Here’s a question that appeared in my inbox late last night:
Happy New Year IT Nerd. I went through an extended power outage due to the ice storm here in Toronto. When I lost power to my computer, I lost some documents that I was working on and had foolishly left open overnight. So I am now convinced that I need a UPS. My question is, how can I pick the right size of UPS?
Happy new year to you as well and thanks for the question.
First you need to figure out what you are going to run off the UPS when power goes off. For most of us, it’s a computer, monitor, and perhaps a external hard drive. Simply take the number of volt amps that each device uses and add them up. Typically, the number of volt amps that the device uses will be on a label on the back of the device. If that label doesn’t list volt amps, no problem. Here’s how you convert to volt amps:
If you see watts listed, use this formula:
watts (W) x 2.0 = volt amps
If you see amps listed, use this formula:
Amps (A) x 230 = volt amps
So let’s say I want to run a Mac Mini with a 85 W power supply and a 20″ LCD monitor that uses 1.25 amps off a UPS. Doing the math, I get 170 VA for the PC and 287.5 for the monitor. That adds up to 457.5 volt amps. Keep in mind that these figures are peak usage. In other words, this is the maximum the device will use. Typical usage should be less than that, but you should always plan around the worst case scenario.
So I now know that I need a UPS that has at least 457.5 volt amps. But you should be thinking of getting a UPS that’s capable of handling more than that as total load that you will be placing on the UPS should not exceed 80% of the unit’s capacity. Plus if you want to add more stuff to the UPS or you want the UPS to power your hardware in a blackout for a longer period of time, you should get way more than whatever figure you come up with. For that reason I typically recommend that you plan for at least 30% on top of the volt amps that your hardware needs. So in my case, that’s 594.75 based on this formula:
total volt amps x 1.3 = recommended size of UPS in volt amps
When you look at UPS models from companies such as APC and Tripp Lite, they have a number that represents the number of volt amps the UPS can power. For example, APC has a line of UPS models called the BackUPS XXX where the XXX is the amount of volt amps the UPS can handle. The same is true for Tripp Lite which has a line of UPS models called AVR XXX. So, if I am looking for an APC the model for me is the BackUPS 650 and from Tripp Lite it would be the AVR 700 U based on their respective lineups. A bigger number of VAs is always preferable. So if you can afford to spend more, you should.
Now most UPS models come with a USB cable to connect to your computer (in other words, make sure the one you pick does) so that when the power gets cut, the computer can get that info from the UPS and shut down the computer based on the parameters that you define. Most operating systems such as OS X or Windows 7 have built in UPS support that does not require you to install additional software. But conversely the software supplied by the UPS manufacturer could come in handy as it will typically tell you the health of the UPS as well as how frequently you have power outages, spikes or surges.
The typical UPS has battery backed and non-battery backed outlets. Plug only the computer, external hard drive, and monitor (if it’s an LCD monitor) into the battery backed outlets. Never plug devices such as laser printers and CRT monitors into those outlets. Laser printers and CRT monitors will drain the battery faster and put a lot of stress on the UPS that will end with you replacing the battery or the UPS as a whole much faster than you should.
Finally, once you purchase your UPS you should make a note of the date you purchase it so that you can either replace it with a new one or replace the battery. Batteries typically last 2 to 3 years depending on the model, and it is sometimes cheaper to replace the entire UPS rather than replace the battery. By keeping an eye on this, you can make sure that you’re always protected.