Choosing a UPS ... scientifically?

[SpacemanSpiff2000]

Hi all. Wondering if there are any folks here who are well-versed in electricity to help me answer this.

I've got a recently self-built server that only lacks some more HDDs and maybe an extra SATA card ... essentially complete. I've also got one of those Kill-A-Watt meters. What I don't have yet for this box is an Uninterruptable Power Supply (only spelled out for the sake of search engines vs. the drivers in brown), so it's leeching off another's.

Yes, most folks just estimate this stuff, and I've seen a few online "calculators" that give wildly different answers to the same criteria. And I don't want to trust something from a UPS maker's site. I just figure I should be able to use the Kill-A-Watt to monitor this thing and know for sure how much UPS it needs.

Where I'm stuck are the Watts and VAs. I can take the time to add up max Watts/component but many UPS's come in VAs only. One site says A*V=VA=W but some makers sell 600VA/340W models, so something's not right there. I put the meter on the server and it never seems to go above 218VA, even when stressed a bit. Seems kinda low to me. Although it does occur to me that maybe it's being limited by being on a full UPS already. Reasonable assumption? Not sure how that part works in order to know.

Essentially what I'm looking for is: Is there a certain way this should be setup (e.g., direct to the wall? doesn't matter?) and what results/numbers do I use from the KAW to help me make this decision based on knowable facts? Some electricity background might be nice, too, but I'm not looking for a college dissertation on the matter.

Thanks! - Spiff

[bob]

In the US it is 110V and a plug is 20A. That makes 220VA. So running from the plug "server and it never seems to go above 218VA" you could run 10 of them. Just know that if you did that the startup draw is greater.
Watts are VA's. That is the VA being drawn like 110 x 20

Unsure but the 600VA/340W model may be the startup draw/sustained draw

Now there should be charts from the maker how long the power can be outputed based on draw. Remember many UPS have fans that require a large amount of the possable storage.

[David M]

All of the older wall outlets are 15 amps at 110 to 120 volts. Many of the newer outlets are 20 amps, but not all. So the power (watts) available at a wall outlet in this country can be anywhere from 15 x 110 = 1650 watts to 20 x 120 = 2400 watts.

VA is potential (volts) times current (amps), which equals power (watts). This formula is true for direct current. You use VA to describe power for AC and watts to describe power for DC.

For AC you multiply volts times amps to get power but then you also have to multiply in a power factor to compensate for inductance or reactance and resistance. The more power efficient a load is, the closer to one will be the Power Factor.

The situation is that the inverter part of the UPS (what converts DC to AC) is not 100% power efficient. In converting the DC power from the battery into AC power, there is a loss of energy which we see as waste heat. So the ratio of the power drawn by the load (the computer) divided by the total amount of power drawn at the wall outlet is called the Power Factor.

The formula for Power Factor is then Power (watts)/ VA (watts) = Power Factor

In order to compensate for this inefficiency, the VA rating has to be higher than the power rating (watts). The greater the difference, the greater the power factor.

So in the case of the inverter you are looking at, you have 340 watts/600VA = 0.56. Or in other words, 56% of the electrical power is kept as electrical power and 100-56=44% becomes power converted to waste heat when electrical energy is being converted from DC to AC at the inverter part of the UPS.

The battery charger part of the UPS will be more power efficient than the inverter.

To answer your question, the power rating in watts (not VA) for your UPS has to be greater than the power drawn by your computer for it to work. Another factor in choosing a UPS is how long it can sustain producing the amount of power that your computer is drawing. It must be long enough to save your work and for your computer to shut down properly.

http://www.labsanywhere.net/circuit/.../lecture12.php

[strollin]

David's explanation is very good but there is more to it than that. Let me explain this way: Wall socket AC in-->Convert to DC-->Charge battery, that's the input part. The output looks like this: DC from battery-->Invert to AC-->Computer. Efficiency comes into play while charging the battery as well as supplying power to the equipment.

However, the efficiency of the unit is not what the OP is asking about. While efficiency will effect how much it costs to operate the unit, it doesn't really mean anything to the OP, he wants to know how much power he needs/how large of a UPS he needs.

You need to know how much power your equipment draws and how long you wish to be able to power said equipment. The design of a UPS is typically to provide sufficient power during AC interruption to keep the equipment running long enough for you to do an orderly shutdown. 10 minutes should be enough, more than that is a safety factor. If you need to keep the equipment powered for extended periods then a backup generator is what you need and the UPS fills the gap between when the AC goes out and the generator kicks in.

So how much power? I would size the UPS to at least twice what the equipment draws so in your case of measuring 218VA, I would go with at least a 450W UPS with a runtime of at least 10 minutes. It's perfectly OK to get a larger UPS then you require, having more capability doesn't hurt anything but your pocket book, having too little means it won't do the job. Be sure to factor in the power requirements for any peripherals you need to keep running such as a monitor, router, external drives, printer, etc...

There are other factors to consider as well. You want the output of the AC to be as close to a sine wave as possible. Less expensive units have inverters that produce a square wave which works but the equipment was designed to run from a sine wave and running from a square wave has a tendency to cause equipment to run hotter since the inversion from square wave AC to DC is less efficient than inversion from sine wave AC to DC.

