Question : Problem: VA rating for power supplies

I am planing on getting the new HEC Zephyr 650W Power Supply
I own a 1000VA stabilizer. since the area of residence (located in Egypt which employs 230 to 240 voltage system) has constant brownout the voltage is often 200V to 185 in the worse senario.

My previous 400watts power supply had an AC input rating of 100 to 240V and the ampere is printed as 10/6.5 A (assuming it means 10 divided by 6.5 the ampere rating should be around 1.54) so the VA rating according to my calculation should be 240V times 1.54 Amperes which is = 370VA, So lets say the VA rating is roughly 400VA

When I did the same procedure to calculate the VA rating for the new 650W PSU something didn't make sense. According to PSU sticker found on this review website (http://www.pcstats.com/articleview.cfm?articleID=2081) the AC input is 100 to 240 and 10A. So 240 multiplied by 10 is 2400VA....?!!!!!! this is a way too large value. I seriously doubt that there is any computer that draws 2400VA (roughly 1700Watts), typically devices that consume 1200W to 1700 are high voltage appliances such as tea kettle, vacuum cleaner, electric oven...etc not computers. Computers would add quite a lot to the power bill if they consumed that much, wouldn't they?

Can someone shed a light on this matter. This is how I have been computing VA rating for power supplies and similarly any other devices such as monitors by multiplying AC input voltage and ampere for quite a time with a consistent results which are not quite the case for the HEC Zephyr 650W Power Supply.

Thanks in advance

a_anis3000

Answer : Problem: VA rating for power supplies

The rating on the input plate is NOT the constant current the device draws ... it's the max current (so you can be sure it is connected to a source with sufficient capacity).   For example, your old 400w supply with the 10/6.5 rating was showing the max current draw at the lower voltage (100v x 10a => 1000va) and upper voltage (240v x 6.5a = 1560va).   The different maxes at different voltage is a function of several things ... power factor; efficiency; and inrush current (which tends to be higher at higher voltages).   Within a few milliseconds, the power supply will stabilize at the current needed to supply the power demands of the system ... a draw MUCH lower than the maximum.

Your new 650w power supply shows a 10amp draw, but does not note what the nominal voltage is for that draw.   It's PROBABLY the rating for the most common voltage ... 120v ... but there's no guarantee of that.   If that's the case, that would be 120v x 10a = 1200va, which is about right for a reasonably efficient unit.  

But the maximum draw isn't what you need to be concerned about, as that only happens for a few milliseconds at turn-on, and all stabilizers and UPS units can sustain a reasonable % overload for that short time.   What's more important is that your stabilizer has enough capacity for the normal load of the computer.

How much power a  power supply actually draws depends on the efficiency of the unit.   The Zephyr is rated at 70% ... so assuming it achieves its rated efficiency,  the constant draw of the unit if it was operating at maximum power would be about 650w/(0.70) = 928 watts.    Thats too much for a 1000va stabilizer ... since these units typically only have about a 65-70% power factor (thus can only supply 650 to 700 watts).   As an example, a 1200va APC UPS unit is rated for 780 watts.

But your PSU will most likely NEVER be drawing its max rated power (if so, you've undersized your power supply) ... so you may be just fine ... you should compute the actual draw of your system components to see just how much power you'll actually be drawing.    But if you want to play it safe, and think you'll be pushing the limits of the power supply, then get a stabilizer with a wattage rating in the 900 watt range ... which means about 1500 VA.

Note:   If you are not building a new system and are simply replacing your power supply, then it's a safe assumption that your system doesn't draw more than a sustained 400 watts (else your old supply wouldn't have worked).   If that's the case, your new unit won't draw more than 400/(0.70) = 571 watts, which is well within the capabilities of your current stabilizer.   In that case you have nothing to be concerned about :-)