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What Size UPS Do I Need For My Computer?

It's dark all of a sudden. Did this ever happen to you?

Screen Dark, Data Lost?
Not For Me – Thanks to UPS

Did it ever happen to you? You are working on a complicated spreadsheet on your computer and all of a sudden… power is out. The screen gets dark, silence follows, and you might hear your neighbor through the wall shout out loud, “Oh, shit!”

I got used to those power outages lasting from 30 seconds to 1 hour. Like during the recent heavy rainfalls in early December. In 95% of all cases power comes back in less than 5 minutes.
  • Other people suffer data loss.
  • Some people even had their hard drives crash or other hardware damaged (Ouch!)

I am not one of them.

After moving to the Canaries I quickly realized that electric power is not as stable as in the big cities where I lived earlier.

Therefore, I got myself a UPS (uninterrupted power supply), a kind of a battery that keeps supplying your PC and monitor with juice even when power fails. Usually you can run up to 15 minutes on battery power with your normal PC or Mac setup.

The reason for writing this article is I want to share with you what I have learned about UPS during the last three years. I just replaced my UPS with a new one because the battery died.

Update: Just added a “How To Video” going through the process.

It's inside the blog post.

  • You should have one for each PC unless it is a battery powered notebook.

  • Only connect your computer equipment (PC, external drives, etc., LCD monitor, modem, …, printer if it is not a laser printer (those can draw way too much current.)

  • A more expensive UPS can signal your computer to shut down when the outage exceeds, for example 5 minutes.

  • A good UPS protects also against voltage spikes and surges, stabilizes the voltage and filters the current.

  • The batteries of a UPS are old fashioned—like car batteries—and should not be handled like modern rechargeable batteries for cell phones or notebooks. They wear out far more quickly.
    Tip: Don't test excessively and too frequently how long your equipment stays on!

    When you test do not just unplug the UPS from the socket, because you disconnect it from the ground that way which can be dangerous! Use the test button on the ground fault interrupter of your main circuit instead.
  • I made it a habit to start saving all open documents and shut down the computer if the power outage has reached 7 minutes. Just to be on the safe side. Consider the power situation in your location when making your plan. I know here, if it is not back within 5 minutes it will take much longer (happens a couple of times a year.)

Now, I hear you asking,

“What ‘size’ UPS should I get?”

Here is my rule of thumb:

(1) I list the power input ratings of all equipment I want to protect.

I do this separately for either Watts or Volt Ampere. Volt Ampere rating preferred if available.

If Volts and Ampere are stated individually, multiply them.

If the input voltage is variable use the smaller number. (low voltage means high current; high voltage means low current; the maximum current is stated, use the low voltage for your calculations.)

Always look at the input ratings of the equipment.

My “How To Video”: How To figure out the right rating for a UPS power supply?

Retweet @johnfurst
John W. Furst for E-biz Booster Blog

The example from the video:

Computer:700 VA 
Monitor:100-240 Volt, 0.5 A
gives 100 x 0.5 = 50 VA
Modem: 25 W
Ink Jet Printer:Power supply 100-240 V, 1500 mA
(1000 mA = 1 A)
gives 100 x 1.5 = 150 VA
Subtotal:900 VA25 W
25 W ~= 25/0.6 VA
Total: 900 + 42 = 942 VA 

The subtotals are 900 VA plus 25 W.

(2) Divide subtotal for Watts by 0.6 and add it to the subtotal of the Volt Ampere.

Total is 900 VA + 25/0.6 VA = 942 VA

Hurray, let's go shopping for a 1,000 VA
(or above) rated UPS

Why not go to Amazon for a first overview?
There are some great product reviews from customers.

UPS Uninterrupted Power Supplies | Electronic (Amazon) (↑)

I deliberately did not repeat the more technical descriptions you find on sites like Wikipedia. You are smart enough to look it up there yourself. Some links below.

For geeks: The best approach is to measure the real and apparent power of all equipment involved. I used to do that originally but just realized that my power-meter is dead now, after not having been used for a couple of years. I know from experience that my rule of thumb puts me on the safe side.

The difference between VA and W ratings comes from capacitors, inductors, and rectifiers in electric circuits like power supplies. In layman terms: The average PC is hungry for extra current, therefore, the factor 0.6.

Power ratings on equipment are usually stated a maximum level. The actual consumption is usually less.

This approach should give you a protection of 15 minutes in the average scenario. You can dive into more technical documentation or simply read some of the customer product reviews on Amazon (link above.)

Further readings:

I bought my UPS in a local computer store last week by the way. And I already needed it once. That means it already paid off for me.

Do you have a UPS already?

If not, start shopping around!

While I did not really plan for it in 2009, I have increased data security in my business significantly last year.

Not only that I am handling my entire email storage at Gmail in the cloud for a couple of month now.

I finally keep records and procedures for handling of my various online accounts up-to-date.

Happy 2010.
Be safe and prosper.

John W. Furst

P.S.: No, I am not an electrical engineer. My father is. Myself? I was quite good in physics. However, no guarantees here.


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This post was mentioned on Reddit by marvin566: Rule of thumb how to estimate the size of a UPS. Protect your computer from power outages.


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alan on :


I was directed to your post when looking for info on this subject. I was pleased to read such interesting information thank you I will certainly be back.

Margaret Gray on :


As a professional author I know that, 'Oh, shittttt!!!!' scream only too well so the information you have just given me is going to be a life saver. Terrific, thanks!

Jennifer M. [] on :


Great Margaret,

Good to hear the info has been useful for you. Thanks for letting us know.


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