[ale] backup generator -- on the cheap

[Bob Toxen]

You need to define "inexpensive".

Some UPSes work with some generators.  If the generator generates true
60Hz sine wave output then any UPS should work.  However some modern
generators only approximate the sine wave (see below).

That said, my Yahama model 2400 (2000 Watts of 120V 60Hz continuous)
works great with my new and old APC UPSes.  (I haven't tried it with
my one off-brand UPS.)  It cost me $1200 plus shipping less a $100
rebate and I'm very happy with it.  It also can power a window AC and
refrigerator at the same time to survive days-long outages following
major weather events.

Suggestions:

  1. Read the details on any generator you're considering to see if it
     uses an AC generator or DC generator driving an inverter.   The
     AC generator should produce a true sine wave that will drive any
     AC device  The DC generator may not.

  2. Consider how quiet the generator (engine actually) is.  Most,
     especially cheap ones are VERY nosy, comparable to a lawn mower.
     Unless you're in the country your neighbors and the cops will
     force you to shut it down after a short time.

  3. I recommend a gasoline-powered generator over a propane-powered
     one because you have an automatic 15 gallon or so reserve in your
     car's gas tank if you have a siphon hose.  The propane ones do
     burn cleaner.  In either case be sure to stock enough fuel to cover
     a long outage.  If you use gasoline, occasionally dump your
     reserves into your car and replenish them as old gas is bad gas.

  4. Consider finding a retail store with display models of generators
     that they will run for you.  Plug in your UPS and a load such as
     a small lamp with a 100W bulb to see if the UPS likes the AC.

  5. Honda makes the quietest generators and are considered of the
     highest quality.  Look for the DB (noise) rating.

  6. To avoid a computer crash you'll have to be able to start the
     generator and switch the UPSes from the wall socket to the generator
     within a few minutes (5-10 depending on the health of your UPS
     batteries).  That includes recognizing a failure and finding a
     flashlight to get around in the dark.

     This means at least keeping a heavy cable from the generator to
     where the UPSes plug into the wall.

     I use 12 AWG cable and have a relay that automatically switches
     the UPSes from the wall socket to the generator when it detects AC
     from the generator.  In the office I have a lamp driven directly
     from the generator so I know when there is generator power (and
     allows me to see) and an indicator light showing if the UPSes
     are receiving AC from either feed (e.g., after the relay).


Technical reasons for generator/UPS incompatibility:

The problem with UPSes is that some are too sensitive to input power,
especially requiring a true sine wave.  Nowadays many generators
generate low voltage DC internally and then use an inverter (like the
second half of a UPS) to generate the 120VAC.

Why?  Because it allows them to vary the RPM of the generator to match
power (wattage -- or Volt Amps for the EEs here) requirements and use
less fuel at lower power draws.  This is unlike a true AC generator
where the RPM must be precisely regulated because the frequency (Hz)
is proportional to the RPM.

This is the same reason why plugging one UPS into another one to get
standby power for a longer time frequently will not work.  (I have
done this successfully with APC UPSes but there is a risk of frying
circuits.)

Bob Toxen
bob at verysecurelinux.com               [Please use for email to me]
http://www.realworldlinuxsecurity.com [My book:"Real World Linux Security 2/e"]

On Mon, May 19, 2008 at 06:25:47PM -0400, Brian W. Neu wrote:
> I've had it with Marietta power.  They're as reliable as a campaign promise.
> 
> Does anyone have a story of producing reliable power from some kind of 
> inexpensive generator set-up?  I've tried some kind of rental deal 
> previously which didn't come remotely close to working.  Is the secret 
> having a true AC->DC->AC converting UPS?
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