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Uninterruptible Power Supplies

ups - display ups
UPS systems improve power quality and provide limited backup power to sensitive electronics. (Photo: David Jones)

Uninterruptable power supplies (UPS) are a class of devices that power equipment in the event of grid power failure, nearly instantaneously, protecting the equipment from damage.  UPS systems vary significantly in their design and functionality, affecting the amount of time they can power equipment, their ability to improve power quality and their cost.

UPS systems are used in a variety of applications, including data servers, computer systems, industrial settings and laboratories.  Because UPSs protect equipment, they are appropriate for any situation where electrical loads may be sensitive to power loss or other power quality issues.  UPS systems are commonly used for computers and servers, for example, because power loss to these loads may result in loss of data or component damage.

Likewise, many types of medical and laboratory equipment are sensitive to interruptions in power supply or poor quality power.  For many health facilities in developing nations, grid power is unreliable or of poor quality, resulting in scheduled or unscheduled power loss for large portions of the day, or fluctuations in grid voltage that may adversely affect equipment.  

In addition to the potential damage to equipment, power loss in hospitals and laboratories leads to downtime, affecting the quality and availability of their critical services.  Thus health facilities often employ backup power systems to meet electrical loads in the case of power loss from the grid.  

UPS systems serve two main purposes: 1) to provide backup power as quickly as possible in the event of power loss and 2) to offer some degree of protection from power quality issues that may damage equipment.  UPS systems will fulfill these goals to varying degrees depending on their design and features, which ultimately affect cost.

Meeting power supply challenges at health facilities is important to ensuring quality healthcare services.  This paper will discuss some of those challenges, explain appropriate uses of UPS systems and differences between UPSs and similar devices.  Finally, lessons learned and best practices regarding the use of UPS systems in health facilities with poor grid access will be presented.


IHFI Experience 

USAID’s Improving Health Facility Infrastructure (IHFI) project addresses problems with energy infrastructure in the health sector in facilities with limited resources.  UPS systems are a critical piece of energy infrastructure in health settings and IHFI has a great deal of experience in implementing such systems in through cost-effective and sustainable approaches.  Much of the information in this article relating to system design, selection and load characterization is directly relevant to IHFI’s activities.

IHFI is confronting power quality and supply problems in Haiti through the use of battery/inverter systems.  These systems are designed to provide backup power to important electrical loads in health facilities, like laboratory and medical equipment, computers, lighting and ventilation fans.  In addition to backup power these systems deliver a constant supply of clean, AC power, free of any voltage or frequency irregularities, to sensitive laboratory electronics.  Essentially, these systems function in the same way as a UPS system, using similar components.  

IFHI’s systems typically comprise a bank of batteries, 2-4 inverter/battery charge controller units, a system monitor and all supporting wiring and electrical hardware.  Each inverter powers a single circuit and is connected to a series of batteries, sized to fit the connected loads.  Usually, one circuit is designated the “no contact” circuit for sensitive laboratory equipment, and has a dedicated inverter that operates like a double conversion UPS, providing clean power at all times.  The remaining inverters power “contact” circuits and are only needed to supply battery power in the event of grid power loss.

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IHFI inverters in Haiti, two units are on “contact” circuits, while the third covers “no-contact” loads. (Photo: Kim Domptail)

This modular system type allows for flexibility in design and sizing; battery bank and inverter capacity can be tailored to the needs of individual facilities.  It is also a cost effective approach to installing the backup power and power quality capabilities of a UPS, but with greater backup time, or autonomy, through increased battery capacity.  In Haiti, backup power capacity is important, as grid power can be unavailable for long periods and the generators on-site are not always reliable.

Another benefit of the modular battery/inverter design is standardization of materials and maintenance procedures.  IHFI has installed approximately 20 of these systems in health facilities throughout Haiti; being able to use a consistent approach to system design and procurement over a diverse set of facilities promotes sustainability and familiarity on the part of facility administrators and energy technicians.  This has also allowed IHFI to implement a standard battery maintenance training program for technicians at every facility.  Were a mixed group of UPS and battery backup systems put in place it would be more difficult to maintain and monitor them in a consistent manner. 

The appropriateness of different types of UPS systems, whether they are battery/inverter systems, or double conversion or standby units, depends on the power supply and quality situation and load characteristics of a particular facility.  In Mozambique, the IHFI project surveyed eight health labs to determine the best UPS option.  The final recommendation was to install large, unitary double conversion UPS units to cover “no contact” loads, rather than battery/inverter systems or multiple smaller UPSs.  

