Evaluation Criteria for Selecting an UPS-Part One



Evaluation Criteria for Selecting an UPS

The process for selecting an UPS consists of eight steps (see Fig.1). These steps are:


Step#1: Determining the need for an UPS,

Step#2: Determining the purpose(s) of the UPS,

Step#3: Determining the power requirements,

Step#4: Selecting the type of UPS,

Step#5: Determining if the safety of the selected UPS is acceptable,

Step#6: Determining if the availability of the selected UPS is acceptable,

Step#7: Determining if the selected UPS is maintainable, and

Step#8: Determining if the selected UPS is affordable.


Important notes:

- The last four steps may require repeating if the UPS does not meet all of the requirements.

- This process does not and cannot provide a “cookbook solution.” Each facility has unique requirements for emergency and standby power. These requirements include:

  • The reliability of the prime power source,
  • The nature of the work done,
  • Local and state regulations governing emergency power, etc.

- The process does not give a single solution that is applicable to all cases but it provides the framework for selecting an UPS for any facility.


Fig.1: The process for selecting an UPS




Step#1: Determine the general need for an UPS.




The process for determining the need for a facility UPS is shown in fig.2. It is mainly depends on:

  • The facility type and usage,
  • The mandating by local, state, or federal laws for the incorporation of an UPS in this facility type.


Fig.2: The process for determining the need for a facility UPS


The number of regulations mandating an alternate power source to ensure safety of personnel and to prevent pollution of the environment continues to increase. Consequently, enforcement agencies should be consulted to determine if an UPS is mandated. They should also be consulted during design and installation to make sure that the UPS is designed and installed in accordance with current applicable regulations.




Step#2:  Determine the purpose of the UPS.




An UPS may be needed for a variety of purposes. These include:

  • Emergency lighting for evacuation,
  • Emergency perimeter lighting for security,
  • Shutdown or continued operation of manufacturing equipment or computer operations,
  • Continued operation of life support or critical medical equipment,
  • Continued operation of communications, and
  • Safe operation of equipment during sags and brownouts.

Some facilities need an UPS for more than one purpose. In any case, the purpose(s) must be known before proceeding because it determines many factors that will drive the amount of power required and the type of UPS that will be needed. These factors are:

  • The acceptable delay between loss of primary power and availability of UPS power,
  • The length of time that emergency or backup power is required, 
  • The criticality of the load that the UPS must bear.




We have two types of applications as follows:

1- Applications cannot tolerate any loss of power, no matter how short the period of time, without loss of life or revenue such as:

  • Hospital life support and safety,
  • Aircraft tracking and landing, 
  • Certain production process controlsÙˆ
  • Data processing equipmentز


2- Other applications may tolerate loss of power for several minutes (or longer) without any adverse effects like:

  • Refrigeration,
  • Heating, 
  • Cooling.


Important notes:

  • For data processing equipment, it may be necessary to maintain power until the equipment can be shutdown in an orderly manner. This process may take only a few minutes.
  • Alternatively, life support, safety, communications and security equipment, and other applications may require continuous power until primary power is restored. Restoration of primary power could take hours or even days. A survey of commercial power outages may be necessary to determine this information. If commercial power outages are historically infrequent and last only a few minutes or hours, it may not make economical sense it may not make economical sense to install an UPS capable of supplying power for several days.


Table-1 lists General criteria for determining the purposes of an UPS and to assist in determining the purposes of a backup power system, diesel generator, UPS, or combination of diesel may vary.  Table-1 is from IEEE 446 Orange Book, Emergency and Standby Power Systems for Industrial and Commercial Applications.


Table-1 General criteria for determining the purposes of an UPS




Definition of the terms emergency power and standby power used in the table-1 as follows:


An emergency power system:

It can be defined as an independent reserve source of electric energy that, upon failure of the primary source, automatically provides reliable electric power within a specified time to critical devices and equipment which, if they fail to operate satisfactorily, would jeopardize the health and safety of personnel, result in property damage, or cause loss of revenue.


A standby power system:

It is an independent reserve source of electric energy that, upon failure or outage of the prime source, provides electric power of acceptable quality so that the user’s facility(ies) may continue operation in a satisfactory manner.




Step#3: Determine the power requirements.




After determining the specific purpose(s) for an UPS, the next step is to determine the facility power requirements (see fig.3). This task is essential because it sets the stage for the remainder of the selection process. Undefined power requirements, or oversight of any initial conditions, could result in the selection of a system that is:

  • Not capable of meeting the current needs of the facility,
  • Costly budget overruns, 
  • Delays in completing the project,
  • Not capable of meeting the future needs of the facility.



