Calculating Volt-Amps (VA)

When we discuss the capacity of a UPS, we use the unit Volt-Amps, or VA. A UPS which supplies 1000VA will power a PC and monitor longer than a smaller 350VA unit. To figure out VA, we multiply Volts by Amps. So for example, 120V (x) 8A = 960VA.

There are essentially four main styles of UPS, whose circuitry ultimately relates to how expensive the unit is, and how well it continues to deliver electricity when the utility power fails. Some of the "higher-end" UPS's are only found in server or hosting facilities, but we'll touch on them since knowing the differences helps when deciding what to get for a particular application. As most UPS's are virtually identical in appearance, you'll really have to pay careful attention to the manufacturer's specs to find out which style of circuitry their uninterruptable power supply is based upon.

The Standby UPS is the most common, and most economical type of UPS available to consumers for the home or office. Typically, a standby UPS will range in capacity up to about 1000VA, and so they are generally best suited for just one computer and monitor. A standby UPS, sometimes also referred to as an 'offline UPS', is a filtered and surge protected power supply, with battery backup. The utility power is the primary source of electricity, and in the event of a power outage the internal battery becomes the secondary power source, supplying DC voltage to an inverter which converts DC voltage to 120V AC.

The switch to battery power may take just an instant, but sensitive electronics can be affected by the momentary drop in power. The rule of thumb though is that most computer power supplies can handle a few milliseconds of 'hold up time', time where the PSU continues to function with no input. In general, the Standby UPS is a cost effective way to keep your desktop PC from shutting off due to the loss of utility power, or even a simple brown out.

A slightly better alternative to the standby UPS is the Ferroresonant Standby UPS. Ferroresonant referrers to the type of transformer used in the uninterruptable power supply, and the reason this style UPS is considered somewhat better is that the transformer provides smoother switching when a power outage does occur. A Ferroresonant Standby UPS still draws its primary power from the 120V AC utility supply, but instead of having a switch (to change from AC mains power to the internal battery supply) the transformer, by design, selects whichever input is still operating. How the ferroresonant transformer exactly operates is beyond the scope of this review, but you can expect this type of UPS to range in size up to about 15,000VA.

A Line-Interactive UPS is typically sized up to 3000VA, and uses a combination inverter/converter to charge the battery backup, and convert DC to AC when a power outage occurs. The advantage of the Line-Interactive UPS design appears to be that the inverter/converter which handles the switching between the 120V AC utility or battery source is always supplying voltage to the connected devices, so switching between utility or battery power is a lot smoother. Additionally, since this configuration provides voltage regulation, it allows the UPS to handle brownouts and moderate voltage spikes in a rudimentary by adjusting voltage to nominal levels. The Powerware 5125 is based on a single phase Line-Interactive UPS style.

The final type of UPS we're going to mention is the Online UPS , and it offers the best power regulation and supply of all the uninterruptable power supply styles we've touched upon. It's also supposed to be the most expensive and least efficient design of all three, but considering that it's best suited to server environments, that's not something most of us need worry about. The reason the Online UPS costs more, and is less efficient is because the primary source of power at all times is the battery. The online UPS works by continually charging a battery which is connected to an inverter, which transforms the DC voltage into 120V AC. Typically available in sizes from 5000VA and up, the reason the online UPS is considered so ideal is that in the event of a power outage nothing happens; the batteries still supply DC voltage to the inverter, and the inverter still converts that DC to AC to supply the computers. The batteries will eventually loose power since they are no longer being charged, but there is no variation in the power, and no switching delays as there can be with the other styles of UPS.

Choosing a UPS to fit your PC

The first thing I would suggest when selecting a UPS is to a have a rough idea of how long you expect it to keep your PC running. A 500VA UPS may only be good for 5-6 minutes, while a 1000VA UPS may provide over 20 minutes of run time for the same computer. Most manufacturers will list how long their UPS batteries will hold up given a specific power load, but since it's pretty difficult for a consumer to accurately measure their PC's true power draw without some tools (hint, that 300W power supply rating is pointless here), these figures should serve only as a rough estimate of what you can expect.

In any event, there are two numbers you should pay close attention to when selecting a UPS, the first is Apparent Power Load (VA) and the second is Wattage (W). In the case of the Powerware 5125 we are reviewing here, it is rated for 1000VA and 700W. Wattage will not always be listed, but the Apparent Power, or capacity of the UPS will always be stated. The larger the VA rating, the longer the UPS will last when the power cuts out.