Circumstances at the labs in Mozambique differed from those in Haiti.  The labs have reliable access to grid power and backup generators were already in place.  The main concern, then, was to protect sensitive loads from power quality problems.  Double conversion UPS units are ideally suited to this situation as they provide clean power but have only a limited amount of backup capacity.


Types of Uninterruptible Power Supplies 

UPS systems provide a comprehensive, modular solution to protecting sensitive equipment from power supply problems.  There are a variety of power quality issues that are commonplace which UPS systems and other, similar equipment address.  UPS systems come in several configurations that offer different levels of protection for a range of costs.  The following is a brief description of each, a discussion of the power quality issues they are built to solve and a comparison of their most important features.


Double Conversion or On-line UPS

A double conversion or on-line UPS system provides full protection and electrical isolation from power quality issues while ensuring an instantaneous source of backup power in the event power loss from the grid.  

This type of UPS offers the highest level of protection by fully isolating connected loads from grid power.  AC power from the grid is converted into DC power before being converted back into AC power again.  This AC power output has perfect voltage and frequency characteristics, therefore addressing all potential power quality issues.  Furthermore, internal capacitors store energy throughout the conversion process, providing a seamless transition from grid to battery power.

The basic configuration of a double conversion UPS is shown below.  This figure presents the essential power conversion components, but a number of other components are involved in the process, including capacitors, transformers and bypass circuits.


double conversion UPS


These systems have a higher cost than either standby or line-interactive UPSs, but give superior protection.  A good deal of energy is lost through the double conversion process, however, so the efficiency of these systems is less than that of the other UPS options.


Standby or Off-line UPS

In a standby or off-line UPS system, the load is supplied power directly from the grid, with no power conditioning or protection other than basic surge protection, or in some cases, noise filtering.  When grid power is lost, power is supplied from the system’s internal battery.

This is the least expensive type of UPS system, and provides the lowest level of protection.   Furthermore, inexpensive models often produce a square-wave rather than a perfect sine-wave when converting DC battery power to AC power, which could be damaging over time to some sensitive equipment.  A schematic is shown below.


standby UPS


Line-interactive UPS

Line-interactive UPS systems offer another level of protection over the basic standby system.  These systems provide a degree of power conditioning by regulating the voltage of the incoming grid power.  This functionality does not provide perfect “clean” power, or isolate loads from all power quality issues, but it does solve basic issues such as under-voltage and over-voltage, which can be common.

As the second-tier option for UPS systems, line-interactive UPSs have all of the functionality of a standby UPS plus some basic voltage regulation.  This class of UPS is priced min-range.  A schematic is shown below.


line-interactive UPS


Each of these UPS technologies is designed to meet the power quality and backup supply needs of electronics equipment in a variety of applications and markets.  Generally, double conversion and line-interactive systems are geared toward business and industry, while standby units are intended for the average consumer computer system.  Despite this distinction, there is a good deal of overlap in the cost and capability of different system types, reflecting the diversity of applications for which they are designed.  The following table compares the three main types of UPS by their typical range of cost, size, efficiency and power conditioning capabilities. 


UPS typeCost (US$)Capacity (kVA)Normal mode efficiencyTypical power conditioning
Standby 50 -500 0.3 - 1.5 97% - 99% Surges, Noise
Line-interactive 80 - 2,000 0.4 - 5 95% - 99% Surges, Noise, Voltage
Double conversion 400 - 15,000+ 0.7 - 20+ 85% - 97% Surges, Noise, Voltage, Harmonics, Frequency


The price of a UPS system is only loosely tied to its overall capacity; battery runtime, power quality and monitoring features also play an important role in determining price.  Another important distinction between UPS units is how they are integrated into a building circuit: at an outlet or hardwired.  Outlet UPSs plug into existing electrical outlets and in turn provide several on-unit outlets to supply supported loads.  Hardwired units are wired directly into an electrical circuit; supported loads are plugged into existing outlets connected to that circuit.  Outlet systems are typically of lower capacity and may be any of the three UPS types.  Hardwired systems are normally high capacity line-interactive or double conversion units.  

Also note that high-end double conversion systems used in data centers and other energy intensive IT applications can have a system capacity of many MW.  These systems are typically specially designed and built to purpose, thus there is no limit to the size and cost of such systems.  

The challenge of selecting the right system to meet the demands of a health care environment is understanding which features are necessary and which are not.  These considerations are discussed in following sections.


