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  1. Introduction

    An Uninterruptible Power Supply (UPS) is an often-overlooked component in today's home computing systems. A good UPS can save you time, trouble, downtime, and damaged equipment when added to your current computer setup. However, not all UPS's are created equal. This FAQ will give you an overview of the differences between the barely-adequate UPS's and the high-end UPS's.


    One of the biggest factors that differentiate UPS's from each other is their topology, which is the circuit design which allows them to perform their function.

    All UPS's share a few things in common: They have input circuitry that typically performs surge protection and EFI/RFI filtering. They have a battery, which is the reservior for reserve power. They have a battery charging circuit to charge the battery. And finally, they have an inverter circuit, which converts the DC power from the battery to AC power to power the connected load.

    The manner in which these components are connected together forms the topology of the UPS. Different topologies have different advantages, disadvantages, and operational characteristics.

    Standby Topology

    In the Standby Topology UPS, the load (in other words, the equipment connected to the UPS that is supposed to be protected from a power outage) is normally powered directly from the line (the utility power source) through the surge protection circuitry and the transfer switch. The battery charger trickle-charges the battery, and the DC/AC inverter is normally shut off. When power fails, the DC/AC inverter is immediately turned on, drawing power from the battery, and the transfer switch switches the load to operate from the inverter instead of from the (now dead) line. The transfer time is typically around 2-10 milliseconds. The battery can now power the load through the inverter for as long as the battery can last. The standby UPS has the advantages of inexpensive construction, and high efficiency when operating on line power. The disadvantages are the switching transient when power fails, and the inability to condition the power to the load when operating on line power.

    Line-Interactive Topology

    The Line-Interactive Topology UPS is very similar to the Standby UPS. However, there is an addition of a tap-changing transformer in the line circuitry. The tap-changing transformer allows the UPS to somewhat regulate the line voltage that is being supplied to the load. For example, if the input voltage is too high, the UPS can change the tap on the transformer to lower the supplied voltage, and if the input voltage is too low (like in brownout conditions), the UPS can change the tap on the transformer to raise the supplied voltage. The line-interactive UPS has the advantages of high efficiency when operating on line power, and this ability to somewhat (not perfectly) regulate the line voltage supplied to the load. The disadvantages are the switching transient when power fails, and somewhat heavier and bulkier construction because of the transformer. A line-interactive UPS must temporarily shift to battery power to change the transformer tap. You will sometimes hear a line-interactive UPS go on battery power for 2-3 seconds and then shift back to utility power seemingly for no reason. In most cases, the UPS was changing the transformer tap to compensate for varying line voltage.

    Online Topology

    The Online UPS is a design that continuously powers the load through the DC/AC inverter. This is sometimes called a "double conversion" UPS because the line power is being converted to DC through the rectifier and then back to AC through the inverter in order to power the load. The advantages of the online UPS is that there is no switching transient when power fails, and the load is fully isolated from the line so there are no voltage variations, frequency variations, surges, or interference that can ever reach the load. Online UPS's typically have a manually-actuated bypass switch that can be used to connect the load directly to the line. This is used during maintenance, such as replacing the UPS's battery pack. The bypass switch allows the load to remain powered while the battery is being replaced. The disadvantages are that an online UPS is less efficient than other type due to the continuous power conversion, is bulkier and heavier due to the much larger rectifier circuitry required, and the components in this UPS suffer more wear and tear due to continuous operation.

    Output Waveform

    Different UPS's have different quality output power due to the design of their DC/AC inverter. The inverter is the circuit responsible for taking the DC power from the battery and converting it to AC power suitable for the connected load. Utility power AC is shaped like a Sine wave, a continuously varying wave that cycles 60 times per second (50 times per second outside the USA).

