Here are the most important characteristics of a BPS:
The VA rating of a BPS specifies the maximum power the UPS can supply, and is determined by the capacity of the inverter. VA rating is the product of nominal AC output voltage and the maximum amperage rating of the inverter. For example, Barbara's 120V APC Back-UPS Pro 650 can supply about 5.4A (650VA/120V). Connecting a load greater than the amperage rating of the inverter overloads the inverter and soon destroys it unless the BPS has current-limiting circuitry. Watts equal VA only for 100% resistive loads (e.g., a light bulb). If the load includes capacitive or inductive components, as do PC power supplies, the draw in VA is equal to Wattage divided by the Power Factor (PF) of the load. Most PC power supplies have Power Factors of 0.65 to 0.7. For example, Robert's APC Smart-UPS 1000 is rated at 1000VA but only 670 Watts, which means that APC assumes a PF of 0.67 when rating wattage for this unit.
The run time of a BPS is determined by many factors, including battery type and condition, Amp-hour capacity, and state of charge; ambient temperature; inverter efficiency; and percentage load. Of those, percentage load is most variable. The number of Amp-hours a battery can supply depends on how many amps you draw from it, which means the relationship between load and run time is not linear. For example, our APC Back-UPS 600 can supply 600VA for five minutes, but can supply 300VA (half the load) for 22 minutes (4.4 times longer). Doubling load cuts run time by much more than half; halving load extends run time by much more than twice.
Utility AC voltage is nominally a pure sine waveform, which is what power supplies and other equipment are designed to use. The output waveform generated by BPSs varies. In order of increasing desirability (and price), output waveforms include: square wave, sawtooth wave, and modified square wave (often somewhat deceptively called near sine wave, stepped approximation to sine wave, modified sine wave, or stepped sine wave?marketers are desperate to get the word "sine" in there, especially for units that don't deserve it). The cheapest units generate square wave output, which is essentially bipolar DC voltage with near zero rise-time and fall-time, which allows it to masquerade as AC. Midrange units normally provide pseudo-sine wave output, which may be anything from a very close approximation to a sine wave to something not much better than an unmodified square wave. The output waveform is determined by the inverter. The inverter is the most expensive component of a BPS. Better inverters?those that generate a sine wave or a close approximation?are more expensive, so the quality of the output waveform generally correlates closely to unit price. Astonishingly, we once saw specifications for a no-name BPS that listed output waveform as "pure square wave," presumably intending to confound buyers with "pure" (a Good Thing) and "square wave" (a Bad Thing).
Although it sounds trivial, battery replacement method is one of the most important characteristics of a BPS. Batteries must be replaced periodically, perhaps as often as annually if you have frequent long power outages. Better units have user-replaceable batteries. Lesser units must be returned to the factory for servicing. It's both much less expensive and much more convenient to be able to replace batteries yourself.
The length of warranty is a reasonably good indicator of the quality of the unit. Better units have a two-year parts and labor warranty, although the battery is usually excluded. Lesser units often carry a one-year warranty, and we have seen many of them fail not long past that time. The cheapest units may carry only a 90-day warranty.
Inexpensive BPSs may provide few or no configuration options. They may, for example, be permanently set to transfer to battery if the input voltage drops below 102VAC or rises above 130VAC. Better BPSs offer flexible options for setting such things as transfer voltage thresholds, warning type (audible, visual, email and/or pager notification, etc.), delay before warning, warning duration, and so on.
Inexpensive units provide few status indicators, typically only an LED that illuminates when the unit is operating on battery. Better units provide detailed LED or LCD status displays to indicate such things as load percentage, battery charge status, overload conditions, and battery replacement required.
All units include some form of overload protection. Less-expensive units often use a fuse, and may need to be returned to the factory if that fuse blows. Better units use a circuit breaker that can be reset by pressing a button.
Most units include two types of receptacle, often differentiated by color. The first sort are backed up by the battery; the second sort are surge-protected only, and are useful for connecting items (such as laser printers) that you want surge-protected but do not want to run from the UPS. Also note that units vary greatly in how many receptacles they provide and how convenient they are to use. Inexpensive units mount a few receptacles on the back panel. Better units provide additional receptacles, and arrange them?either by spacing or by making the receptacle a female connector on a short extension cord?so that connecting a power brick or oversize plug does not block other receptacles.
There are two aspects to BPS manageability:
All but entry-level BPS units include a network interface port. By connecting that port to a serial port on the computer?which usually requires a nonstandard cable?and running automatic shutdown software supplied with the OS or the BPS, you can allow the BPS to shut down the computer in an orderly manner during a power failure before battery power runs out. If your computer runs unattended, automatic shutdown is a valuable feature. Some BPS models support automatic shutdown via a USB link. If yours does, make sure the unit also supports serial connection, or it will be unusable with Windows NT 4 and other OSs that don't support USB. Note that if you share one BPS among computers, you will be able to shut down only one of them automatically unless you purchase expensive hardware designed to distribute the automatic shutdown signal to multiple computers.
Simple Network Management Protocol (SNMP) can be used to centralize monitoring and control of a large network. In that environment, having SNMP-capable BPSs is important, but in typical home-office and smallbusiness environments, SNMP support is a nonissue. Inexpensive BPSs do not support SNMP. Midrange and high-end SPSs may include it as a standard or optional feature. If SNMP is an issue for you, make sure that the BPS manufacturer supplies an MIB that is usable by your management package. If you don't know what an MIB is, don't worry about it.
Here are some BPS characteristics that are promoted by marketers but are largely meaningless:
This item is pertinent only if you use shutdown software, either that bundled with the OS or that provided by the BPS manufacturer. The shutdown software provided with a modern OS recognizes most common BPS models, and can usually be configured to support oddball requirements from off-brand BPSs. Most people use the automatic shutdown software bundled with the BPS, as it is usually more functional and supports specific features of the BPS model. In that case, the only thing that matters is that it runs on your OS, which it is likely to do unless you're running something relatively uncommon such as OS/2. Linux support, formerly rare, is now common. OS certification should be at most a checklist item.
Typical BPSs have nominal switching times of 2 to 4 ms. That's best case. Under adverse conditions, such as an extended period of low-voltage, partially discharged batteries, and so on, transfer time can be longer. A typical BPS might list worst-case transfer time of 8 ms, which should be within the hold-time of any decent power supply, even operating under adverse conditions. Shorter is obviously better here, but don't give nominal switching time too much weight.
Most BPS makers include a connected equipment warranty, typically for $25,000. In theory, if your equipment suffers damage attributable to a fault in the BPS, the BPS company pays to repair or replace it. That sounds good, but the truth is that few people ever collect on such warranties. There are so many exclusions and limitations, including the fact that the coverage is often subrogated to your home or business insurance, that such warranties are all sizzle and no steak.