Offline-usv line-interactive example Bladeups1 
Example of a standby UPS. Example of a line-interactive UPS. (Photo: Hundehalter, available under a Creative Commons Attribution-Share Alike license.) Example of a rack-mounted, double conversion UPS.



Power Quality Issues 

Perfect AC sine wave.

Power quality refers to the adequacy of a power supply’s voltage, frequency and waveform characteristics.  Electrical equipment is designed to use electrical power with certain characteristics, like 12 or 24 volts, or, when designed for AC power, 50 or 60Hz frequency.  Similarly, power supplied by the grid, or some other source such as a generator or battery bank, is intended to meet a certain voltage level or frequency.

Most power systems operate on alternating current (AC) power as opposed to direct current (DC) power (including the grid).  AC electrical power is represented by a sine wave, where the current regularly changes direction.  The rate of this change in direction is the current’s frequency, which is typically 50 or 60Hz (50 or 60 changes per second).

Electrical equipment is designed to consume power with particular voltage and frequency characteristics, in the form of a perfect sine-wave.  Power supplies, however, are never perfect, and there are inevitably variations in the voltage, frequency and waveform of AC electricity.  For most common appliances and electrical devices, these variations are acceptable, but some medical and laboratory equipment are unable to tolerate less-than-perfect power, due to their complex and sensitive circuitry.  While not all power quality problems will lead to immediate damage, cumulative effects over time will harm equipment or result in less efficient operation.

One of the major purposes of a double conversion UPS system is to provide perfect power to sensitive loads, like those often found in a health lab.  Discussed below are a number of common power quality issues, including power loss, which health facilities must be prepared to address in order to ensure the safety of critical equipment.  The issues listed below are loosely categorized, with more exact definitions available based on the cause and duration of each issue.



Power interruption

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A power loss can last anywhere from milliseconds to days, depending on the cause of the interruption.  In the event of loss of grid power many health facilities have some backup power source, typically a generator.  Although backup power may be available, it will not be instantaneous, resulting in a brief loss of power – furthermore, generator start up often produces a spike in voltage.  UPS systems protect equipment against such threats, smoothing out the transition from grid to backup power.

Voltage sag/ Undervoltage

ups - voltage sag

Voltage sag is a decrease in the utility power voltage lasting as long as one minute.  This power problem can adversely affect sensitive electronic loads.  UPS systems or voltage regulators are needed to address voltage sags.

Voltage swell/ Overvoltage

ups - voltage swell

Voltage swell is the opposite of voltage sag, an increase in the utility power voltage lasting as long as one minute.  Similar to voltage sag, swells can be destructive to sensitive electronic equipment.

Voltage transient/ spike/ surge

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A voltage transient is a short, sudden, and sometimes extreme, change in voltage or current.  These types of power problems can be caused by lightning strikes as well as other issues in grid operation.  Transients can damage all manner of electrical equipment, including lighting.  Sensitive electronics may be damaged, data loss may also result.  UPS systems, as well as basic surge protectors, insulate equipment from transients.


ups - noise

Noise is characterized as unwanted, random signals in the power supply.  Noise is often caused by the equipment connected to an electrical circuit.  In most cases, noise in unavoidable, as it is not due to malfunctions but rather anomalies in the way electronic components operate. 

Harmonic distortion

ups - distortion

Harmonic distortions are alterations to a pure sine-wave.  Such distortions are due to non-linear loads.  Computer power supplies are a common cause as well as lighting ballasts and variable speed drives, such as those found on high-efficiency air conditioners.  Harmonic distortions can cause overheating of equipment and wiring, as well as efficiency losses.


Related Devices 

In addressing backup power needs and power quality issues, there are a range of technologies available in addition to UPS systems.  For each class of device, functionality and cost will vary depending on the intended application (larger, or more robust equipment is typically more expensive).  Health facilities can benefit greatly from utilizing these technologies in a manner appropriate to the availability and quality of their power supply, and the extent to which funding and human resources can be dedicated to their energy system.

Various types of equipment used for backup power and power conditioning are briefly described below.  Most of these devices are actually components of UPS systems, but can be utilized outside of a UPS to achieve similar results.



An inverter is a basic piece of power equipment responsible for converting power from DC to AC.  Inverters are a component to any UPS system as they are required to convert DC power from the battery to the AC power needed to run equipment.  In double conversion UPS systems, inverters are used to condition grid power to a perfect sine-wave.

