UPS wattage

[muno]

I have some UPS that says "252W". Now, my PSU is 300W.
Should that mean anything to me? I plugged it in the wall, and the computer. Taking out the wall cable didn't do anything, the computer stayed on. This was just a brief test I did, like few seconds.

If I had a 250W PSU and the manual of the ups would say '5minutes uptime', would this time be divided by 1,2 meaning 4minutes uptime with 300W PSU?
Or does that have anything to do with it?
What do the milliampers mean on the UPS?
-M

[DVNT1]

Quote:

Watt Ratings differs from Volt Amp Ratings
The power drawn by computing equipment is expressed in Watts or Volt-Amps (VA). The power in Watts is the real power drawn by the equipment. Volt-Amps is called the "apparent power" and is the product of the voltage applied to the
equipment times the current drawn by the equipment. Both Watt and VA ratings have a use and purpose. The Watt rating determines the actual power purchased from the utility company and the heat loading generated by the equipment. The VA rating is used for sizing wiring and circuit breakers.

The VA and Watt ratings for some types of electrical loads, like incandescent light bulbs, are identical. However, for computer equipment the Watt and VA ratings can differ significantly, with the VA rating always being equal to or
larger than the Watt rating. The ratio of the Watt to VA rating is called the "Power Factor" and is expressed either as a number (i.e. 0.7) or a percentage (i.e. 70%).

The power rating of the UPS have both Watt ratings and VA ratings. Neither the Watt nor the VA rating of a UPS may be exceeded. In most cases, UPS manufacturers only publish the VA rating of the UPS. However, it is a standard in the industry that the Watt rating is approximately 60% of the VA rating, this being the typical power factor of common loads. Therefore, it is safe to assume that the Watt rating of the UPS is 60% of the published VA rating.


How to avoid sizing errors

Using APC sizing guidelines or and APC Configuration can help avoid these problems, as the load power values are verified. Equipment nameplate ratings are often in VA, which makes it difficult to know the Watt ratings. If using equipment nameplate ratings for sizing, a user might configure a system which appears to be correctly sized based on VA
ratings but actually exceeds the UPS Watt rating.

By sizing the VA rating of a load to be no greater than 60% of the VA rating of the UPS, it is impossible to exceed the Watt rating of the UPS. Therefore, unless you have high certainty of the Watt ratings of the loads, the safest approach is to keep the sum of the load nameplate ratings below 60% of the UPS VA rating.

Note that this conservative sizing approach will typically give rise to an oversized UPS and a larger run time than expected. If optimization of the system and an accurate run time are required, use APC sizing and Worldwide Web
configuration tools.


Conclusion

Power consumption information on computer loads is often not specified in a way that allows simple sizing of a UPS. It is possible to configure systems that appear to be correctly sized but actually overload the UPS. By slightly over
sizing the UPS compared with the nameplate ratings of the equipment, proper operation of the system is ensured. Over sizing also provides the side benefit of providing additional UPS backup time.

from http://nam-en.apc.com/cgi-bin/nam_en...YWdlPTE*&p_li=

[dunbar]

You're close to the limits of your UPS.

Just because a PSU says 300W does not mean you are drawing 300W out of the UPS. I'm going to take a few liberties to explain this as simply as it can be, so PM me if you want to haggle over details.

Most PCs I have used do not actually pull 100% of the rated wattage from the PSU outputs, more like 50-70%. Thus, the PSU, rated for 300W output power, is more likely delivering say 210W through the PSU outputs.

No PSU drawing 210W input power is going to deliver all 210W to the output of the PSU, because there is power wasted in the process of converting wall power to DC voltages. The better PSUs are >80% efficient, thus 210W output / .80 will be 262.5 W input to the PSU..... this is what the 250W UPS is delivering to the PC as it runs.

Gulp.

Ok, I took a few liberties: I assumed the PSU was a PFC design, PFC = Power Factor Corrected, which means there is minimal harmonic current content (aka minimal current distortion), and I also added monitor power to the PC PSU, which isn't what is actually happening, but even the modern monitor is roughly 80% efficient as a power conversion device, so the astute observer should see the parallel loads (monitor and PC) seen by the UPS can be treated as one converter. And non PFC designs will push the RMS equivalent a bit higher than the 262.5W I calculated.... more of a problem.

You need a larger UPS, IMO

[muno]

Thanks dunbar, simple enough.

Thank you too dvnt1 but I didn't understand that
-M