PSN-L Email List Message

Subject: Re: Gas Generator recommendations
From: twleiper@........
Date: Sat, 12 May 2001 03:23:05 -0400

On Fri, 11 May 2001 12:03:31 EDT ChrisAtUpw@....... writes:
> In a message dated 11/05/01, twleiper@........ writes:
> > Except you want the 5 HP engine to put away at about 1500 RPM with 
> > 
>        Since we are considering fairly high power systems, providing 
> two 12 V 
> storage batteries for a 24 V system and two 750 W alternators to 
> charge them 
> 1,500 W is probably OK for the computer and other 
> equipment 
> being considered, but it is not a lot of power to run a whole house 
> as well.
The 24V is a very attractive option, but you quickly get back into
money unless you are quite lucky. 24V inverters are not easy to come
by, and two 12v ones are probably cheaper. About ten years ago I WAS
lucky in that I could tell that a non-descipt box with a giant cannon
on one end was, in fact, a 2KW inverter used to power the
outlets in an airliner. This is a very nice unit with sine wave output,
regulation (including load controlled cooling fan speed), buss
and all weighing about 10 lbs. It run on 24 volts as well. I bought it
for $20
and, after reverse-engineering it by drawing out schematics, put it to
in my SWL /seismo shack to power almost the same loads Larry is talking
about. A couple computers and 160W of flourescent lighting. The average
load is about 500W if my 20" Sun monitors are off, 1KW otherwise. The
bank  consists of two 110 AH PowerOne Gell cells in series, and is also
used to power an R-392 military tube receiver that has been on for ten

I charge the batteries and power the radio with a 5 amp current limited
power supply. When power is interrupted, a simple adjustable "delay-on"
relay "falls off" and switches the load. The inverter runs at all times
thanks to its "sense" input, remains syncronized with the line. It draws
little power in the no-load state, and the relay is easily fast enough
the switching supply in the computers. When power is restored, the relay
times out about a minute and then switches the load back to the line. I
also have a simple lightning / static detector amplifier system that will
switch to the inverter and cut the modem lines when thunderstorms are

For the rest of the house I have a very conservatively rated 5 kw
four cylinder water cooled gas generator from 1960 with a Hercules
engine and Leyland generator. It is 120V, and, before I built my
transfer switch, I had a manual transfer switch that powered both legs in
parallel, thus cutting out all the big loads like the range, oven and

Being military, the generator has a 24v charging and cranking system
which I could use to charge the inverter batteries, but I don't. The
just keeps the computers going while the generator starts.

Later, I installed a 100A sub panel with just the standby loads so the
generator could start automatically without being over-loaded. The
gets regular exercise under load of about a half hour every other week
in the summer and every week during the winter. Actual outages average
about 15 hrs a year, but there are years where the actual time may have
only been minutes, and one Nor'easter year it ran 80 hrs for one storm

The 5KW generator easily powers a fridge, a freezer, oil fired boiler,
pump, hot water circulator (heat water with oil and/or 80 gal electric) a
large TV and a couple computers, as well as enough lights to move around
in the 6,000 sq ft home. I have a KWH meter on the generator, and the
cumulative average load has been 2.1 KW at night and half that during the
The only caution in this example is the fact that this particular
has greater than average capability to start motor loads because of the
high torque of the four cylinder (13HP) engine and rather large rotating
mass...the whole thing weighs 750 lbs. A "lawn mower" type generator
of the same 5KW size would probably require you to stagger your motor
loads. One load you should avoid is ceiling fans. These are series-wound
and draw an amazing amount of current while starting, as does the the oil
burner blower motor. If the head pressure has not had enough time to
bleed down, your fridge will hammer down a small generator as well.

In any event, having worked with both these power sources for many
years, and helped friends and family members design and install or
restore similar systems, I would not recommend the inverter for anything
other than what Larry started this thread about...resistive and light
loads. Certainly no motors beyond drills and sewing machines. But you
will be suprised what you can do with 1 KW of load, and that would easily
be supported by the 12V 80A alternator and putt-putt Briggs Stratton. And
if you want to power intermittent bigger loads just get a 2 to 3 KW
and plenty of battery (current) capacity. Then you can use the hair
hot plate, electric frying pan (remember them?) or coffee maker. The
putt will deliver its kilowatt and the batteries the rest. Speaking of
my favorite local scrap yard has main-frame UPS pulls that are Gel in the
80 to 100 AH range for about $20 each. Some of these are rated for a
ten year service life and may only be half used. They have skids of them.

Sorry for the long post, but trust me, it will work.


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Larry Cochrane <cochrane@..............>