They are also necessary accessories to PV systems and battery banks, as both produce DC power – requiring conversion to AC before powering conventional AC loads.  Thus inverters may be small circuits internal to UPS systems or other equipment types, or may be stand-alone units capable of handling large quantities of power, as in the case of a bank of batteries or a PV system.


Rectifiers perform the opposite function of an inverter, converting AC power from the grid into the DC power needed to charge batteries.  This type of circuit is essential to any equipment running on DC power, including most electronics and computers; for example, computer power supplies include a rectifier.  In UPSs rectifiers make up part of the charge controller, which uses grid power to keep the internal battery fully charged.  In double conversion UPSs, the rectifier is the first step in the double conversion process, filtering out noise and other power quality problems from the AC grid.

Battery bank

Batteries store chemical energy that can be converted directly to electrical DC power when connected to an electrical circuit.  All UPS systems include a battery to store energy for when grid power is lost.  Similarly, large battery banks can be constructed to provide backup power to entire building circuits for long periods of time.  

Batteries used in such systems are rechargeable, and, depending on the type of battery being used, may be charged and discharged thousands of times.

Charge controller A charge controller is needed to charge a battery, including that of a UPS.  Battery charging is a multi-phase process that involves applying a series specific voltages to the battery leads.  If a battery is not charged properly, its life can be shortened substantially or it can be ruined completely.  Charge controllers are responsible for carrying out this process. 
Power conditioner A power conditioner is a device that addresses one or more power quality issues, including voltage sags or swells (e.g. automatic voltage regulator) and harmonic distortions (e.g. harmonic filter).  Double conversion UPS systems perform the same functions as power conditioning equipment, but go further by providing a degree of backup power in the form of a battery.
Automatic Voltage Regulator (AVR)

An automatic voltage regulator outputs a target voltage within a narrow range from a larger range of input voltages, addressing voltage sag and swell.  Such devices can be based on transformers or electronic components.

Voltage regulators are internal components of some UPS systems, especially line-interactive types.  As stand-alone units they may also incorporate other power conditioning capabilities such as surge protection, noise reduction and harmonic filtering.  AVRs perform many of the same functions as line-interactive and double conversion UPSs, but do not have any backup power capacity, leaving connected loads exposed to interruptions in power.

Isolation transformer

Transformers are commonly used to change the voltage of a power line (step-down transformer), or in the case of an isolation transformer, to isolate loads from power quality issues and maintain supply voltage at the correct level.  Isolation transformers are sometimes used in hospitals to isolate sensitive loads, filtering out noise and providing proper voltage.

Isolation transformers are also used as components of some UPS systems, normally line-interactive UPS.

Surge protector Surge protectors are simple devices that limit the allowable voltage to a safe threshold.  Designed to protect against power spikes or transients, these equipment will ground any spike in voltage that could damage connected loads.  Surge protectors are common pieces of electrical equipment and are often integrated into power strips, providing basic protection for electronics.  These devices have no other power conditioning capability and are unable to address issues of voltage sag or swell or sine-wave distortions.  All UPS systems have this basic functionality, although in in a standby UPS this is typically the only type of protection provided.

A generator produces electricity by burning fuel in a reciprocating engine, in much the same manner as a car’s alternator generates power for on-board electronics.  A generator is a common choice for backup power supply as they can run for long periods while fuel is available.  Generator output is controlled in such a way as to maintain a constant voltage and current, but such controls are not perfect and power quality is generally an issue for generators.

UPS systems are important equipment for protecting sensitive loads when generator power is frequently used.  At the initial loss of grid power, generators can automatically start up.  This start-up, however, is not instantaneous, and this brief interruption in power may damage equipment or lead to data loss.  Additionally, such start-ups are often characterized by voltage spikes.  UPS systems are designed to ensure a smooth transition in these situations.

Inertia Wheel Inertia wheels are a form of mechanical energy storage, rather than chemical energy storage as in the case of batteries.  Inertial wheels are typically made up of a rotating drum connected to a motor/generator.  When power is available, the inertia wheel is charged by speeding up the rotation of the drum.  Energy is stored in the rotational inertia of the drum itself.  When electricity is needed, an inertia wheel powers its generator, creating electricity but slowing the speed of the drum.  Such energy storage devices can be more efficient than batteries with faster charge/discharge times and low in maintenance.  They are most often used at grid-scale or in other large UPS applications.


ups - inverter ups - avr ups -surge protector
Inverters are an essential to a battery bank, and must be sized appropriately. Voltage regulators are used to ensure power quality where additional backup power in not needed. Surge protectors are commonly intigrated into power strips, or designed for single outlets.



