Netters,
I am running four computers in a small network at home.
It might be extremely important to mention the details of
these systems, for example any significant consumers of
power in them since website calculators can be wildly
inaccurate.
All the four
mini-tower boxes are sitting side-by-site. There is only one monitor
that is shared using a KVM switch.
What kind of monitor? It makes a difference.
I am looking at getting a UPS system that must be adequate for these 4
computers.
Does it really "need" to be a single UPS? I ask because
often the consumer grade around 500VA types are
significantly less expensive per VA, sometimes under $30 per
on sale or after rebates. I picked up my last for $20 but
that sale is over so it depends on how much looking you want
to do, I ususally buy ahead of the perceived need instead of
impulse buying.
However, I am confused with various numbers that are thrown
around.
1. On trip-lite UPS website calculator, it claims that it machine must
take 77 watts on an average. This assumes each computer has its own
monitor.
Are you sure you are interpreting this right? I ask because
it can't be right, you will not have as low as 77W per
system with a monitor, except maybe the most miserly of
energy conservative designs (Via Epia, etc) only a single
drive, and a medium to smaller LCD monitor. The average
budget OEM system sold today can exceed 77W without a
monitor. The low end systems' average "might" be as low as
77W if it's mostly sitting idle, but that's not how you'd
want to budget your power, as it certainly can't be sitting
idle in the middle of shutdown sequences.
2. Another website claims that the consumption should be around 50
watts per computer.
Why didn't you just mention the system details?
50W is also, probably too low. A conservatively set up
PIII/Celeron system could be around that figure at idle, but
again you'd be looking at the lower power state, not the max
(per system) you'd want to budget for.
3. There is typically a power loss of 20% in the power supply.
Therefore, the actual power drawn is 77/0.8 = about 100VA.
There is typically 25%, sometimes more loss in the power
supply. 80% is a current goal, and one many systems don't
meet in best light, let alone when close enough to idle to
draw so little power as estimated above.
4. I saw one more website that had a VA graph for a P4 computer+5
drives. This graph indicates an initial surge of 200VA (when the
computer was turned on) but an average of 170VA.
You need to find some other website, one modern system with
one monitor should have roughly 500VA consumer UPS. With a
business/commercial UPS, they are a bit more conservative,
sometimes, and you might be able to budget down closer to
300VA if the momentary peak current is sufficient (else the
UPS just turns off the moment it tries to switch over to
running the systems).
Questions:
1. So many different results for power consumption. What do you think
is a reasonable value?
1500-2000VA, depending on exactly what it is.
2. If the power consumed is less then 80 watts by a computer, why does
a computer have a power supply rated at 300 watts, for example?
Because the PSU rating is it's peak output, or if
"sustained", it still means the MAX it can sustain. You
always want a PSU capable of a little more than you'd
actually use, this applies to any power supply for anything.
However, many PSU labeled as 300W can't even supply 300W,
it's just a daydream put into words stamped onto a label.
On the other hand, PSU also spec higher wattage only because
of the available current per rail, a PSU will try to appeal
to a wider audience by supporting systems with more power
requirement on one rail than another, instead of having to
simultaneously make several different PSU with a similar
total wattage, requiring separate designs, assembly lines,
parts, testing stations, etc. You are more likely to find a
PSU with a rated wattage closer to the system's peak
hypothetical requirement (perhaps allowing for a slight
upgrade in some cases) with an OEM that can better match
each PSU they spec to a particular system configuration.
Plus, since a PSU manufacturer isn't trying to sell an OEM
unit in retail channels (unless divered to surplus channels,
so the manufacturer had already made the sale), they can be
more conservative about rating it, not trying to be
(marketing/specs) competitive with other retail psu with
agressive, sometimes even fradulent wattage or current
ratings.
3. Now, here is my calculation:
4 computers = 400VA
1 DSL router = 50VA (I am guessing. Can't be more than that)
1 Network switch = 50VA (I am guessing)
I don't need to protect the monitor.
But you need it powered still? So you can't ignore it?
I saw a 550VA UPS for $69. Looks like a good deal. Are my calculation
right to make this deal work?
Absolutely not, but you have failed to provide even basic
info about the systems, only data from websites that can't
possibly be right.
Two systems may be reasonably paired with a 600-750VA unit,
sometimes a little smaller if they are old enough systems or
pretty spartan, with only one or two LCD monitors. Then
with UPS as with any PSU, there's the accuracy of the
manufacturer's ratings and how they can be applied.
If you can get by with a modest 750VA unit for 4 systems
it'll be because it was a commercial unit, essentially
costing as much as a higher VA rated but less robustly
designed unit for consumer or SOHO. Generally as mentioned
above, you're looking at above 1000VA units just to keep the
UPS turned on, let alone for how long...
You didn't even mention your needed runtime, another
important factor. With an industrial UPS you might find
larger batteries for some application to extend runtime
compared to the other VA rating, but in general the lower
end products (even from a respected manufacturer) will have
a correspondingly lower runtime at any given load from a
lower VA rated unit. So suppose you want 30 minutes, a
2000VA unit may not be able to do it, but in all the details
we don't have, lies exactly what you might need for a goal
like that.
