Numerous posts have been made to newgroups on this subject. At the
bottom of my answer you will find a Google Groups link which will lead
you to dozens of such posts. Since posts to newsgroups are made by
experts and amateurs alike, the advice one finds on newsgroups is of
the "take with a grain of salt" variety, and should be regarded with
caution, particularly when one is dealing with expensive and delicate
While the newsgroup discussions are worth an examination, I have found
information that should be much more trustworthy than newsgroup advice
from persons of unknown credentials. This comes straight from APC; the
information below, particularly regarding the positioning of the dip
switches on the back of the unit, may be the key to solving your
This document is "Answer ID 1358," from the APC Web site. I found it
by visiting the APC Support Center online, and searching the APC
Using generators with APC's Back-UPS, Back-UPS Office, Back-UPS Pro,
Generators are not rated or designed to handle non-linear loads such
as computer equipment or other electronic equipment. Using non-linear
loads with a smaller generator will result in the output waveform of
the generator to be very distorted. This distortion combined with
variance in the frequency of the output power will most likely result
in the UPS constantly operating On-Battery or frequently transferring
back and forth from On-Line to On-Battery. To prevent voltage
distortion and complications with a UPS, the generator must be
selected so that it will supply the peak current required by the
computer load, not only the average current. It has been determined
experimentally that this typically requires that the generator have a
rating at least 2 to 3 times the rating of the entire load to be
connected to the generator. However, due to wide differences in output
impedance for generators, it is not possible to specify an oversizing
factor that guarantees compatibility with all systems. If a UPS is to
be used with a small gas-powered generator, then APC recommends using
either a Smart UPS or "S" model Back-UPS Pro**. These models are
recommended because they can be desensitized (consult Owner's Manual
and/or PowerChute plus User's Guide) to allow the unit to be more
tolerant of voltage distortion and, therefore, make less transfers to
battery. APC does NOT recommend using the Back-UPS, Back-UPS Office,
Back-UPS Pro PNP, or Powercell units with small gas-powered generators
due to the inability to adjust their sensitivity.
If you already are using a Back-UPS with a generator and the UPS is
constantly going to On-Battery operation:
Most standard Back-UPS units can be adjusted to accept a lower input
voltage so that they would go to battery less often. On the rear of
the unit there are 4 "dip" switches (consult owner's manual).
Normally, a Back-UPS will go to battery when the input voltage is at
or below 103 volts ac (vac). If you move dip #2 to the up position,
the UPS would go to battery at 98 vac. If dip #3 is moved to the up
position, then the unit would go to battery at 93 vac. And if both 2
and 3 are moved up, then the unit would not go to battery until 88
vac. If the unit continues to go to battery after adjusting the dip
switches, there may be other problems with the output power of the
generator. If the power from the generator cannot be improved, it is
recommended that the Smart UPS or "S" model Back-UPS Pro** be used
If you already are using a Back-UPS Office, Back-UPS Pro PNP, or
Powercell UPS with a generator and the UPS is constantly going to On
Battery operation: These models of UPSs cannot have their sensitivity
adjusted. If the output power of the generator cannot be improved
(i.e., by reducing the overall load of the generator), then it is
recommended that the Smart-UPS or "S" model Back-UPS Pro** be used
**To determine if your Back-UPS Pro is an "S" model look on the rear
of the unit at the white bar code sticker. This sticker contains both
the "model number" and "serial number". If the model number ends with
an "S", then your Pro can have its sensitivity adjusted through the
PowerChute plus software or through a terminal program. Consult the
PowerChute plus User's Guide located on the PowerChute plus CD. If the
model number ends with "PNP" or "U", then your model cannot have its
Note about Building-size generators - Generally, generators used to
back up an entire building or facility, such as those used for
hospitals, are sized large enough and are of good enough quality that
any APC UPS will operate fine.