UPS Terms, Specs and Features 

When selecting an appropriate UPS system for health facility applications it is useful to have a basic understanding of the common product components and features available.  Table 3 presents a listing of terms, specifications and features that may be encountered when researching UPS products.


Runtime Runtime is the amount of time the UPS can operate loads on battery power.  This depends on the size of battery included with the unit and the availability of battery extension to increase runtime.  This figure may run anywhere from a couple of minutes to several hours and is usually reported at both full and half load.  A lower load results in longer runtime, and as a general rule of thumb, a UPS battery at half load will run three times longer than at full load.
Automatic data network shut down A useful option when powering servers or computers, this function automatically shuts down computer equipment safely through the installed operating system, ensuring that all data is saved before the UPSs battery runtime is complete.
Battery extensions Many UPSs allow for battery extensions, external battery packs that can be connected to the UPS to increase the system’s runtime.  These battery packs are offered by the UPS manufacturer so it is important to understand what battery pack models are compatible with the UPS, and ensure that enough battery extensions can be added to meet equipment runtime needs.
Data port Many UPS systems offer some way to interface, or connect, with other equipment, such as computer or other power equipment.  These connections usually come in the form of a USB, RS-232 or other data port.  This functionality can be beneficial, for instance, in reporting UPS alarms or other operating data to a central point and for monitoring the performance of the UPS systems.
Bypass switch

A bypass switch provides a direct connection between the input and output lines of a UPS, circumventing the system’s internal components.  This feature is important in case the UPS fails or runs out of battery power and is unable to supply any power at all to connected loads.  No backup power or power conditioning is available when in bypass mode.

An automatic bypass switch will move connected loads to normal grid or generator power automatically when the UPS fails.  A manual bypass switch is used for maintenance purposes, when components need repaired or replaced.

Delta conversion A particular form of on-line UPS that does not perform full double conversion when connected to the grid. Rather than using a rectifier and an inverter, a delta conversion UPS utilizes a transformer and an inverter to produce “clean” power for connected loads, resulting in greater efficiency.  Such systems match all of the abilities of a traditional on-line or double conversion UPS while also being able to do power factor corrections, all with greater efficiency.
EMI/RFI noise filtering Electromagnetic interference (EMI)/Radio-frequency interference (RFI) noise filtering removes uncontrolled frequency variations common in facility wiring.  Noise is generated by other loads on the same distribution system, such as an air conditioner, or from the incoming grid power.  Because noise can be damaging to sensitive medical equipment this feature should be considered important for any health facility UPS system.  All double conversion UPSs filter noise, for standby or line-interactive UPS systems it is important to ensure that this option is included.
Transfer time (milliseconds) Transfer time is the amount of time, in milliseconds, that it takes for the UPS to switch from grid to battery mode, rather, the amount of time connected loads will see and interruption in power.  For off-line and line-interactive units, transfer times typically range from 4-25 milliseconds.  On-line UPS units should have a transfer time of 0 milliseconds, as their internal capacitors allow for in-line energy storage.
Cold start operation Turning on a UPS before being plugged into the AC line is called cold starting.  Under this operation the UPS will run on battery power only.  This function can be useful if a UPS needs to be added during a power outage or to ensure that the battery is working properly.
Hot swappable batteries Hot swapping allows a UPS battery to be replaced without disconnecting the UPS or its connected loads from AC power.  If a battery requires replacement, it can be hot swapped without any equipment downtime.  If a UPS is not hot swappable, the system should be disconnected before battery replacement (8).
Battery recharge rate Battery recharge rate is typically described in the number of hours to complete recharge.  If power outages are too long or too frequent to provide adequate time for battery recharging the life of the battery will decreased and the effectiveness of the UPS compromised due to inadequate capacity.  External battery packs and a separate battery charger could allow for battery to be recharged more quickly if necessary.
Voltage transfer set points In all types of UPS systems, loads are transferred to battery mode if the incoming voltage goes above or below specific set points.  In the case of standby and line-interactive UPS systems, this acceptable voltage range is fairly narrow, as these systems have no other mode of protecting against harmful voltage variations.  Double conversion units have an built-in capacity for voltage regulation during normal operating mode; a double conversion unit will only transfer to battery power when the incoming voltage is beyond its ability to correct.
Efficiency Efficiency figures for UPS systems refer to the efficiency at which grid power reaches the connected loads.  The power conditioning processes inside UPS systems inevitably lead to energy losses, even in standby systems with minimal capabilities.  UPSs typically have efficiencies between 85%-99%, with increased efficiency as the connected load reaches the system’s capacity. 
Redundancy Because UPS systems are so important to protecting critical loads against power failure, it is common practice to use redundant, or extra, systems in case a UPS fails or needs to be serviced.  Redundancy is an important issue in large-scale UPS applications, such as data servers, where UPS systems are networked together to backup large loads.  In a typical medical or laboratory setting UPSs are not networked.
Number of outlets In outlet-type UPS systems, equipment plug directly into the unit itself, for these units it is important to ensure that an adequate number of outlets are available for the number of equipment the system is intended to support.  Large UPSs are hardwired into the electrical circuit, so existing wall outlets are protected.