    DC/AC inverters are generally built in 3 different types that produce very different power:

    Square Wave Inverter

    This is the least expensive type of inverter to produce. The waveform it outputs is a square wave, formed by outputting 120V (240V outside the USA) for the first half of the waveform, and -120V (-240V) on the second half of the waveform. This type of waveform is suitable for powering devices which have their own internal power supply that converts incoming power to DC, like a computer. This type of waveform is not suitable for powering devices with high inductive or capacitive loads, like electric motors, appliances, printers, and surge suppression devices. This type of waveform is hard on the DC power supplies in computers, and should not be used to power them except for the few minutes needed to perform a controlled shutdown of the computer after power has failed.

    Stepped Sine Wave Inverter

    This type of inverter attempts to better simulate utility power by providing a waveform that is a closer approximation to a true sine wave. Most of the rules for a square wave inverter also apply to the stepped sine wave inverter, although this inverter is not nearly as harsh on computer power supplies. A good stepped sine wave inverter can safely power a computer for several minutes after a power failure without worry about overly stressing the power supply. Some manufacturers also call this type of inverter a Pulse Width Modulated Sine wave inverter.

    Pure Sine Wave Inverter

    The highest quality power (and least efficient) inverter outputs a true sine wave that is indistinguishable from utility power. All devices will operate properly from this inverter, including motors and appliances (see key points section for some restrictions). This inverter is not very efficient, so the UPS requires a larger battery for the same run time as a similar UPS that uses a stepped sine wave output. This inverter also produces a fair amount of heat while in operation.

    Surge Suppression

    Almost all UPS's have a surge suppression and filtering circuit in them that filters out surges, spikes, electromagnetic interference (EMI), and radio frequency interference (RFI) from the power. The surge suppression circuit uses metal oxide varistors (MOVs) to protect the load from surges and spikes. The surge suppression capability of a UPS is measured in Joules (J), and is a rating of the amount of energy that the MOVs can absorb while only allowing some small portion of the surge/spike (generally 5%) to be "let through" to the load. For example, a short-duration 5000V spike could be absorbed by the MOVs while allowing only 5% of the spike voltage (250V) to be felt by the load. Most electronics will survive a 250V spike with no damage. MOVs with higher Joule ratings could absorb spikes that are longer in duration. Better UPS's with better MOVs have a lower "let through" percentage, in some cases as low as 0.3%.

    Most UPS's have several outlets on the back, with some outlets protected by the UPS for power failure (i.e. these outlets are connected to the inverter), wheras other outlets are protected only by the surge suppression circuitry. All outlets are actually protected from surges and spikes, but the UPS bank is also battery-backed, whereas the surge-protection-only outlets will lose power in the event of a power failure. This is designed so that you can plug all devices into the UPS, but make intelligent decisions as to what equipment needs battery protection and what doesn't. Generally, the CPU, a monitor, and possibly external hard drives are protected by the battery, whereas printers, scanners, cameras, speakers and other non-essential peripherals are only plugged into the surge-protected outlets.


    Many UPS's have some type of interface to the computer so that the computer can monitor its status. The interface on many UPS's is USB, on others it can be serial using a 9-pin DB9 connector, while enterprise-level UPS's can have a 100Base-TX Ethernet interface that can be monitored with SNMP. If your UPS comes with a communication interface, the UPS manufacturer generally provides software that can be installed to monitor the UPS. Many UPS's do not need their own software, and can be monitored by Windows XP using Windows' built-in UPS service.

    Real Power, Reactive Power, and Apparent Power

    When discussing power in terms of AC waveforms, there are several types of power.

    Real power is that we are most familiar with. Real power is the measure of how much energy per second is being used by a load to do real work (e.g. perform a computational task, generate heat, spin a hard drive platter). Real power is measured in watts, and is the product of voltage (in volts), current (in amps), and a fractional factor that determines the ratio of real power to apparent power. The fractional factor is called the power factor (pf).

    P(real) = V * I * pf

    Reactive power is an unseen quantity. It is the measure of power that is flowing between the source and the load that is being used to set up magnetic fields (in transformers or inductors) or electric fields (inside capacitors). This power is stored inside the magnetic field of the inductor or the electric field of the capacitor. The stored power is then returned to the source later in the 60 Hz cycle. Even though the power gets returned to the source because it didn't do any real work, we have to account for the reactive power because the current necessary for it must flow. Reactive power increases the amps that must flow without increasing the watts used. Reactive power is measured in VARs (volt-amps-reactive).