APC: Using generators with APC's Back-UPS, Back-UPS Office, Back-UPS
Pro, and Powercell
If the link above is truncated, or does not function, please use this
TinyURL: Abbreviation of APC link above
You may want visit the APC Knowledge Base to examine other documents
regarding APC's advice on UPS usage with generators. The link below
will take you directly to the APC Knowledge Base; a search using the
keyword "generator" will bring up five pages of hits, totalling 91
documents. Many of these apply only to specific APC models, so it
should not be necessary to read them all. I have read through several
of the more general documents, and the one linked above seemed to me
to offer the best overview of your situation.
APC: Knowledge Base
I hope this information helps you to resolve your problem. If anything
in my answer is not clear, or if any of the links do not function,
please do not hesitate to ask for clarification.
My Google search strategy:
Google Groups: "generator" + "ups" + "computer"
Request for Answer Clarification by
14 Sep 2002 12:34 PDT
I've discovered the anwer you posted experimentally and more or less
described it as prelude to my question in the posting. What I am
looking for is a solution other the adjusting the dip switches on the
back the APC UPSs that I am already using. The voltage regulators
built into the APC units is not adequate for this task, hence we need
The APC is basically peddling their products and tailor all their
solution towards that end. Therefore the answer you posted is not
I am looking for some device to place between the generator and the
APC UPS units that will filter the power and or store the power so
that the generator produced power will keep the UPSs charged without
throwing off the voltage regulators built into the UPSs.
Clarification of Answer by
14 Sep 2002 13:01 PDT
Since you did not initially indicate that you had already consulted
the APC Knowledge Base, and made no mention of the dip switch
adjustments, I had hoped that the information I located would help.
APC offers free online support which can be very useful. Their
responses are not the typical "form letter" style support, but involve
actual interchange of information with knowledgable individuals. You
can describe your problem online here:
Monday through Friday, you can also utilize APC's free online chat
service, described at the bottom of this page:
I will discuss the particulars of your question with a local
consulting engineer who has been very helpful to me in troubleshooting
problems related to computers, telephones, and other electronic
devices. After I speak with him, I will let you know what his advice
is on this subject.
It will be helpful if I can tell the engineer exactly what equipment
you are now working with; if you can list the model numbers for me,
I'll pass this data along to him for evaluation.
Request for Answer Clarification by
14 Sep 2002 14:05 PDT
We need to keep a minimum of 3 and maximum of ten workstations, 3
small servers, a Meridian phone system, 1 laser printer operational
during multi hour power outages.
All our equipment is is equipped UPSs of different sizes, all from
APC. We purchased a 10hp gas powered generator outputting 30AMPS
(will post make model etc on Monday). Before wiring the generator
into an alternate power circuit, we tested it and found that the power
drove the UPS crazy. They kept switching between green and yellow and
than went unto red. The voltage regulators within the UPSs were not up
to the task. Also one of the computer screens flickered but all the
systems stayed up. We shut down before the batteries died.
Would a line conditioners between generator and the UPSs solve the
problem? During the past six years we had one multi hour power
outage. We want to be able to keep operating in case of an attack.
What would be a complete solution that a small business could afford?
Clarification of Answer by
14 Sep 2002 14:27 PDT
Thank you for the additional details. I will pass the information
along to the consulting engineer I mentioned earlier, and I will post
his response as soon as I receive it.
Clarification of Answer by
15 Sep 2002 18:41 PDT
Below is a discussion of the generator/UPS situation, written by a
licensed Professional Engineer who has, for nearly twenty years,
operated his own business as an engineering consultant, in addition to
installing, troubleshooting, and repairing computers, telephones, and
other electronic equipment. I hope this will be helpful in working
toward a solution to your problem.
Notes on the compatibility between generators and UPS units:
UPS units designed for computer applications fall into two general
categories. In the smaller sizes typically used for individual
workstations and servers, the most common design is the "offline",
"single conversion", or "standby" system. During times when utility
power is present, that utility power is directed through the unit and
powers the load. Some utility power is also used to power the float
battery charger that maintains the energy stored in the batteries.
