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.