Volt amps vs watts?

[Red Squirrel]

Ok this is confusing me. I see some 1000VA UPSes that are 500 watts, and others that are like 1000 watts, but according to what I've read Volt amps are the number of amps times number of volts, but that's what watts are, so what's the difference between VA and Watts?

 

[Continuity27]

Volts X Amps = Watts

I dunno what VoltAmps are...

 

[SirBrass]

When speaking about units of measure, VoltAmps (VA) IS V*A == Volts*Amps == Volts x Amps == Potential Difference multiplied by Current = Power = Watts.

So, VoltAmps would logically be watts. But it obviously isn't in these cases. My mind is befuddled as to why as well. Unless they mean VoltAmps as the RMS power and Watts as the peak (dunno why they'd do that). That's a shot-in-the-dark guess anyway.

 

[Red Squirrel]

Yeah according to what I've read VA are V * A but that can't be, since Watts is that, and VA ratings on UPSes is always different then the Watt ratting. And it does not "vary directly as", seen some 500VA units with 500 watts, some 1000VA with 300Watts. really weird.


Edit: so when buying a UPS what should I assume is the "real" watts?

Ex: I'm wanting to plug about 300Watts worth of stuff, so do I need 300VA or 300Watts? (obviously I want higher to not put it at full load, but I'm just theoricly speaking here)

 

[Goi]

Logically they're the same, but there's a reason why UPC ratings are in VA rather than W. I just don't remember what that reason is. Try google?

 

[Navid]

Power is RMS voltage multiplied by RMS current multiplied by the Cosine of the angle between voltage and current.

In case of Direct Current (DC), there is no phase difference. So, the phase is 0. The Cosine of 0 is 1. So, in case of DC, power is the same as Voltage multiplied by current. In case of AC, which is the case of a UPS, you have to keep the angle in mind.

That Cosine of the angle between the voltage and current is called the power factor (PF).

 

[OrganizedChaos]

without getting into really complicated math such as trig. for your purposes you assume 0.7VA=1 watt
that will give you enough play to not have to worry about complicated stuff

 

[Red Squirrel]

So if I get this right, the VA is the wattage it can handle at the "worse" case wave shift difference? Or am I'm clueless about what I'm talking about? lol I sort of get the AC /angle concept but not really. I know AC alternates from 120 to -120 60 times per second in a sine wave so if the amperate wave is not in sync with the voltage one then watt readings are differrent... something like that.

 

[SirBrass]

Ok, so VoltAmps is the power, but it's just a way of saying that they didn't convert the power to DC (like the power is converted into when it gets to the power supply), and so there is a phase difference between the voltage and the current.

Edit: Navid, I congratuate you. I totally forgot about the power factor. You have put this 4th year EE major to shame.

 

[Navid]

Originally posted by: RedSquirrel
So if I get this right, the VA is the wattage it can handle at the "worse" case wave shift difference? Or am I'm clueless about what I'm talking about? lol I sort of get the AC /angle concept but not really. I know AC alternates from 120 to -120 60 times per second in a sine wave so if the amperate wave is not in sync with the voltage one then watt readings are differrent... something like that.

120 is actually the root mean square (RMS) value of the voltage. The peak voltage is actually 169.7V. So, the voltage goes from +169.7V to -169.7V as you say 60 times per second.

If the load is resistive, the current will be in phase with the voltage. But, in general, a load is not resistive. There is some reactive component to the load. That reactive part results in the phase shift. In an extreme case, if the current is 90 degrees out of phase with respect to the voltage, the power delivered will be 0.

The VA rating of a UPS gives you the maximum current the UPS can deliver (the VA figure divided by 120V).
The power rating of a UPS (W) tells you the maximum power the UPS can deliver.
You have to make sure that what you are going to put on the UPS can live happily with both of those figures. The bottle neck is not always the same. Sometimes, it is the VA figure, sometimes, it is the W figure.

 

[SirBrass]

Actually, all loads are resistive, but some have reactive components as well (described as imaginary values when written down mathmatically. yay for Euler!). I'm not sure if I know of a load that isn't an ideal capacitor or ideal inductor that doesn't have a resistive element to it.