These units may be called single conversion since at any point in
time, either AC is converted to DC (utility power present, batteries
being charged), or DC is converted to AC (utility power absent or
inadequate, batteries being discharged by the inverter circuit).
Larger UPS designs (and some smaller ones) offer the "online" or "dual
conversion" system. This design involves running the inverter
circuitry on a 24/7 basis to make electricity for the load. At the
same time, utility power is being used to simultaneously power the
inverter, and keep the batteries charged. This scheme involves
simultaneous AC to DC and DC to AC operations, thus the term "dual
Dual conversion UPS units are more expensive, but are more forgiving
of faults in the utility power (or local generator substitute).
Greater variations in frequency, waveform, and voltage can be
tolerated without disturbances visible to the load or the user.
Single conversion UPS units are cheaper and generally completely
adequate when used with utility power as a means to bridge short
interruptions in that power. These are often sized to give the user a
10 to 15 minute run time on batteries.
When users desire to have the ability to continue operation of their
computer loads for durations beyond what the energy in batteries of
the individual UPS units can provide, the most common solution is to
obtain a fuel powered generator. The idea is to use the 10-20 minute
run time provided by the UPS batteries to get the generator running
and stabilized, and to transfer the UPS power input circuits off of
the utility power grid and onto the generator output. How successful
this scheme is will depend upon the performance of the generator and
upon how the generator and the UPS's interact.
Most people have seen work lights running on a small generator. Just
looking at that example of a generator in use, it would appear and
first glance that the generator power is equivalent to utility power
(except for the amount available). However, things are not quite that
simple. Incandescent loads such as lights and heaters are very
forgiving loads. This type of load has a zero power factor (i.e. VA
equals watts), and if the voltage sags slightly, the lights just dim a
bit. If the frequency changes a few Hertz up or down from 60 Hz, the
average user wouldn't even notice that.
The typical combination of a UPS and the computer/monitor it's running
present a very different type of load compared to a lighting load that
uses the same amount of energy. Computers and monitors have switching
power supplies inside that present a non-zero power factor load. This
means that VA and watts are not the same. For a computer and monitor
combination, the VA requirement will be on the order of 1.4 times the
The power factor issue is primarily of interest when one is sizing the
UPS units and the generator. Both UPS units and generators have VA
and watt limits. To design a system properly, one must make sure
there is a reasonable margin of oversize for both considerations.
Utility power is delivered to the users as a very low impedance or
"stiff" source. In other words, except for drawing so much power from
the utility source that you activate a circuit breaker or fuse, there
is little you can do with the load that has any significant impact on
the voltage or frequency delivered to that load. Generators are not
like that unless they are very large compared to the load they are
If you have 1000 watts of computer gear that you want to run on a
generator, but your generator is "house sized" and can deliver 20 KW,
then the generator's output will be essentially like utility power as
far as the UPS/computers are concerned. It's more likely however that
your generator will be much smaller physically and electrically, so
the fact that locally generated power is being substituted for utility
power will make a difference.
When the capacity of the generator and the size of the UPS/computer
loads are fairly similar, one will often experience interactions
between the generator and the UPS. The most common problem noted by
the user is frequent and un-ending oscillations of the UPS's mode from
AC operation to battery, and back again. This is caused by the fact
that the generator's output does not match the utility power output
closely enough in terms of voltage, frequency, and waveform.
The solution to this problem may be achieved by adjusting one or more
of the following three factors:
UPS type, i.e. standby vs. on-line design
UPS input voltage/frequency sensitivity
For any given combination of generator size vs. load size, the problem
be will reduced if on-line UPS's are used instead of standby UPS's.