Sizing and Selection 

Given the range of options for backup power and power quality equipment, selecting the most appropriate choice for a health facility requires a careful examination of the facility’s loads, power supply and energy management capacity.  Important considerations to take into account during this process include: “How large are the sensitive equipment loads as compared to the total facility load?”; “What power supply issues does the facility face; quality, availability, both?; “What resources (money, manpower) are available to manage and maintain energy equipment?”.  Depending on these considerations, the most suitable system may be a UPS unit, a battery/inverter system, a voltage regulator or some other configuration of power conditioning and backup power equipment. 

In order to address these questions, and develop a cost-effective energy system to meet facility needs, Powering Health has laid out a six step approach, focused on analyzing energy supply and demand, identifying appropriate use of technology and ensuring energy system sustainability through management and maintenance.  Powering Health also provides Energy Audit and Power System Optimization tools to assist in the process.


Load Characterization

ups - load labels
Circuits are labeled by load type and number of outlets.

With regard to UPS systems in particular, contact and no-contact loads is an essential concept.  Health facility loads can be characterized in one of three ways: non-critical loads, contact critical loads and no-contact critical loads. 

Each type of load places different requirements on the backup power system.  Non-critical loads do not require battery backup power, but should be supplied by a diesel generator when grid power is unavailable.  Contact critical loads can be directly supplied by grid and generator power, but require battery backup in the event that this primary power is disrupted.  No-contact critical loads should always be isolated from grid or diesel power due to fluctuations or spikes in voltage that can occur.  Rather, they should be supplied solely and constantly by a double conversion UPS or other source of clean power, which acts as a buffer between the sensitive equipment and the unreliable primary power source.

These different load categories can be characterized by their back-up power and power quality needs, which, in turn, point to suitable power supply equipment.  These considerations are outlined in the following table.


 Non-critical loadsContact loadsNo-contact loads
Example equipment Air conditioners, low-priority area lighting, TVs Priority area lighting, cold chain refrigerators, incubator, centrifuge data servers and computers, emergency lighting, blood analyzer, microscope, fire suppression
High quality power Preferable Preferable Required
Transfer time to emergency power Emergency power no required 10 seconds or less Instantaneous (0 seconds)
Automony/runtime (backup power time) No backup required Variable, from a few minutes to an hour Variable, enough for the longest power outage
System reliability N/A 99% >99%
Monitoring Not necessary Non necessary Recommended
Power supply equipment options Automatic voltage regulator, surge protector Standby UPS, line-interactive UPS, "contact" battery/inverter system Double conversion UPS, "no-contact" battery/inverter system


As in Powering Health’s six-step approach to energy system design, the first step in selecting an appropriate back-up power or power quality technology is to understand the size of the current facility load, identifying contact and no contact loads.

This process begins with an energy audit to inventory, categorize (i.e. non-critical, contact, no-contact) and quantify all existing loads as well as their operating hours.  Also, as in step two of the approach, future additions or changes in facility loads should be considered before going ahead with system sizing.

Powering Health offers a number of resources to aid in conducting an energy audit and obtaining load estimates, see the following tools and information:

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Energy Audit Tool

A spreadsheet tool and guide designed to be an overall off-grid energy information package to help energy experts and procurement officers collect and analyze information, plan PV and generator systems in off-grid health centers, and develop specifications and bidding documents.

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Load Analysis and Example Calculations

A fundamental part of energy management, and the first step in improving a health facility energy system, is an electrical load inventory.  An electrical load inventory is a listing of all electricity-consuming equipment in a facility, everything from light bulbs to expensive lab equipment to cell phone chargers.



Power Supply Characterization

UPS systems are recommended for any sensitive electronic equipment that may be damaged in the event of power loss, or other power anomalies; even when grid power is reliable.  While UPSs are important regardless of level of power quality a facility receives, an assessment of power quality is useful in determining the size, configuration, runtime and additional features of the UPS system.

