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    Uninterruptible Power Supply (UPS)

    An uninterruptible power supply (UPS) is a type of power supply system that contains a battery to maintain power to provide power to electronics in the event of a power surge or outage.

    What is a UPS used for?

    Typically UPS power keeps a personal computer (PC) running for several minutes after a power outage, enabling users to save data that is in memory and shut down the computer properly. Many uninterruptible power supplies now offer a software component that enables you to automate backup and shut down procedures in case there’s a power failure while the user is away from the computer.

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    Common power problems addressed by a UPS

    A UPS will help protect computers, servers, and other electronic devices from different types of power supply problems, many of which can cause severe damage to hardware, software, and storage. Here is an overview of some of the most common power problems that can be minimized or eliminated by using a reliable UPS device:

    • Blackouts: When the electrical supply stops entirely, it is called a power blackout. Blackout can arise from issues with the electrical grid, maintenance downtime, or uncontrollable events such as severe weather. In most cases, “blackout” refers to a large-scale power interruption; however, only a small area or single building may be affected. Smaller interruptions of power are commonly referred to as power outages, which are typically more frequent. 
    • Brownouts: Fluctuations in demand and supply of electricity can cause brownouts, which results in a voltage drop. During a brownout, users will get electricity, but not at the usual voltage levels. A brownout can cause severe damage to a computer system, servers, and other electronic devices.
    • Power Surge: A power surge is the opposite of a brownout. As the name suggests, a power surge results in a temporary increase in the voltage of the power supply. A power surge is especially damaging to larger electrical appliances.   
    • Frequency Variation and Noise: When the power supply fluctuates it can cause frequency variation and noise, which can degrade or disrupt the performance of computers and other electrical devices. Most high-quality UPS systems are designed to eliminate frequency variation and noise.

    Features to look for in selecting a UPS


    An uninterruptible power supply system generally offers multiple outlets, allowing usrrs to maintain battery backup power to more than one device, along with additional outlets for surge protection.


    One of the most important features of any type of UPS is its capacity or runtime. Users should select a system with enough total wattage and battery runtime to support the computer itself, along with any attached storage devices.

    Generally, compact UPS has a runtime of around 90 minutes, while high runtime UPS can run for over 4 hours. If using a UPS for a personal computer setup, users will need around 200-300 watts total capacity, while a server room may require more than 2500 watts depending on the number and size of servers that connect to the UPS.

    Form factor

    Uninterruptible power supplies are available in different form factors including desktop, tower, and rackmount. Each form factor offers certain advantages and disadvantages.

    The desktop form factor is the most compact and can easily fit on an office table, but has limited mounting slots and capacity. When using a UPS with a tower form factor, you get more mounting slots and capacity, but the design is not compact. The largest form factor is rackmount UPS, most commonly used for server racks. It requires extra space but offers larger capacity and mounting slots.

    Energy-saving features

    UPS devices that are Energy-Star certified or come with “eco-mode” or “standby mode” offer maximum energy savings. Some energy-efficient UPS devices also feature a bypass mode which allows the power supply to bypass the inverter and rectifier and feed directly to the IT load. This minimizes any power losses from rectifier and inverter circuits.

    Secondary UPS features

    There are several secondly UPS features to consider when choosing a UPS. Some high-end UPS come with the software used for remote monitoring and management of the UPS. Some UPS devices include LCD displays to view battery conditions, load, and other information. 

    Users have choices as well in the type of battery used by the UPS.  Lead-acid batteries are more common, but Lithium-ion batteries provide better longevity. Users should strongly consider also consider buying a UPS that features replaceable batteries. This means that when the battery gets worn out, the user can simply install a new battery rather than buy a new UPS.

    Types of UPS

    There are three basic types of UPS systems: the standby power system (SPS); the on-line UPS system; and the line-interactive UPS

    The standby UPS is the most common type of UPS used for desktop computers and other individual electronic devices. The line-interactive UPS is more commonly used in small business and server situations.

    Standby Power System (SPS)

    The standby power system monitors the power line and switches to battery power as soon as it detects a problem. The switch to battery, however, can require several milliseconds, during which time the computer is not receiving any power.

    Standby Power System (SPS)
    Standby UPS, produced by Schneider Electric’s Data Center Science Center

    On-Line UPS System (Double-Conversion)

    An on-line UPS avoids these momentary power lapses by constantly providing power from its own inverter, even when the power line is functioning properly. In general, on-line UPSs are much more expensive than SPSs.

    Line Interactive UPS

    With line-interactive UPS, the system is powered with a conditioned current that eliminates any voltage fluctuations. As alternating current (AC) passes through the voltage regulator, any fluctuation in the power is corrected. Most line-interactive UPS systems feature a battery backup mode that activates when power failure is detected. The advantage of line-interactive UPS is high reliability and high efficiency through voltage conditioning. However, the available power capacity of this type of UPS might be limited.

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    UPS wave outputs: which one to get

    Uninterruptible power supplies are available with two types of wave outputs in UPS systems: pure sine wave and simulated sine wave. It is important to understand the difference between the two so you can pick the right UPS for your needs.

    The utility power supply is typically in the form of the pure sine wave. However, if there is power supply disturbance such as blackouts, brownouts, or frequency issues, the wave output can be disturbed and the UPS will try to “clean” the disturbance. This is where the sine wave becomes an important consideration.

    A pure sine wave offers cleaner and smoother output, which is vital for sensitive equipment. The curve of a pure sine wave has smooth edges. A simulated or modified sine wave is less stable and smooth and can result in decreased equipment efficiency and performance. The modified sine wave has rectangular-shaped edges.

    Keep in mind that both types of UPS systems will produce pure sine waves until the battery back mode is active due to power supply issues such as blackout or voltage issues. A UPS that offers pure sine wave is going to be significantly more expensive compared to modified sine wave UPS systems. When the UPS has to generate power from its battery reserves, the output waveform becomes a concern.

    Wave output is particularly important when the UPS is connected to sensitive equipment such as mission-critical servers, medical equipment, laboratory equipment, and telecommunication devices. Systems like these need pure sine wave capabilities. However, for basic IT devices such as home entertainment systems, personal PCs, and media centers, a modified sine wave UPS is generally sufficient.

    UPS battery types

    UPS systems rely on one of three main types of battery to provide backup power:

    • Valve Regulated (VRLA): This is the most common type of UPS battery. VRLA batteries consist of a sealed battery with a valve to release pressure if the internal pressure of the battery gets high. This type of UPS battery is also referred to as a sealed lead-acid (SLA) battery. 
    • Open Vented (VLA): These types of batteries are designed to have plates flooded with electrolyte acid. They offer longer life compared to VRLA and are most suited for larger installations that require high amperes.
    • Lithium-Ion: These types of batteries are not as common in UPS systems but are becoming more popular as they offer better reliability compared to VRLA and VLA. The better reliability comes from its internal battery management and monitoring system. However, you can expect to pay more for the lithium-ion battery for a UPS compared to VRLA and VLA batteries.

    Battery maintenance and testing

    A UPS battery’s service life can be maximized by minimizing excessive cycling, harsh temperatures, and improper charging. A battery that has electrolytes might need periodic top-off service. For the testing of the health of larger UPS battery systems, an AC/DC load bank and battery load testing by properly trained personnel is required. 


    This definition was reviewed and updated in April 2022 by Ali Azhar.