity using equipment and can affect software operation and electronic data.
  • Current Harmonics – Distortions of the normal sine wave form of the alternating current electricity provided by the utility at 60 Hertz or cycles per second. Harmonics tend to exist in multiples of the 60 Hertz grid frequency, such as 3x (180 Hertz) and 6x (360 Hertz). Some types of electricity using equipment cause harmonics, especially electronics, by distorting the current wave form. Harmonics can cause significant voltage distortion and in a “three-phase system” harmonics may cause overheating of the “neutral” line which can lead to damage or fire.

Synergies with Other Benefits and Uses

An attractive facet of this value proposition is the prospect of adding UPS functionality to storage that is deployed for other purposes at low incremental cost. To add UPS functionality, a storage system would need to have several minutes of additional discharge duration (run-time) and the necessary electronics to address poor power quality. UPS functionality could be added to storage deployed for user bill management and for on-site photovoltaics integration. UPS functionality could also be added to utility-owned storage used for community energy storage.

Conclusions and Observations

UPSs provide billions of dollars of benefits annually. They are proven and well known. As storage technology improves, and as electricity storage becomes more common for other benefits and uses, many more prospective users may want the reliability, convenience and safety that UPSs can provide.

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