 

[Jiggz]

OK, here's the "Layman's Term" when defining VA and Watts. In DC world, and also with transformers, power is measured in terms of VA (volt-amps). When it comes to AC power and they have built in reactive compensating circuit to reactive currents (currents fed or used by capacitors and inductors) then the power is measured in Watts. Which by the way is also the same as the product of the voltage and current and including the power factor of the circuit. Power factor is the only difference when to use Watts or VA when measuring power in electrical ckts. Depending on the design of the electrical generating or handling equipment, it can have or not have a reactive compensating circuit. Electrical loads are always measured in Watts, since this is the actual power consumed by the equipment. Watt is called "True Power" while VA is called "Apparent Power". Apparent means, the total voltage and amperage generated or used. For example, if an AC transformer is rated 1000VA, it can handle a total load of 1000 watts; if the load has a power factor of 1 (which is a purely resistive load). But if the load has a power factor of 0.8 then the total wattage it can handle goes down to 800 watts. PF depends on the inductive and capacitive load in the circuit. Typically, electrical loads are inductive in nature. Motors are usually 0.7 to 0.8 PF. Flourescent ligts uses about 0.50 PF. There are very few pure resistive loads like the incandescent bulbs. Elec power generators are usually designed with 0.8 PF inductive.

 

[CalvinHobbs]

er correct me if i'm wrong but since when in DC you use VA? i thought it was in AC that these things come up, i.e W,VA, VAr.

 

[Navid]

Originally posted by: calvinHobbs
er correct me if i'm wrong but since when in DC you use VA? i thought it was in AC that these things come up, i.e W,VA, VAr.

Power has always been measured in Watts.
Whether electricity is AC or DC makes no difference. Power can even be non-electrical.
Power, in general, is the energy consumed (or delivered) in unit time. In other words, one Watt is one Joule in one second, where Joule is the unit of energy.

http://www.google.com/search?hl=en&lr=&oi=defmore&q=define:Watt

 

[Red Squirrel]

Wow I think I somewhat understand actually. But this seems like the type of material one would want to take a course on to really understand. Why cant we just all use DC and simplify things. lol. There's some good reasons mains is AC, actually, minus the complications part.

But that said, a 600W / 1000VA UPS should handle 500 watts nicely, then. (technically 600 but it's not good to push it due to startup power)

 

[Navid]

Originally posted by: RedSquirrel
Why cant we just all use DC and simplify things. lol. There's some good reasons mains is AC, actually, minus the complications part.

The main reason for using AC is that AC voltage can be changed easily using transformers. To change DC voltage, you will need circuitry that is much more complicated, and less efficient, than a transformer.
Voltage needs to be changed dramatically, in different stages, all the way from a power generator to your home.

But that said, a 600W / 1000VA UPS should handle 500 watts nicely, then. (technically 600 but it's not good to push it due to startup power)

It should be OK as long as the 500W figure is the total power you need the UPS to support. Keep in mind that you have to take the monitor power into account. If you plan to plug in anything else other than the PC and the monitor, you need to add the power of those components too.

 

[Zepper]

Just a reminder - when your UPS is in battery mode, NEVER power up the attached equipment. Only power up when running from AC. Inrush currents of over 100A are possible at power on...

.bh.

 

[imported_whatever]

Originally posted by: Navid

Originally posted by: RedSquirrel
Why cant we just all use DC and simplify things. lol. There's some good reasons mains is AC, actually, minus the complications part.

The main reason for using AC is that AC voltage can be changed easily using transformers. To change DC voltage, you will need circuitry that is much more complicated, and less efficient, than a transformer.
Voltage needs to be changed dramatically, in different stages, all the way from a power generator to your home.

But that said, a 600W / 1000VA UPS should handle 500 watts nicely, then. (technically 600 but it's not good to push it due to startup power)

It should be OK as long as the 500W figure is the total power you need the UPS to support. Keep in mind that you have to take the monitor power into account. If you plan to plug in anything else other than the PC and the monitor, you need to add the power of those components too.

yes, and because of the monitor issue, LCDs are strongly reccomended when using UPSs

 

[Red Squirrel]

Originally posted by: Zepper
Just a reminder - when your UPS is in battery mode, NEVER power up the attached equipment. Only power up when running from AC. Inrush currents of over 100A are possible at power on...

.bh.

100A? are you sure? Since most circuit breakers or fuses are rated at 15 amps, some 20 in comercial buildings, or is it so fast that it does not trip?

Mind you I realize what you're saying not to power stuff on in battery mode, especially something like a printer or AC unit since startup on those is crazy.

 

[Continuity27]

Originally posted by: RedSquirrel

Originally posted by: Zepper
Just a reminder - when your UPS is in battery mode, NEVER power up the attached equipment. Only power up when running from AC. Inrush currents of over 100A are possible at power on...

.bh.

100A? are you sure? Since most circuit breakers or fuses are rated at 15 amps, some 20 in comercial buildings, or is it so fast that it does not trip?

Mind you I realize what you're saying not to power stuff on in battery mode, especially something like a printer or AC unit since startup on those is crazy.

Actually I think 20 is still rated for private use, commercial uses more usually.

 

[Mark R]

100A? are you sure?

Yup, that's definitely right. For most 'home grade' PSUs it will probably be less, as there are various regulations limiting it. Most home PC PSUs will generally be limited to about 50 A at 110 V (100 A at 230 V).