Fig.3 the process of determining the required power capacity.


However, to determine the required power capacity, you need to get answers for the following questions:

The first question: how much power is needed?

Power requirements can be divided into two categories:

  1. Critical,
  2. Non-critical.


Critical power is used for items such as:

  1. Emergency lighting for evacuation of personnel,
  2. Security systems,
  3. Central computing systems,
  4. Signaling systems,
  5. Medical equipment, and
  6. Other functions that could cause loss of productivity, or result in injury or a life Threatening situation.


Non-critical power is used for items such as:

  1. General lighting,
  2. Escalators,
  3. Coffee makers, etc.


The Second question: how much (if any) to oversize the unit?

 Oversizing serves two purposes:

  • First, it provides the capability to handle surges in power requirements due to peak demands caused by starting machinery, switching power supplies, etc.
  • Secondly, it provides for future growth. Over time, power demands may rise due to the addition of equipment or personnel, increases in productivity, and other reasons.


UPS Oversizing rule of thumb

A general rule of thumb in oversizing is to increase the initial power requirement by 30 percent.

If oversizing cannot be justified, the UPS should be selected and the installation designed such that future expansion can be accommodated at the least possible cost.




Step#4: Select the Type of UPS.




Selecting a particular type and configuration of an UPS depends on many factors that must be considered according to a facility’s particular requirements. These factors include:

  1. The purpose of the UPS,
  2. The required power,
  3. Cost,
  4. Safety,
  5. Environmental,
  6. Availability, and
  7. Maintenance.


Note that the selection process (see fig.1) is iterative. The type and configuration of the UPS initially selected is based on the purpose and power required. If the selected UPS is not acceptable based on one or more of the remaining factors, another type or configuration must be selected and the evaluation repeated.


The criticality, of the loads will determine the necessary availability of the UPS. Based on the criticality the UPS capacity or configuration can be selected. the following table helps in selecting the proper UPS configuration:


Type of Load

Redundancy Level

Configuration of UPS

Non-Critical Load



Critical Loads



Critical Loads



Critical Loads

Fault Tolerant System






Step#5: Determine if the safety of the selected UPS is acceptable.




Safety is basically governed by the electrical codes and standards as adopted by government and commercial agencies, and good judgment on the part of the design and installation team.

Batteries pose special safety concerns for the facility manager. Safety problems associated with lead-acid batteries include:

  • Spills of sulfuric acid,
  • Potential explosions from the generation of hydrogen and oxygen, 
  • The generation of toxic gasses such as arsine (AsH3) and stibine (SbH3).


All of these problems can be satisfactorily handled with the proper safety precautions like:

  • Good battery room ventilation as guided in National Fire Protection Association (NFPA) 70, National Electrical Code (NEC). Hydrogen accumulation is usually not a problem if good air circulation around a battery is present.
  • Wearing face shields and plastic/rubber aprons and gloves when handling acid is recommended to avoid chemical burns from sulfuric acid.
  • Set warning devices to alarm at 20 to 25 percent of the explosive level of the flammable gas Mixture of hydrogen and oxygen (formed during overcharge of lead-acid cells) noting that the gas mixture is explosive when hydrogen in air exceeds 4 percent by volume.
  • If relatively large batteries are confined in a small room, an exhaust fan(s) should be used to constantly vent the room or should start automatically when hydrogen accumulation exceeds 20 percent of the lower explosive limit.
  • Finally, the materials used in the battery container should be fire retardant.




Step#6: Determine if the availability of the selected UPS is acceptable.




Availability definition:

  • It is the amount of time a piece of equipment is available to perform its function divided by the total time the equipment is needed.
  • It is also defined as “uptime” divided by “total time.”


Thus, if an air conditioner is required 12 hours each day, the availability would be 90 percent if it is out of commission an average of 1.2 hours each day.

Normally, the required availability for UPS is 98 percent. The inherent or designed-in availability is usually expressed as follows:





Ai is inherent availability

MTBF is mean time between failure (a measure of reliability)

MTTR is mean time to repair (a measure of maintainability)


For example, assume the MTBF and the MTTR of a single UPS unit are 500 hours and 20 hours, respectively. The inherent availability of a single unit configuration would be:

Ai = 500 / (500+20) = 0.962

The inherent availability of a two-unit configuration where only one unit is required would be:

Ai = A1 + A2 – (A1 x A2) =0.999

The inherent availability of a two-unit configuration where both units are required would be:

Ai = A1 x A2 = 0.925


From the equation for Ai, it is obvious that availability can be increased by:

  • Increasing MTBF and MTBF is a measure of reliability,
  • Reducing MTTR.


and The availability could be increased by:

  • Increasing the reliability or
  • Reducing the MTTR.