The problem with that solution is often the fact that all the UPS's
were bought and installed first; the generator addition is now being
contemplated and it's not desirable to purchase a whole new set of
If you oversize the generator capacity significantly (by 2.5 to 3x
compared to the total of all the loads), that will reduce the output
impedance of the generator as seen by the loads. This may well reduce
the variations in voltage, frequency, and rate of change of frequency
(slew rate) to levels low enough that the UPS units will consider the
power equivalent to utility power and not cycle to batteries.
If the generator is somewhat oversized, but perhaps not quite enough
oversized, relief might be achieved by changing the sensitivity of the
UPS input power monitoring circuits. Not all UPS models provide a
means to do this, but many of the medium and better grade units do.
One would have to consult the technical information available for the
UPS's in question. Adjustment may be by DIP switches in some units,
or it may be done by using configuration options available in the
software that runs on the host computer. While the main purpose of
such UPS software is to orchestrate an orderly shutdown of the
computer prior to loss of battery capacity, the software packages
usually have some monitoring and configuration features too.
Using some power consumption estimates for computer workstations,
server, monitors, phone systems, and laser printers, one can obtain an
idea of what the generator requirements might be for a given
installation. Take the example of 10 mid-tower computer workstations
with Pentium-III processors inside and connected to 17" CRT type
monitors. That would be about 91 volt-amperes (VA) for each computer
and about 119 VA for each monitor. Add to the configuration three
Dell Poweredge 2400 file servers with single processors at about 323
VA each. Add to that a single 17" monitor and a keyboard/video/mouse
(KVM) switch for another 130 VA. Lets throw in a single laser printer
at about 630 VA, and 12 station phone system at about 280 VA. This
all adds up to approximately 4109 VA. Assuming a power factor of
about 0.70, that load could also be expressed as approximately 2876
A single phase 120 volt output generator producing 4109 VA would need
to be able to produce 34.24 Amps continuously. One manufacturer of
UPS units (APC Corp.) recommends that a generator be sized between 2
and 3 times the actual load in order to have a stable interaction
between the generator power source and the UPS loads. Based upon that
guideline, the appropriate generator for this application would need
to be able to deliver about 10,272 VA.
One way to confirm that generator sizing is a problem in this
application is to back calculate what size load you could run based
upon these guidelines with a 30 Amp output generator. By dividing the
2.5 factor into 30 Amps, one gets a load rating of 12 Amps or 1440 VA.
The next step would be to set up a test where you place some
UPS/Computer loads on the generator that total to about load. Once
the test connections are made, start the generator and wait for it to
stabilize. Then you could turn on your test loads sequentially until
they are all energized. If that test results in stable operation of
the generator and the various UPS's, then you have confirmed that you
have a generator sizing problem.
Request for Answer Clarification by
15 Sep 2002 22:20 PDT
This is excellent progress. Thank you very much. Could your engineer
friend recommend some specific makes and models of UPSs for the work
stations and servers and a couple generator recommendations? Is there
a voltage regulator that could be placed between the generator and the
Clarification of Answer by
15 Sep 2002 22:59 PDT
I will ask my colleague, the engineer, if he can suggest some makes
and models of equipment that would be appropriate to your needs.
Approximately what sort of budget is available for improving your
system? (Just a ballpark figure would help.)
One more question: do you have a monitor for each server, or a common
monitor switched among the servers? This will affect the estimate of
your power consumption.
Request for Answer Clarification by
16 Sep 2002 06:43 PDT
There are 15 inch monitors on each server. We could replace 2 with a
switch as suggested no problem. I was hoping to do it for under
$5000. I saw some items on ebay under "generators" and "line
conditioners" and backup power that could be useful but I do not have
enough knowledge to know.
Clarification of Answer by
16 Sep 2002 08:06 PDT
Thanks for the additional information. I've passed it along to my
engineer friend for his recommendations.
Bless his heart, I think I am going to have to buy him lunch a few
times for his assistance!