There are a number of power quality tests available, designed to quantify issues such harmonic distortion, power factor, frequency variation and voltage variation.  In determining UPS requirements at a health facility, the two most important power quality indicators will be: 1) grid availability, and 2) voltage variation.

Understanding these two factors, combined with a quantification of the facility’s critical loads, is necessary to choose the correct type and size of UPS system.  The length of power outages will point to the amount of battery runtime necessary to power equipment until the grid or generator is restored, while the frequency of outages may affect the battery’s charging, and could necessitate additional replacement batteries.  Voltage variation will point to the correct UPS system type (off-line, line-interactive, on-line) based on the level of grid isolation needed to provide good quality power.

An assessment of grid availability is mostly based on past experience.  Interviews with facility personnel, logs of generator runtime, or in some cases utility data, will be useful in evaluating the extent of grid availability.  Administrators and technicians should have a good idea of how frequently and for how long, power outages occur.  In some cases, power is supplied on a schedule, meaning that grid loss is frequent and predictable.

Assessing voltage variation requires on-site measurement.  Grid power entering the facility should be recorded over a period of at least a couple of hours, with longer measurement times providing greater confidence in the assessment’s conclusions.  A data logger is hooked up to the laboratory circuit in order to capture current, power demand and voltage across all three phases.

Logged data can then be analyzed using computer software or spreadsheets.  In this analysis, voltage within each phase, and between phases, is compared to identify over-voltage, under-voltage or other variations.  Voltage data will yield minimum, maximum and average values for each phase; these values should not vary more than 2% for any one phase, or between phases.


Battery Considerations

UPSs transfer to battery mode when power is lost or a voltage set point is crossed, selecting a UPS that is appropriate to the grid power conditions at the facility therefore important.  If data logging shows frequent voltage swings a UPS should be chosen that has some form of automatic voltage regulation, such as a double conversion unit or many line-interactive units, otherwise the UPS will frequently switch to battery power, lowering its lifespan.

Depth of discharge is difficult to control, as the UPS will operate in battery mode until grid or generator power is restored.  This too, however, can be managed by correctly sizing the UPS and selecting an appropriate runtime capacity.  Reducing the load on the UPS will increase runtime, lowering the depth of discharge on the battery over short periods.  This can also be accomplished by adding external battery packs.  The required runtime should be determined based on the typical length of grid or generator power loss.



Like a battery bank or any other energy system, the process for sizing a UPS system begins with a calculation of the supported load.  In the case of UPS systems, the supported load may be a single piece of equipment, several pieces of related equipment or one or more electrical circuits.  UPS units are usually classified by their output capacity range in volt-amperes (VA )(e.g. 350VA – 750VA, 10kVA – 40kVA). Therefore, loads should be measured or calculated based on nameplate data in kVA (Volts x Amperes).

A UPS with a capacity range that covers the supported load should be selected, with some greater capacity if additional loads are expected in the future.  It is also common to oversize a slightly oversize a UPS system just to ensure enough capacity for all connected loads; a general rule of thumb when oversizing is to add an additional 10% of the load to the system capacity.

Another important consideration regarding UPS system size is the effect on the facility power load.  UPS systems have power consumption beyond that of their connected loads.  Due to inefficiencies in power conversion and battery charging, UPSs will add to the overall system load.  The exact efficiency of any given system will vary depending on its type, quality and features.  Double conversion systems have the lowest efficiency because of losses from the double conversion process.  

It is worth considering efficiency when comparing UPS systems, but battery charging is generally not included in reported operating efficiencies.  Charging efficiency will play a greater or lesser role depending on the frequency of battery use.  As a general rule of thumb, expect a conservative 20% increase in power load due to the UPS (10).


System Selection

In addition to the three main types of unitary UPS system (standby, line-interactive and double conversion), much of the related equipment referenced above can be integrated into systems that are also able to address power quality and backup power needs.  Inverters, battery banks, voltage regulators and even inertia wheels may make up the components of a cost-effective UPS solution that does not rely on a pre-packaged unit.  General characteristics of those options are outlined in the following table.