The reliability could be increased by:

  • Selecting a more reliable unit,
  • Derating the unit (i.e., use a unit capable of providing more power than needed - when used, it will be operating below its capacity thereby reducing stresses),or
  • Use redundancy.


MTTR could be decreased by:

  • Selecting an inherently more maintainable system or
  • Perhaps by improving diagnostics, training, or procedures.





So, Availability is a function of :

  1. Reliability and
  2. Maintainability.




1- UPS reliability.


Reliability can be expressed in many was as follows:

  • It can be expressed as the probability that the item will perform as intended for a specified period of time,under a stated set of conditions.
  • It can be expressed in terms of the number of failures in a given time (the failure rate), or as the time between failures (for repairable items), or time to failure (for “one-shot” or non-repairable items.


Reliability is a function of:

a- The design of the UPS:

Redundancy reduces the overall failure rate of the UPS because one failure does not cause the UPS to fail.

Redundancy definition:

It is the duplication of elements in a system or installation for the purpose of enhancing the reliability of the system or installation.


Fig.4 Redundancy improves system reliability


The example in fig.4 illustrates why this so. two units may be placed in parallel where each is capable of supplying 100 percent of the load or three units may be placed in parallel where each is capable of carrying 50 percent of the load. The earlier case where one of two units is required is the most reliable. This is shown through the following equations:

The reliability where two of three units are required to supply 100 percent of the needed power, is given by the following equation.

Reliability = R(t) = 3e-2lt-2e-3lt


l is the failure rate of each unit and the units fail exponentially

t is the time over which the system must operate


The reliability where one of two units is needed to supply 100 percent of the power, the reliability is

given by the following equation.

Reliability = R(t) = 2e-lt-2e-2lt


b- The configuration selected:

Another significant factor in UPS reliability is the configuration. The different UPS configurations were discussed in previous articles.


c- The parts used (like battery):

The reliability of the UPS is certainly affected by the battery selected. In choosing an UPS, criteria that the buyer might use in evaluating the batteries are shown in table-2.


Table-2 Criteria for evaluating UPS battery



For Reliable Performance & Longer Life

The thickness of the positive plate

Thicker is better for durability.

The material used for the battery posts


Copper inserted posts operate more efficiently and cooler, and require less frequent re-torqueing than do lead posts

How the batteries are tested at the plant

Cells should be tested together

Capacity at which the batteries are shipped

from the factory

Anything less than 100% makes it debatable whether 100%

can ever be achieved

Tolerance to temperatures above 77 F

The higher, the better

Frequency at which a boost charge is



Less frequent is better

Frequency at which testing is required

Less often for short times is better


d- The environment in the UPS room:

Environmental factors such as excessive heat, cold, humidity, and/or dust can all have a significant effect on the UPS reliability.


Reliability the six-nine rule


  • Reliability and availability data on UPS components can be obtained by contacting the manufacturer or, if the information is not available, by referencing the Institute of Electrical and Electronics Engineers (IEEE) 500.
  • Due to the nature of their critical loads, UPS systems are desired to maintain a reliability level of 99.9999 (commonly known as the six-nine rule). This should be taken into account as one attempts to design or select an UPS system. The selected UPS system, in combination with the overall system, should maintain reliability level of 99.9999 or higher. Therefore, it might be necessary to design and select a system with greater redundancy, in order to achieve the "six-nine" goal.





2- UPS Maintainability.


Maintainability can be expressed in many was as follows:

  • It is the relative ease and economy of time and resources with which an item can be retained in, or restored to, a specific condition.
  • It can be expressed as the probability that an item can be restored to operational condition in a stated time, the number of repairs that can be made in a specific time (repair rate), or the MTTR.


As already shown by the availability equation, reducing the time to restore a system after it has failed is another way to increase availability.


Major factors determining the ease and economy with which maintenance can be performed are:

  • Means for identifying the component(s) that caused the system to fail (i.e., diagnostics) which is known as capability,
  • Accessibility of components, 
  • Skills and resources needed to repair the system (through in-place repair of the failed component(s) or by removing and replacing them).


a- capability

This capability can consist of built-in test, manual troubleshooting procedures, or troubleshooting using external test equipment.


b- Accessibility

Once the component(s) that caused the system to fail are identified through the diagnostic capability available for the system, maintenance personnel must gain access to those components. To some extent, the accessibility is determined by the installation as well as the system design.