Request for Answer Clarification by
16 Sep 2002 09:50 PDT
Here is an item on eBay with a starting bid of $500 buy it now of
$1950 (probably will accept around $1,500) that would solve the UPS
issues than we would only need to add a suitable generator. "Large
Deltec UPS Power Backup 25 KVA Warranty
Item # 2053290473" The other item is LieBert Uninteruptible Power
Item # 2053631172 Model 311. If you guys solve my problem using eBay,
I will double the fee to $100.00
Clarification of Answer by
16 Sep 2002 10:03 PDT
I've emailed the information on the Ebay items to my engineer, and
will post his response as soon as I receive it.
Clarification of Answer by
16 Sep 2002 10:23 PDT
Just after I sent the Ebay information, I received the message below
from my engineer friend. He included the text of an online chat that
he had with an APC support person. I haven't heard back yet regarding
the Ebay items.
Here's the most recent email from the engineer:
>Could your engineer friend recommend some specific makes and models
of UPSs for the work stations and servers
Sizing a UPS involves two major considerations. The first is
selecting a unit that has adequate VA and wattage capabilities to run
the load in question. This is controlled by the capacity of the
inverter circuitry (sizes of the transistors and heat sinks, etc.)
The second consideration has to do with the amount of energy stored in
the batteries. This translates to run time. The user has to come up
with a run-time requirement. Of course a typical minimum of just 10
minutes get you through blips and other brief events. Some folks have
other factors that will cause them to specify some longer run time (30
minutes, 2 hours, whatever) interval.
Longer runs times means more capital expense when buying the UPS and
larger recurring expenses when the batteries need to be replaced every
three to four years.
>and a couple generator recommendations?
That is a work in progress. I will try to have a chat with a fellow
at Clifford Power Systems this morning.
>Is there a voltage regulator that could be placed between the
generator and the UPS?
I doubt it, but I will raise that very question in a live chat I have
going on right now with one of the APC tech support fellows. Here is
the transcript of our chat.
Please wait for a site operator to respond.
You are now chatting with 'Harry Ellison'
Harry Ellison: Hello Engineer. Welcome to APC Interactive Online
Technical Support. I'm Harry, your online, live APC Technical Support
Engineer. How can I assist you today?
Engineer: I had a couple of questions about the SmartUPS 1000 units
that I have a couple of. Is the "1000" a measure of this unit's power
delivery capacity? If so, would it be 1000 Watts (like at a power
factor of 0.7 or so), or would it be 1000 VA.
Harry Ellison: The 1000 in the Smart-UPS 1000 is the VA capacity of
Harry Ellison: 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.
Harry Ellison: 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.
Harry Ellison: 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
Harry Ellison: UPS have both Watt ratings and VA ratings. Neither the
Watt nor the VA rating of a UPS may be exceeded.
Harry Ellison: 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.
Engineer: For the purposes of sizing a UPS to run a pile of computer
gear, I have noticed in researching the power requirements of various
items such as workstations, servers, and monitors that most seem to
have an "A" meaning ampere rating. Knowing the voltage input is 120
VAC in most cases, I can multiply the amps times the voltage to arrive
at the VA loading for that device. However, would the assmuption that
the power factor is 0.7 be about right for those sorts of devices. I
want to create a spreadsheet for sizing where I track estimated VA and
watt requirements so that when I run my totals I can verify that I've
neither exceeded the watt or VA capability of a given UPS.
Harry Ellison: Yes, you are right, you would need to multiply Amps
*120V to arrive at the VA loading of the UPS unit.
Engineer: Are you comfortable with a power factor of 0.70 for
workstations, servers, and monitors?
Harry Ellison: Yes, the power factor of 0.7 is correct.
Harry Ellison: Is there anything else I can assist you with?