[David M]

Originally Posted by strollin David's explanation is very good but there is more to it than that. First off, where David talked of the inverter he should have said converter. Let me explain this way: Wall socket AC in-->Convert to DC-->Charge battery, that's the input part. The output looks like this: DC from battery-->Invert to AC-->Computer. Efficiency comes into play while charging the battery as well as supplying power to the equipment. However, the efficiency of the unit is not what the OP is asking about. While efficiency will effect how much it costs to operate the unit, it doesn't really mean anything to the OP, he wants to know how much power he needs/how large of a UPS he needs. You need to know how much power your equipment draws and how long you wish to be able to power said equipment. The design of a UPS is typically to provide sufficient power during AC interruption to keep the equipment running long enough for you to do an orderly shutdown. 10 minutes should be enough, more than that is a safety factor. If you need to keep the equipment powered for extended periods then a backup generator is what you need and the UPS fills the gap between when the AC goes out and the generator kicks in. So how much power? I would size the UPS to at least twice what the equipment draws so in your case of measuring 218VA, I would go with at least a 450W UPS with a runtime of at least 10 minutes. It's perfectly OK to get a larger UPS then you require, having more capability doesn't hurt anything but your pocket book, having too little means it won't do the job. Be sure to factor in the power requirements for any peripherals you need to keep running such as a monitor, router, external drives, printer, etc... There are other factors to consider as well. You want the output of the AC to be as close to a sine wave as possible. Less expensive units have inverters that produce a square wave which works but the equipment was designed to run from a sine wave and running from a square wave has a tendency to cause equipment to run hotter since the inversion from square wave AC to DC is less efficient than inversion from sine wave AC to DC.

On boats at least they are called inverters, a device which converts DC to AC. From what I have seen they are also called inverters on land as well. http://www.amazon.com/Whistler-Pro-2...inverter+2000w

I went into the explanation about inefficiency, power factor and power loss as heat because it directly relates to the conversion between VA and Watts, which the OP was also asking about.

I do state that the battery charger is not 100% efficient, but more efficient than an inverter.

More precisely, an inverters raw waveform is a square wave. Electrical "tricks" are done such as lopping off the corners of the waveform to make the square wave closer to that of a sine wave. There are also true sine wave inverters, which do have true sine waves or have sine waves that are very close to a sine wave.

I think that a UPS that can produce twice the AC power of what is needed is overkill.

I answered the OP's original question in the last paragraph regarding what factors would make for a suitable UPS for him.

[strollin]

Sorry David, I meant to expand on your excellent post, not contradict, you apparently took offense. I removed a line from my post that I felt may have been construed as critical.

Conversion is used to describe the process of going from AC to DC while Inversion describes the process of going from DC to AC. Converting AC to DC is much easier and very straightforward while Inverting DC to AC is much more complex.

[David M]

Originally Posted by strollin Sorry David, I meant to expand on your excellent post, not contradict, you apparently took offense. I removed a line from my post that I felt may have been construed as critical. Conversion is used to describe the process of going from AC to DC while Inversion describes the process of going from DC to AC. Converting AC to DC is much easier and very straightforward while Inverting DC to AC is much more complex.

No offense taken. I agree with your last paragraph.

Cheers!

[tmathews]

I don't know if applies to this case, but in the electric power industry, Watts is used for power that's consumed at the load (like in a resistor), Vars are used for reactive power (energy that's moved around but not consumed, like charging a capacitor then pulling it out later), and VA = Watts + Vars. From the 600VA/340W spec, I'd read it as being able to drive a 340W resistive load with a power factor (PF) of .56 (where most computer power supplies are in the .95 PF ballpark). 218VA for your server sounds pretty reasonable to me so the 600VA/340W unit should work nicely with it with a pretty decent safety margin.

[David M]

What is the difference between Voltage-Amps (VA) and watts and how do I properly size my UPS? ? Power Solutions

[Petef56]

Originally Posted by strollin So how much power? I would size the UPS to at least twice what the equipment draws so in your case of measuring 218VA, I would go with at least a 450W UPS with a runtime of at least 10 minutes. It's perfectly OK to get a larger UPS then you require, having more capability doesn't hurt anything but your pocket book, having too little means it won't do the job.

I agree with the "doubling" concept. I've always done it that way and never regretted it. Having all that head room allows the unit to run with less stress and the batteries seem to last very long. I typically get 4-5 years out of my UPS batteries. Lead Acid batteries degrade more dramatically if you discharge them below a certain level. I believe that level is something like 50% or 60%. So "doubling" of the required minimum size is very beneficial in that respect.

http://www.pacificlanworks.com/gener...ttery-705.html

Excerpt...
Do not utilize your UPS above 80% of its rate capacity. Once you exceed it, your runtime battery life will be shorten and if there is a power failure, a fully loaded UPS will drain and discharge much faster, and the lifespan of the batteries will also be shorten.

---pete---

[glc]

It never hurts to oversize a UPS. What's most important in my opinion is get a "line-interactive" unit to filter out power spikes. I have a 1250 watt APC that I paid $150 for at Sam's Club. I have 4 computers (generally only 2 of them are on at any one time) and a LCD monitor on it, and I get about 30 minutes runtime with a power failure.