Access to certain areas of an UPS installed in a very limited space may make repair very difficult even though the UPS design is very maintainable.


(c) Skills and resources

Even a system that has the most reliable and built-in diagnostics and is designed for total accessibility will not be economical to maintain if:


1- Highly skilled personnel:

When highly skilled personnel are required, the cost of maintenance increases and Training costs also increase.

Solution:  personnel will need only low to moderate skills and a minimum of training.


2- Extensive and expensive equipment are required:

If many expensive test equipment and tools are needed to support the UPS, this means that the UPS system is not economical.

Solution: the only tools required will be common hand tools normally found in a facility maintenance shop and the number and cost of test equipment will be minimal.





Step#7: Determine if the selected UPS is maintainable




Maintenance determines most of the operating cost of an UPS, how?

  • Done correctly, maintenance can ensure that the UPS stays reliable.
  • Poorly done, maintenance can compromise reliability and safety.


A system may be inherently maintainable (i.e., good access, good diagnostics, etc.), but other factors can determine whether or not the system can be maintained. These factors include:


1- Availability of trained personnel

Availability of personnel is a function of:

  • The total number of maintenance personnel,
  • The hours of facility operation (dictates the number of shifts during which maintenance personnel may be required), 
  • Training schedules.


2- Availability of spares and parts

Availability of spares (to replace removed components) and parts (to repair in-place or removed components) depends on the total number initially procured. Replacement and repair then depends on:

  • The rate spares and parts are purchased and
  • The rate at which the spares and parts become obsolete.


3- Location of repair facilities for removed components.

Location of repair facilities is important as follows:

  • If they are located a significant distance from the UPS, transportation may increase the time (and the cost) to restore the UPS to operation.
  • In many cases, the facility manager may choose to do only that maintenance performed directly on the UPS (i.e., in-place repair and removal and replacement) with internal resources. All other maintenance (i.e., repair of removed components) would be done by the UPS manufacturer or a third party.
  • Alternatively, the manager may contract out all of the maintenance.


The decision should be made primarily on the basis of:


1- Cost

The cost of hiring and training personnel should be compared to that of contracting out all or some of the maintenance. The expected number of repairs should be considered.

On the other hand, many contractors require time (typically 24 hours) to respond to a problem. This additional time may result in large losses of revenue. Even if internal repair is more expensive, the added cost may be more than offset by the savings in revenue.


2- Availability

A study of high failure rate components and the stock levels needed to minimize down time is also important. The contractor might be able to maintain the required stock levels at a lower cost and might be able to make repairs in a shorter time.




Step#8:  Determine if the selected UPS is affordable




Although discussed as the last step in the selection process, affordability is often a limiting factor in the selection of an UPS. It is placed last because the pricing of the UPS can only be done when the type, configuration, and sizing are known. These parameters cannot be known until the other 7 steps have been completed.

When considering the cost of an UPS (or any product, for that matter), it is best to consider the total cost, or life cycle cost, that will be incurred. (See fig.5) that describes the process for determining affordability.


Fig.5 the process for determining affordability


For an UPS, the total cost includes:

  1. The purchase price and Installation cost,
  2. Operating and support costs, and
  3. Disposal costs.


1- The purchase price and Installation cost


The acquisition and installation of the UPS typically constitute the greatest portion of the life cycle cost as follows:

  • The cost of any needed auxiliary equipment must also be considered. At a minimum, a manual bypass switch is required to disengage the UPS from the input power and load during maintenance or repair. These switches are typically integrated into a static UPS but are standalone items with rotaries. In the latter case, the switches require additional cabling and support hardware. More sophisticated systems having automatic monitoring, switching, and control functions requiring additional components, adding to the complexity and cost of the system(s).
  • As part of the installation costs, the costs associated with needed facility modifications should also be considered. If the UPS is operated under adverse conditions, availability will suffer.
  • Specific precautions must be taken for dirty, hot, cold, corrosive, explosive, tropical and other adverse conditions. Additional air conditioning might be required for the UPS (or for the facility due to heat loads from the UPS). Rotary units may require additional or special ventilation equipment to purge toxic fumes from working areas.  A rotary UPS or engine generator used in a cold climate will probably require thermostatically controlled lubricating oil, coolant heaters, and radiator louvers. All equipment manufacturers specify the temperature extremes, humidity, and other conditions for which the UPS was designed.
  • Floor units (usually static UPS for computer system backup) may require strengthening of the floor to support their weight. Large UPSs may require the construction of a separate building to house the unit.