Engineer: When making plans to run a collection of UPSs and their
associated loads on a gasoline or diesel generator (thinking in the
10-12 Kw range), I noticed your KB articles suggest oversizing the
generator by a factor of 2x to 3x in order to get stable results (i.e.
no oscillation of the UPSs on and off battery for example). Can you
point me to some good sources for generators in that size range that
produce adequate quality products.
Harry Ellison: Though I am not a product champ in generators, I have
heard of Mitsubishi and Caterpillar....
Engineer: When dealing with the instability problems that arise when
folks try to run APC UPSs on undersized inexpensive generators, is
there any device whatsover that could be placed inbetween the
generator output and the UPS inputs that could be of any benefit.
Harry Ellison: No, there are no devices which can be put between the
generator and the UPS.
Harry Ellison: You would need to adjust the "Sensitivity settings" on
the rear end of your UPS unit to medium or low. To reduce UPS
"Sensitivity", press the "Sensitivity" button on the rear panel. Use a
pointed object such as a pen to press the button.
Harry Ellison: Press it once to set the UPSs sensitivity to reduced.
Press it again to set the sensitivity to low. Press the button a third
time to reset normal sensitivity. When the UPS is set to normal
sensitivity, the configuration LED is brightly lit. When it is set to
reduced sensitivity, the LED is dimly lit. When it is set to low
sensitivity, the LED is off.
Engineer: Thanks for the help. I think I have enough info now to size
the UPSs once I determine the run time requirements. I will try to
find some smart generator people to talk to about their products. We
have a couple of local generator and emergency power contractors that
may be able to specify and source a good generator for our
application. Thanks for all your help.
Harry Ellison: You are most welcome. It was a pleasure chatting with
Harry Ellison: Have a nice day. Please feel free to contact us again
if you need any further help.
Harry Ellison: Good bye and take care.
Clarification of Answer by
16 Sep 2002 16:44 PDT
From my engineer friend, here are some more thoughts about your
First, about the two UPS's you mentioned that are currently listed on
>Deltec UPS Power Backup 25 KVA Warranty
>Item # 2053290473
This is a really big UPS system similar (probably bigger than) to the
Ferrups system they have installed at one of my customer's sites. That
Ferrups unit probably has 30 computers (but no monitors) connected to
it. Note that this unit requires 3-phase power. I do not know whether
3-phase power is available at your location. For enough money, you can
get 3-phase power anywhere, but we're on a reasonable budget here, so
it's an issue if this item were to be considered.
This UPS is much larger than what would be required by your needs at
this time. My initial calculations suggested that you would need to
run about 4.1 KVA or about 3.0 KW of load. This Deltec system is about
six times that size.
>LieBert Uninterruptible Power Supply (UPS)
>Item # 2053631172 Model 311
This unit is somewhat smaller, but still about 2.75 times the
calculated requirement. It's always good to oversize a bit for
expansion, but that much oversize seems too much unless there are
specific expansion plans already known. This also appears to be be a
3-phase unit, and will end up costing quite a bit before the bidding
A common issue with both of these "central UPS" solutions is that an
electrical contractor would be required to place dedicated outlets in
all the locations where the loads to be protected are. All these
circuits would be brought back to a new emergency power panel board so
that one of these central UPS units could serve them. As I will
discuss later, it may be necessary that wiring of that sort be done in
Two of the factors to consider with central UPS solutions is you have
a single point of failure and one very expensive pile of batteries.
When the time comes that the system declares the batteries are faulty,
one will have to write a good sized check to make things right again.
If the UPS electronics fail, then the whole system is down.
Distributed UPS units have certain advantages to consider. Over time,
they too will have battery failures, but as years go by, the failure
will tend to fall out of synchronization. From time to time you will
need new batteries, but not all of them, all at once. This allows you
spread out the battery expenditures. Another advantage of individual
UPS units, especially for servers, is you can interconnect the UPS
data port to a serial port on the server and run software that will do
an orderly shutdown of the server in case the batteries run low and
you haven't got either utility power or generator power for some
The next issue to consider is what exactly do you want to be faced
with, when the power goes out. In the most deluxe situation, the UPS
units start working in milliseconds. After a few seconds of utility
power loss, the generator auto-starts. A few seconds later, the
generator stabilizes and the automatic transfer switch moves the UPS's
from utility power to generator power. All the user has to do is watch
it work and make sure it all happens.