TechnologyDouble conversion UPSBattery/inverter systemAutomatic voltage regulatorFlywheel UPS
Cost (US$/VA) 0.18 - 1.10 1.40 - 2.40 (per W) 0.07 - 0.50 1.60 - 3.50
Capacity (kVA) 0.4 - 20+ 1.3 - 2.2 (scalable) 1 - 2,000 10 -10,000
Normal mode efficiency 85% - 97% 90% - 93% 98% - 99% 97% - 99%
Backup time minutes - hours (with battery expansion) hours - days no backup less than 1.5 minutes
O&M US$ 500 per year and battery replacement every 5 - 10 years US$ 300 per year and battery replacement every 5 - 8 years N/A US$ 500 per year maximum
Life 10 year 20 years with maintenance and battery replacement 15 years 24 years
Typical power conditioning Surges, Noise, Voltage, Harmonics, Frequency Surges, Noise, Voltage, Harmonics, Frequency (in no-contact configuration) Voltage Surges, Noise, Voltage, Harmonics, Frequency


These devices are all viable alternatives to addressing the power supply needs of a health facility and each has power conditioning capabilities.  Determining which solution will be most cost effective in the long term depends largely on the backup power requirements, total system size and maintenance competencies of the facility.

Ultimately, selection of the most appropriate type of UPS system must account for all of the factors discussed above: load characteristics, power quality issues, system size and battery runtime.  If little or no backup capacity is necessary and high quality clean power is not essential line-interactive UPS or even an automatic voltage regulator are relatively low-cost options.  These devices compliment backup generators to protect loads from voltage swells, sags or transients.

In the case of sensitive laboratory, medical or computer equipment, a double conversion system is essential.  While these systems are more expensive and less efficient than standby or line-interactive systems, their ability to protect sensitive equipment from damage is unmatched.  For large loads, flywheels may be available as an alternative to battery backup in a double conversion UPS.  Flywheels are generally more reliable and easier to maintain than batteries, but with less backup time.

Packaged UPS systems, however, are not a true backup power solution; they provide power for brief gaps in grid or generator power supply, typically no more than 30 minutes.  If primary power supply is infrequent or unreliable, a battery bank may be a practical option to supply backup power for long periods.  Packaged UPS systems can often be coupled with expansion battery packs to increase system runtime, but a battery banks have greater scalability and can provide equal protection.

Further consideration needs to be given to the size and number of UPS systems used.  A single, hardwired UPS is able to provide clean power to any number of laboratory devices; the same results, however, could be accomplished by several smaller, outlet-connected units.  While a single, large UPS provides simplicity, it is also likely to be more costly to procure and install and will leave loads unprotected if out of service.  Multiple, smaller systems provide redundancy and scalability but can clutter laboratory space and may complicate monitoring and maintenance.



UPS systems often have internal, sealed batteries that must be replaced over time. (Photo: Hundehalter, available under a Creative Commons Attribution-Share Alike license.)

The maintenance required for UPS systems is generally low.  Battery health is the greatest concern in ensuring the overall effectiveness of a UPS system.  Confidence in the UPSs ability to provide necessary runtime therefore depends on good battery maintenance.  Maintaining UPS batteries entails periodic cleaning and testing as well as proper replacement at the battery’s end of life.

Most UPS systems use low maintenance, sealed lead-acid batteries.  These batteries require simple types of preventative maintenance such as confirming that terminal connections are tight and removing corrosion.

It is also important to check battery health from time to time to ensure that sufficient capacity is available to backup loads.  UPS systems connected to monitoring software continuously track information on the state of charge and other parameters indicating battery health and performance.  Smaller UPS systems typically provide a “test” button, that when pressed will perform a deep-discharge/recharge of the battery as a matter of routine maintenance.

UPS batteries, like most lead-acid batteries, have a life of about 3-5 years.  Actual battery life depends greatly on operating and environmental conditions like the frequency and depth of discharge and the ambient temperature.  These factors can be managed through proper sizing and UPS selection, which is discussed in greater detail under UPS selection.


UPS Standards 


International Electrotechnical Commission (IEC)
 UPS IEC 62040-1: Uninterruptible power systems (UPS) - Part 1: General and safety requirements for UPS – Uninterruptible Power Supplies
 UPS IEC 62040-2: Uninterruptible power systems (UPS) - Part 2: Electromagnetic compatibility (EMC) requirements – Uninterruptible Power Supplies
 UPS IEC 62040-3: Uninterruptible power systems (UPS) - Part 3: Method of specifying the performance and test requirements – Uninterruptible Power Supplies


Underwriters Laboratory (UL)
 Medical Equipment
UL 60601-1: Medical Electrical Equipment, Part 1: General Requirements for Safety – Medical Equipment
 UPS UL 1778: Uninterruptible power systems (UPS) – Uninterruptible Power Supplies

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