UPS question

Chuck Swiger cswiger at
Wed Aug 11 16:44:43 UTC 2010

Hi, Ryan--

On Aug 11, 2010, at 8:51 AM, Ryan Coleman wrote:
> Total: 495W
> According to a calculator if I enter all that information:
> It says that it will use 693VA.

That sounds reasonable.  The better PSUs have "80 Plus" certification for efficiency, and that's better than the typical wall warts used for modems and switches and the like commonly manage.  (The efficiency they're assuming is a bit over 70%; using 80% would be around 600VA.)

> Enter that into
> It requires Amps... 495W  / 120 voltage = 4.125 amps... doesn't seem right but...
> 192 hours... that's not right, right?

Assume for discussion their number was right.  In order to get 495W of output load, the UPS needs to provide 693 volt-amps of juice to your equipment.  After the inverter and 10:1 stepup transformer used to convert 12VDC or whatever the UPS batteries are charged to up to 120VAC, the current needed would be 5.77 amps.  However, the 12VDC battery source itself would be getting a draw of 57 amps (ideally; again, the inverter+transformer themselves might only rate about 90% efficiency for very good quality UPS, so would be drawing more like 60 or 65 amps).

A standard APC/Tripplite/whatever 700VA UPS tend so have a lead-acid battery reasonably similar to a car battery, and typically will have around 100 amp-hours of charge; they'd probably give you 90 minutes of backup time.  But you can look up the detailed specs of specific models and work from their amp-hour (or watt-hour) ratings-- actually, I think I'm guestimating more from what a 1200VA unit might provide, and a 700VA model is probably going to provide more like 40-60 minutes of power...


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