However, it's worth noting that you will ONLY get this surge when the main power switch is switched on (i.e. the one on the back of the PSU (if there is one) - not the normal 'standby' switch that you would normally use to switch on a PC). The reason for the surge is the reservoir capacitors in the PSU.

If you think about AC - you'll see there are periods when the voltage drops down around zero, reaching zero instantaneously. However, the electronic components in the PC need continuous smooth power, so the PSU has to have some kind of energy store in it - that's what the capacitors do. Under normal circumstances, the caps get recharged 120 times a sec (60 Hz x 2 - because they can charge on the positive and negative peaks of the wave), so they only need a small top-up. But when mains power is first applied the caps have to charge all the way from empty - the result a big surge.

The surge itself is extremely brief - only a few ms, but it can be a problem. If you plug/unplug equipment it can cause sparks which burn or corrode the connections, or it can damage electronic equipment which is exposed to it (like UPSs). Circuit breakers and fuses have a designed in time-delay before they operate, so they won't trip on such short surges (unless they are massive).

Anecdote: At HS I was computer prefect, and I was throwing other pupils out of the lab at closing time. To get the message across I turned off the main power switch on the main fuseboard. Once the room was cleared I turned the switch back on, only to have a couple of the breakers trip out immediately. The room had about 50 workstations on about 8 circuits. There was a problem though, there was still no power, even with the breakers reset. The combined surge from the 6 or 7 workstations on each circuit was not enough reliably to trip the breakers on each circuit. However, the combined surge from all 50 had managed to blow-out the breaker supplying power to the entire building.

 

[Zepper]

Yes, over 100A is possible, but for a VERY short time. I see the inrush current on the Zippy 700 is limited to 65A. I can't imagine what the inrush on such a humongous PSU would be w/o a limiter... :shocked:

.bh.

 

[Miramonti]

Originally posted by: Mark R

100A? are you sure?

Yup, that's definitely right. For most 'home grade' PSUs it will probably be less, as there are various regulations limiting it. Most home PC PSUs will generally be limited to about 50 A at 110 V (100 A at 230 V).

However, it's worth noting that you will ONLY get this surge when the main power switch is switched on (i.e. the one on the back of the PSU (if there is one) - not the normal 'standby' switch that you would normally use to switch on a PC). The reason for the surge is the reservoir capacitors in the PSU.

If you think about AC - you'll see there are periods when the voltage drops down around zero, reaching zero instantaneously. However, the electronic components in the PC need continuous smooth power, so the PSU has to have some kind of energy store in it - that's what the capacitors do. Under normal circumstances, the caps get recharged 120 times a sec (60 Hz x 2 - because they can charge on the positive and negative peaks of the wave), so they only need a small top-up. But when mains power is first applied the caps have to charge all the way from empty - the result a big surge.

The surge itself is extremely brief - only a few ms, but it can be a problem. If you plug/unplug equipment it can cause sparks which burn or corrode the connections, or it can damage electronic equipment which is exposed to it (like UPSs). Circuit breakers and fuses have a designed in time-delay before they operate, so they won't trip on such short surges (unless they are massive).

Anecdote: At HS I was computer prefect, and I was throwing other pupils out of the lab at closing time. To get the message across I turned off the main power switch on the main fuseboard. Once the room was cleared I turned the switch back on, only to have a couple of the breakers trip out immediately. The room had about 50 workstations on about 8 circuits. There was a problem though, there was still no power, even with the breakers reset. The combined surge from the 6 or 7 workstations on each circuit was not enough reliably to trip the breakers on each circuit. However, the combined surge from all 50 had managed to blow-out the breaker supplying power to the entire building.

LOL, dang that must have been embarassing! Fortunately it sounds like it was late and probably not too many people around. :laugh:

 

[Navid]

Originally posted by: Zepper
Just a reminder - when your UPS is in battery mode, NEVER power up the attached equipment. Only power up when running from AC. Inrush currents of over 100A are possible at power on...

How long do you think this inrush current will flow?

Let's say that the inrush current is 100A and flows for 50 milliseconds and the power factor is 1.
So, the inrush power will be 100A X 120V= 12000W!

Then, the energy required will be 12000W X 50/1000 s = 600 Joules (unit of energy) since power is energy in unit time.

Now, let's see how much energy is stored in a low-quality UPS battery.
A UPS that can provide only 100W for only 2 minutes contains 100W X 2 X 60s = 12000 Joules.

So, the energy wasted by the inrush current is only 5% of the energy stored in the fully charged low-quality UPS battery.

There should be no problem with a healthy decent UPS if you turn on the equipment even when there is no AC power as long as the UPS battery is charged.

 

[Mark R]

How long do you think this inrush current will flow?

Potentially long enough to damage the transistors in the UPS inverter.