2- Operating and support costs

Operating and support costs of the UPS are usually secondary to the costs of purchasing and installation.

However, they can be substantial and should be included as a factor in the final selection.

Operating and support costs include:

  • Cost of fuel,
  • Maintenance cost,
  • Replacement parts cost, and
  • Taxes.


3- Disposal costs

Finally, when an UPS reaches the end of its life, disposal costs will be incurred. Disposal may simply consist of:

  • Dismantling the UPS and
  • Selling the parts to a recycling company or
  • Dumping it at an approved refuse site.

For UPSs containing dangerous or environmentally unsafe chemicals or materials, disposal is much more complicated and expensive.

For example, disposal of lead acid batteries must be performed according to all federal, state, and local regulations. Lead-acid batteries should be reclaimed to avoid the regulatory requirements for hazardous waste treatment and disposal facilities. Reclamation may be included as part of a procurement contract for replacement batteries or contracts may be placed with a permitted reclaiming facility. In disposing of spent batteries, the facility manager must ensure that batteries meet all radioactive contamination requirements for uncontrolled release.



In the next Article, we will explain the Example for UPS selection & UPS system ratings and sizing. So, please keep following.


Subject Of Pervious Article


Applicable Standards for UPS Systems

What is a UPS?

Why do we need a UPS?

UPS Rating

Classification of UPS:

1-Voltage range,

2-No. of phases,

3- Mobility,

4- Technological design,


Classification and Types of UPS – Part One

5- Physical Size/capacity,

6- Form factor/ configurations:

6.1- “N” System Configuration

Classification and Types of UPS – Part Two

6.2- “N+1” System Configuration, which includes:

  • Isolated Redundant Configuration (N +1)
  • Parallel Redundant Configuration (1+1)
  • Parallel Redundant Configuration (N +1)
  • Parallel Redundant Configuration (N +2) and so on

6.3- Parallel Redundant with Dual Bus Configuration (N+1 or 1+1)

Classification and Types of UPS – Part Three


6.4- Parallel Redundant with STS Configuration

  • Parallel Redundant Configuration (1+1) + STS
  • Parallel Redundant Configuration (N+1) + STS

6.5- System plus System 2(N+1), 2N+2, [(N+1) + (N+1)], and 2N

Classification and Types of UPS – Part Four



7- According to UPS Topology

7.1 Off-line or Standby UPS,

7.2 Line Interactive UPS,

7.3 Standby-Ferro UPS,

7.4 Online Double Conversion UPS,

7.5 The Delta Conversion On-Line UPS.

Classification and Types of UPS – Part Five




8- According to UPS Distribution Architecture

8.1 Centralized UPS Configuration,

8.2 Distributed (Decentralized) UPS Configuration,

8.2.1 Distributed UPS-Zonewise Configuration

8.3 Hybrid UPS Configuration.

Conventional (Monolithic) Vs Modular UPS System:

  • Deploy UPSs in parallel,
  • Deploy UPSs in Series,
  • Use modular UPS products.


Classification and Types of UPS – Part Six


Three Basic Configurations Of Mains And Bypass For A UPS System:

  • Single mains,
  • Single mains without bypass,
  • Dual mains.

9-According to Use of transformers with the UPS

  • Transformer based,
  • Transformer less UPS,
  • Transformer less UPS with external input/ output transformer.

Classification and Types of UPS – Part Seven



Transformer Arrangements in Practical UPS Systems:

1-Transformer options for the “single mains” configuration

2-Transformer Options for the “Dual Mains” Configuration

Classification and Types of UPS – Part Eight


3- Transformer options for “single mains without bypass”

Classification and Types of UPS – Part Nine

Components of Online Double Conversion UPS:

1- Rectifier,

2- Inverter,

3- Energy Storage system:

3.1 Battery

Components of Online Double Conversion UPS– Part One



3.1.1 Battery Configurations

Serial Strings,

Parallel Strings.

3.1.2 Battery Size and Location

3.1.3 Battery Transition Boxes

3.1.4 Battery Monitoring

3.2 Energy Storage System – Flywheel

3.3 Energy Storage system – Super Capacitors

3.4 Hydrogen Fuel Cells

4- Static switch

Earthing Principles of UPS Systems

Components of Online Double Conversion UPS – Part Two




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