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Knowledge is Power: The Importance of Understanding The Main Components of Your UPS

Page updated on: July 9, 2024

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What Are the Main Components of a UPS?

As complex devices tasked with ensuring clean power and continuous uptime to your critical load, uninterruptible power systems (UPSs) are comprised of a variety of critical components that wear out during normal operations. Left unchanged, these parts are subject to failure. Understanding how the key elements of your UPS work ── as well as the importance of engaging in routine preventive maintenance for your UPS system ── will help you to more easily identify potential problems while minimizing the risk of systems failure.

Why You Need to Know the Components of Your UPS

UPS systems come in all shapes and sizes; from large online UPS models that keep data centers and hospitals up and running to single-phase UPS solutions that protect businesses, schools and other applications.

Regardless of size or topology, all UPS systems include four main components: the system batteries, the automatic transfer switch (ATS), the rectifier and the inverter. While this quartet represents the core components, there are additional UPS parts that also require attention and routine maintenance. Scheduling regular service visits is critical to preserving the functionality of your overall UPS system ── and understanding the basics of how these components work is the first step to ensuring optimal ongoing performance.  

UPS Batteries

As the heart of any uninterruptible power supply (UPS) system, batteries provide emergency power to the connected load during a utility power failure, or when power anomalies cause fluctuations in the incoming power supply. Every battery system contains at least one string, and depending on the UPS configuration, multiple strings of batteries may be added to increase runtime and/or redundancy. Because battery strings are connected in a series, if a single battery goes bad, it can cause the entire string to fail.

In smaller UPS designs, batteries are generally housed inside the unit, while larger UPSs often include standalone cabinets for the system batteries.

Overview of ATS, Rectifiers, and Inverters

In addition to the UPS’s batteries, it’s important to understand the role played by the three other primary components: the ATS, rectifiers and inverters.


An ATS is a device that automatically transfers power from the primary source to a backup source during a power outage. It is typically used to provide resilience for smaller uninterruptible power supply units below 10 kVA that are unable to operate in a parallel configuration. An ATS includes two AC input power sources (‘A’ and ‘B’) so if one fails, the load is automatically and instantaneously transferred to the other.


The primary role of a UPS rectifier is to convert power from AC to DC power. Rectifiers are capable of accepting a wide range of input voltage fluctuations, enabling the system to handle overloads or surges without having to engage the batteries.

UPS rectifiers are also responsible for recharging the system batteries while the DC power routes to the inverter. Depending on the size of the UPS, the rectifier may incorporate the battery charger. However, with smaller UPS systems (below 3 kVA), it is not uncommon for the rectifier and battery charger to be separate components. 


In an online UPS, the inverter is a key aspect of the double conversion process, which works as a filter during power anomalies such as surges, spikes and electrical noise. After the rectifier converts input power from AC to DC power, and DC power is routed to the inverter, the inverter then converts the DC voltage back to AC output, which is needed to power the critical load.

In a line-interactive UPS, the inverter is part of the output. While the AC input is usual, the inverter works in reverse to charge the battery and turn to battery power when the input fails.

Other UPS Components

In addition to a UPS’s four primary components, there are several other parts that are critical to its operation. These include:

Static Bypass Switch

Providing an important line of defense in the event of a failure, a UPS’s internal static bypass switch automatically closes the circuit and allows the incoming power to divert around the rectifier, batteries and inverter. Although the resulting power supply is not filtered or conditioned as an online double-conversion UPS would normally provide, the bypass switch allows critical systems continue functioning even if the UPS’s components fail.

External Maintenance Bypass

An external maintenance bypass switch enables a UPS system to be electrically isolated by removing it from the critical power circuit. The switch is used to safely perform UPS maintenance, repair or UPS replacement without any disruption to the connected load.


One of the few UPS components that are mechanical in nature, fans also wear out over time and eventually need to be replaced, usually after six or seven years. However, the lifespan of UPS fans can vary based on factors such as ambient temperature, humidity, particulates, clogged air filters and the capacity under which the UPS system is operating.

UPS Capacitors

Devices designed to store and release electrical energy, UPS capacitors are responsible for smoothing and filtering voltage fluctuations. Ranging in size and type, the number of capacitors varies depending on the kVA rating of the UPS system. Even small standby UPSs have dozens of capacitors, while large three-phase models can have hundreds. Like battery systems, capacitors degrade over time and for similar reasons, such as excessive current, overwork and heat.  As their materials age and degrade, capacitors lose their ability to perform effectively. On average, most capacitors need to be replaced every five to eight years.

Contact Unified Power for UPS Power Solutions. We provide immediate help for UPS failure and mission critical power services to help you keep your UPS in the best condition possible, preventing failures and unexpected downtime.

UPS Maintenance Services from Unified Power

Regardless of UPS topology, the components inside of UPS equipment wear out with normal use, making them susceptible to failure. However, engaging in regular service performed by trained and experienced service personnel enables potential issues to be identified and resolved before they become expensive problems, greatly minimizing the risk of UPS failure. Unified Power offers an extensive range of UPS maintenance service plans and maintenance contracts, including battery preventive maintenance, to meet virtually every need and budget. In addition to UPSs, our services include generator repair and maintenance, battery recycling, battery monitoring, UPS rentals, site surveys and more.           

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