A cheaper solution is more like this: The power fails and the UPS's
start working in milliseconds. The users consider the situation and
start wondering if the generator will be needed. Somebody goes over to
where the generator is and starts fiddling around with it, probably
using a flashlight held by a co-worker. After about 5 minutes, it's
agreed that power is going to be gone for a while, so somebody starts
the generator. Others are busy laying extension cords (stored for just
this occasion) through the offices and down the halls to link the
generator with all the UPS's. Workers unplug their UPS's from the wall
outlets and into their extension cord outlets. As long as all the
UPS's have enough run time to keep the loads up, all you've lost is
the time spent getting set up.
You could switch off monitors to extend the run time of the UPS's.
That can be a trap if your UPS's are too small. When you turn the
monitor back on (even if only to see what your doing for shutdown
purposes), the inrush current associated with turning on the monitor
my cause a small UPS to shutdown, thus dropping power suddenly to the
If you want to avoid the extension cord hassle, then you need
dedicated outlets for each UPS location. These outlets would be wired
to a separate panel (just like described above with the central UPS
examples). This time, however, we are doing this to make a central
point of connection between the generator and multiple UPS's instead
of a central point of connection between a single UPS and multiple
Automatic transfer switches and auto-start functionality is usually
included in bigger generator systems. An electrical contractor will be
required to do the interconnection work. Under no circumstances should
one ever connect the output of a generator system directly to the same
wiring as is served by utility power. This situation would mean that
the generator output could back feed outside of the building and
possibly injure utility workers who thought they were working on dead
Generally, one should try to get UPS units that are generously sized
in relation to the loads that will be connected to them. This give
longer run times and prevents the monitor inrush current problem
discussed earlier. Used UPS units can be bought in the 600 - 2200 VA
range for reasonable amounts of money. Used units may come with dead
batteries, so one should know the cost of and plan for battery
replacement. That can be taken into consideration when determining how
much one will pay for a used UPS.
Your load requirements seem to total out to about 4100 VA. Allowing a
little excess capacity for calculation errors and/or growth would
suggest that at least 5000 VA be used as the starting point for sizing
the generator. Generator dealers will agree that substantial
oversizing is required when UPS's are involved. Doubling the 5000 VA
figure and using the result to estimate the required KW capacity of
the generator suggests a unit of about 10 KW capacity would be
Another important consideration when selecting a generator is
determining what fuel(s) are going to be used. Portable generators for
contractors running lights and drills are usually gasoline. Most of
those aren't really stable enough for computer applications. Other
fuels often used are diesel, natural gas, and propane.
One must decide based upon fuel availability (natural gas isn't always
a choice, propane suppliers may or may not be competitive in a given
area), safety (gasoline is particularly hazardous), esthetics (diesel
smells worse), and noise considerations.
Just to propose an example unit, consider the Kohler model 11RMY unit.
This unit is rated for 11 KW when running on propane and 9.5 KW when
running on natural gas. This unit comes with a 100 Amp transfer switch
and has an outdoor enclosure. This generator can be bought for around
$4800.00 (new.) To that figure one must add in the cost of electrical
work to tie in the transfer switch and install dedicated circuits. It
will also be necessary to arrange for the fuel connection and a
concrete pad may need to be poured to support the unit. Information
about this unit can be found here:
information can be obtained from a Kohler dealer such as "House of
Generators," who can be reached at 800-987-4484. Other generators can
be found on Ebay, as well. As an example, here is a 12 KW diesel unit
that sold on Ebay earlier today: