I want to buy a generator to hold up a small computer network and one or
two other things, total load about 5 or so kilowatts 240V. At present
the computers and monitors are connected to APC UPS units but these
obviously only provide a limited hold up time, and sometimes our power
cuts last all day which plays havoc to planned work!
There are any amount of small generators offered on Ebay with so called
automatic voltage regulators, and some with inverters. Please could
someone give me some advise about what I should be looking out for, and
what to avoid!
Very many thanks.
The starting point is to try to reduce the power consumption, if possible.
Let's look first at a battery solution. Which avoids all the problems
and costs of a generator and can easily be configured to automatically
take over supply.
5kW for, say 8 hours, is about 2000 AHr at 24 volts. This could be met
by 40 6v Trojan T-125s plus charger plus inverter. If you can get the
power consumption down, you can manage with fewer batteries. Also, if
set up with a genny, as below, fewer batteries are needed.
Now the genny situation.
Inverter gennies are good for continuous running on varying loads and
are much more fuel efficient on light load. Basically, the engine runs
at a speed determined by the load and can go to tick-over at very light
load. The inverter produces a constant output voltage and frequency,
irrespective of engine speed. So they are great for, say, powering a
house, where the loads can vary. You don't need this as you will only be
running it now and again and never on light load.
The waveform from low power gennies (eg a few kW) on or near full load
is broader and flatter than mains power - this can cause a UPS to reject
it and refuse to switch over from battery backup. For 5kW loading via
UPS, I would suggest a 10 - 12.5kVA generator as a minimum. It probably
won't be an inverter-type at this sort of power. AVRs don't do anything
to the waveform shape, so won't fix the UPS problem.
One setup to consider is a smaller set of Trojans linked to a very
high-output charger and inverter, plus a genny. The inverter
automatically takes over, without a flicker, if the mains fails and you
can size the batteries to run what you need for 4 hours or so. Also have
a genny which you can connect to the high-output charger. Start the
genny if the power has been off for a couple of hours and you need power
for more that the next two hours. The charger produces enough output to
power the loads *and* recharge the batteries. So the genny is always on
full load, when running. When the genny load starts to reduce as the
batteries reach full charge, stop it and run off the batteries again..
The biggest thing to watch out for is buying too small a genny. Next
thing to watch out for, in a system only using UPS and a genny, is not
being able to get the genny started fast enough and the UPS not "liking"
the output from the genny and either refusing to switch over, or, worse,
cycling back and forth between battery and genny. A set of batteries and
inverter are far, far easier...and usually the power comes back on
before you have to start the genny.
In his posting of Mon, 1 May 2006, =?UTF-8?B?UGFsaW5kcuKYu21l?= writes
Many thanks for the very comprehensive posting, much appreciated.
I should just say that this is for a one man office work shop and so
funds are rather limited. The computer part of the load amounts to
3-4KW 240v UK and the additional being 4.5KW for HID lighting and
running the central heating etc.....but this is not critical but would
be nice to be able to run these in the event of a long power failure.
At present the UPS units hold up the computer related stuff for about 40
minutes. Most of the power outages amount to less than one minute, but
when the failure is longer then it can last for hours or all day
occasionally. This is always a worry as what I produce has tight
deadlines involved and sod's law the long failures happen at the most
inconvenient time! Any hold up is obviously costly of course in simply
being unable to carry on working as normal.
Not too sure what you mean here..... Are you saying that I am going to
have a problem with all AVR regardless of their capability? BTW I have
contacted several sellers of these cheapish small generators and they
say their kit will run computers without any problems but I am very
Yes I like the simplicity of the battery and inverter route!
A small genny will run computers directly, usually without any problem.
They are typically designed to take a very wide range of input
voltage/frequency/waveform. Many bits of modern electronics have switch
mode supplies which work off any voltage between 100 - 260 v and almost
any frequency, even down to dc..
However, UPS typically are more demanding than computers! Off load, the
genny output may be good enough and the UPS transfers the load from
batteries to genny. At that point, the genny waveform changes and the
peak voltage drops - although the RMS voltage may stay in spec. The UPS
senses that as being out of spec and transfers the load back to the
batteries. At which point the load comes off the genny and its output
waveform changes back... and so does the UPS...
A good AVR will sense the RMS voltage from the genny, which will always
be in spec. A cheap AVR may sense peak voltage but takes time to
respond, if it is a transformer tap changer. By which time the UPS has
already reverted to batteries... The AVR then senses that the peak
voltage has risen out of spec and changes tap.. The whole system is
A genny without UPS and with/without AVR will often drive computer
systems with no problem at all....
But I would still suggest using the genny only to power a high-output
battery charger and use the batteries + inverter to power the systems.
Not all computer equipment is tolerant of out of spec supplies..
Basically, you can set up so that your systems are always powered by the
inverter - at which point you can sell all the UPS you presently have,
as they won't be needed any more...
In his posting of Mon, 1 May 2006, =?UTF-8?B?UGFsaW5kcuKYu21l?= writes
Many thanks for your suggestions and advise. I need to keep this
simply or in other words...cheap but effective <G> I have been having
a re-think and would like to hear your thoughts on this please. I
have no problems in dealing with the short outages indeed my UPS units
will happily keep the computers, monitors and room lighting going for
about forty minutes which is ample to finish a particular job and make a
normal shut down.
Following this it strikes me that if I organise myself I could then
start the standby generator and power up the computers, monitors, HID
lighting etc. whilst not as neat as powering up a generator and not
having to close everything down and restart them, there are cost
advantages in this alternative.
Please let me know what your thoughts are to this alternative way of
All the best
I can only recommend either going for:
1) a big genny that will supply the system, UPS and all - which
shouldn't need you to shut everything down before swapping over.
2) going for a battery + inverter + small genny solution.
The above will work, no matter what your system.
Anything else would require a lot of details on each load, each UPS, etc
as it would need to be designed to match your system.
As someone suggested, if you go to an electrician/electrical engineer
used to designing backup power systems, he may present you with many
one drawback to the "small" generator when used to power computers through
UPS units is that if the UPS units find the voltage or frequency out of
tolerance they remain firmly in operation until the batteries run dry and
then switch themselves and their load off.
as this is for business purposes i suggest you get 3 or more quotes from
qualified electricians for materials and instillation. this will get you
the right size, the right fuel, the right transfer switch and grounding, and
a source of future maintenance.
sure this will cost more then getting some questionable product on e-bay but
consider the problems if the electrical inspector, fire marshal, insurance
company, and OSHA (or equivalent) dislike some aspect of the system.
one feature that i feel is important to specify is "permanent magnet
im also a fan of natural gas and propane fueled back up generators as the
fuel does not go bad after time elapses.
if your budget is really tight your electrician may be able to find a used
or reconditioned unit. some places (i.e. government entities) replace these
as funds become available whether they need it or not.
In his posting of Sun, 30 Apr 2006, TimPerry writes
Thanks for your reply to my posting. A few years ago I tried a
generator to power up a UPS and the UPS kept jumping in and out so after
a few minutes I gave it up and returned the generator. Mind you it was
in all probability not fitted with an AVR as it was sold through my
local builders merchants for powering up power tools and site lighting
well, i like to keep thing short and simple. it kinda means there is a big
permanent magnet in the generator as opposed to a wound field
a poor mans solution which I used in the "great fiasco" a few years back.
if your UPS is 12 volts drive motor vehicle to within jumper cable distance.
remove cover of UPS. apply cable to battery of UPS and motor vehicle. leave
engine running for duration.
alternately, obtain DC/AC inverter with as close to true sinewave output as
you can find. install in motor vehicle. run long extension cords as needed.
Why add a generator when you actually need a larger UPS. Buy a RV
battery, battery charger, and DC/AC Inverter. These can be purchased
for about $200. The RV battery will supply a reasonable load for 12
hours or more. These batteries are designed for long continuous loads
as required for RV's and golf carts. A car battery is designed for
high output for a short duration while a marine battery is designed for
both high output for a short time and low output for a long duration.
We used a RV battery for a remote cabin and were able to run a 27 inch
television and vcr on and off for about a week. It is amazing just how
long one of these batteries will last for low loads, and a computer is
a relatively low load. If one battery is not enough add additonal
batteries in parallel - they cost about $100 each. This is a lot less
expensive than buying and maintaining a generator. Also, small
gasoline generators have small gas tanks and have to be refueled about
every 4 hours or so and they are noisey and have to be located out of
doors or ventilated. Propane generators are about 2 times the price of
gasoline generators. The gasoline generator we have only runs 3 1/2
hours on one full gas tank. You can buy a 400 watt DC/AC inverter at
Walmart made in China for about $60 or less. If one is not enough buy
two and connect them in parallel
Thanks for your advise. I need around 4KW for part of the business and
on occasions a further 4KW, both are 240V 50cycle. I struck me that
the generator route was a pragmatic one but I was going to use a diesel
generator with a large tank. I am however considering the inverter
route as it does have a certain simplicity to it.
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