Get the Right Size Uninterruptible Power Supply (UPS) for Your Home Office

Get the Right Size Uninterruptible Power Supply (UPS) for Your Home Office
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One Way to Choose a UPS

The major manufacturers of uninterruptible power supply equipment have kindly put product guides or “selectors” on their websites. Supposedly, entering some basic information about your setup will painlessly lead the buyer to the smart choice. If this sounds too good to be true, it is!

I used exactly the same specifications for type of computer including processor, size of monitor, peripheral equipment needing protection, desired runtime on battery and allowance for future expansion on three UPS makers’ sites. Results ranged in cost from a low of $79.99 to a high of $1,600.00. As for the Volt-Amps (VA) ratings of the recommended equipment, the low went from 650VA to 3,000VA. This is worth repeating—the buying guide calculations were based on exactly the same equipment across three manufacturers.

More importantly, the low-end uninterruptible power supply models would have drastically under-supported the actual power draw of the involved equipment. Both the middle and high-end models would have seriously over rated the need for protection and given far longer run times than would ever be needed.

The Smart Way to Select a Battery Backup Power Supply

To end up with the optimum size for your specific computer setup, you will have to calculate the power load that will be applied to the battery when needed during an outage. This is not always easy to do. In the best case, the computer, monitor, and any other externals you want to protect should have somewhere on each piece of equipment either the VA rating or the amperage. In the best case, there may even be a figure for Wattage. If you can’t locate this data on the equipment, try the specification pages of whatever documentation that came with the models. Failing this, try the manufacturer’s websites.

If all you have are amp ratings, then multiply this number by the house current, presumably 120 volts in the United States. The result equals volt-amps. A more accurate sizing process will involve adding up the total wattage of the equipment. With personal computers and peripherals, the wattage is normally equal to about 65% of the VA. When I used these numbers for a specific calculation on each of the three websites, I came up with the best option, depending on the maker, being a UPS between 1200VA and 1300VA.

With this kind of data in hand, you can shop in stores or online for a more appropriately sized UPS. It is better to err a bit on the side of over-estimating backup than under-estimating. For this reason, unless trying to pare the cost to the bone, look for an UPS that is rated at about 60 percent above the total equipment draw. For example, if your calculations add up to 900VA, look for a 1500VA UPS. This will provide some margin of safety and a longer runtime on backup.

As for what needs to be protected with the backup system, this is a matter of individual preference. At a minimum, the computer and monitor should be connected to the UPS. An external backup hard disk running constantly also needs protection to allow for proper shutdown. Printers should not be connected to a UPS since their load would require a much heavier battery backup.


Swarthmore College Computer Society

Photo credit Uninterruptible Power Supply credit Charles R. Anderson

This post is part of the series: Adding an Uninterruptible Power Supply to Your Home Office

An uninterruptible power supply (UPS) provides peace of mind, protection of electronic equipment and may prevent loss of important information in open computer files in the event of power failure. This series covers why you need a UPS, how to select the right size, and home wiring considerations.

  1. Do You Need an Uninterruptible Power Supply in Your Home?
  2. Purchasing the Uninterruptible Power Supply Your Home Office Needs
  3. Uninterruptible Power Supplies and Electrical Wiring Concerns