    Apparent power is what we compute when we multiply volts times amps. It is the combination of the real and reactive power. Apparent power is measured in VA (volt-amps).

    P(apparent) = V * I
    P(apparent) = SQRT( P(real)^2 + P(reactive)^2 )

    Power factor (mentioned above) is a ratio of the real power and the apparent power.

    pf = P(real) / P(apparent)

    UPS Ratings

    UPS manufacturers rate their UPS's in terms of apparent power (measured in VA). The real power that a UPS can provide is always less than this. It is vitally important when selecting a UPS that you determine how much real power you need to provide, and make sure that the UPS can provide enough real power for your connected load. You will need to look in the manufacturer's specifications for the UPS you are considering to find the real power rating (in watts).

    Other Caveats

    When using any UPS, there are some rules that should be observed:

    - Never plug any surge suppressor or power strip into the load side of a UPS. Especially with square wave and stepped sine wave inverters, the surge suppressors on the load side can cause the UPS to drain its battery faster than expected, or cause the UPS to shut down or trip a circuit breaker.

    - Never plug any device into a UPS's battery-protected outlets that has a high inrush current (i.e. a device that draws a high amount of power when first turned on). This includes most printers, especially laser printers. These devices can cause the UPS to shut down or trip its circuit breaker because the power they draw at start-up exceeds what the UPS can deliver.

    - Be careful with the amount of load you connect to the UPS, and make sure the load doesn't exceed the UPS's real or apparent power ratings. Most UPS's have some type of total load indication or at least an overload indicator light to let you know that the UPS is overloaded.

    - Different models of UPS may have the same apparent power rating but a different real power rating. This is because the UPS designs are different and one can handle more real power than the other. A prime example is the APC Smart-UPS 750 vs. the Smart-UPS 750 XL. Both are rated 750 VA, but the XL model can handle 600W while the non-XL model can only handle 500W.

    - Different models of UPS may have the same apparent power rating but vastly different run times. The power rating is determined by the size and design of the inverter, while the run time is determined mainly by the battery size. The APC Smart-UPS 750 and Smart-UPS 750XL are both rated at 750VA, but when powering a 300W load, the XL model can run it for over 45 minutes, while the non-XL model has only 10 minutes of run time.

    - Your computer power supply rating is much greater than the amount of power your system actually draws. Many computers today have a 600W or higher power supply, but actual draw from the line is generally 300W or less, so you don't need to spend the money on a 1500VA UPS. To find out the actual amount of power your computer is using, use an inexpensive watt meter like the Kill-A-Watt.

    Comparison Chart

    The following chart shows the major manufacturers of UPS's, and compares the various model series they make. For this comparison, the following rules were used:

    - I did not research model series for enterprise rack or whole-data center applications. Basically, if the series didn't include at least one unit rated under 1000VA, I did not include it, since those units would likely not be used in a home/home office environment.

    - Within a series, product specifications can vary, especially in terms of the communications interface and surge protection capability. Check the specifications on the exact model you're considering.

    - I made no attempt to judge the engineering quality of the different manufacturers, I only looked at their technical specifications.

    - In some cases, a few of the specifications had to be inferred by looking at other units in the same series.

    - The color codes are highlighted with red/rose being a low-quality specification, yellow being a medium quality specification, and green representing the highest quality specification. Remember that in most cases with UPS's, higher power quality means less efficiency, so that is something to consider when choosing a UPS.

    Manufacturer's Links

    The following are links to manufacturer's web sites where you can research your UPS.

    American Power Conversion (APC)
    Belkin UPS
    Liebert UPS
    Minuteman UPS


    I hope this guide is helpful in your choosing of your UPS.

    Version History

    1.0.0 - 02/04/2008 - Initial Release
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