[aprssig] 8 hour Backup Power Rule

Ray McKnight shortsheep at worldnet.att.net
Tue Mar 18 17:41:37 EDT 2008

>Here's a more practical question - at what level of load does a 
>generator produce the most power per unit of fuel?


>From my experiences in sailing, and from info I've read from various
emergency power forums and generator manufacturer web sites, most
Small engines are at peak efficiency somewhere between 50-75% load.
That's not a true scientific measurement, simply a comparison of fuel
Consumption to load.  What you need to test on your system is the point
At which fuel consumption rises dramatically.  At too low of a load you
Don't see much reduction in fuel consumption, but you aren't getting as
Much work from the engine as it could provide.  Someone mentioned near
This is probably true for large commercial systems.  Small gennies like
Generally use, typically 5000-6500 watts which have 10-12hp engines, are
Always exaggerated as far as load capacity.  They advertise things like
"peak load" which seems like the old ads for car stereo speakers
1000 watt power handling capacity when in reality the $29 speakers were
really only about 20 watts RMS!  So the genset makers want you to think
you're getting more for your money, when the ratings are really hyped at

that engine running at 100% load or even greater.  By that I mean once
engine either begins overheating or running hotter than it should, or
the governor can't maintain the engine speed needed to produce 60
I ALWAYS have a voltmeter and frequency meter on my gennie so I can see
Things are normal.  And I ALWAYS have a UPS in the circuit when running
Any sort of electronics.

You can expect to use 1/2gal / hr at 1/2 load on a 5kw gas generator.
At today's gasoline prices this means you'll be spending about $40/day
To have electricity and need at least 2 5gal gas cans, probably more.

Now this causes most of us an even bigger headache.  Any good quality
Should have a sensitivity circuit allowing you to adjust the trip
If voltage or frequency is all over the place as most small gensets will
Be as loads are switched in and out, the UPS will sense that as a
Another factor is the UPS charging circuit which can easily draw 10amps
More, so when the UPS starts charging it may cause the gennie to choke,
Then the UPS trips, the genset stabilizes because the UPS is not
The UPS thinks all is okay and goes back into standby but also the
Circuit kicks back in briefly, so the genset chokes, and on and on and
You see the UPS kicking in and out every 10-20 seconds and hear the
Surging.  So you need to adjust the UPS trip points to make it less
sensitive to gennie voltage fluctuations.  

Be aware that any change in engine RPM correlates to a change in
Most small gensets operate at 3600 rpm.

You'll need probably at least 1000kva UPS to run anything like a 100w HF
Or anything drawing 500+ watts.  So expect a 10amp load from the UPS
when charging.  This can be even higher, maybe 15 amps for larger 120v
I even have a 3000kva unit that runs off 120v and needs 20amps.  So your
UPS should be on a separate circuit all by itself.  The "1000" rating of
a UPS is NOT the wattage rating of the connected load.  That's typically
25-30% less.  My APC 1400's say 975watt load I think.  Or was that the
1200's, I need to check.

When sailing, it's terribly inefficient to run the main engine to charge
Batteries.  Most will wait until they need to use the engine for
As the alternators/generators only put 5-10% load on the main engine.
Diesels especially don't like idling or very low loads, they begin to 
Build up deposits in the engine.  Gasoline engines aren't as bad but
it's still not good to run them near idle for extended periods.  So a
good solution for sailing is having a small engine just for battery
charging which runs at least 50% load.  You can use the same principals
with portable
Gensets.  It's impressive when someone shows up at Field Day with that
Military surplus generator, but if you only need 5kwh of juice why
All that noise and waste the fuel?

One last thing on generators.  Depending on where you are you may be
To either noise of spark arrest regulations.  During emergency response
Can likely get away with almost anything, but as the situation gets
Control some good intentioned enforcement person might find nit picky
To look for.  Load generators cause fatigue and stress to try and be
sensitive to that aspect too.  Don't need to aggravate an already bad

Ray - WB3ABN
Kingston, WA

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