Index Home About Blog
From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Minimum Gen for 7100 BTU Roofair?
Date: Thu, 07 Sep 2000 07:02:35 -0400

SomeOne wrote:

> In article <8p6qck$>, "Melissa S. Frye"
> <> wrote:
> >I run my 600 watt microwave off my Honda EX1000 without a problem - but I
> >don't run anything else at the time :)
> >
> >I have the same question about the 7100 - let me know if you get an answer.
> >
> ><> wrote in message
> >news:8p4u8u$ps5$
> >> HI,
> >>
> >> Can someone tell me what the minumum generator is for a 7100 BTU air
> >> conditioner?  I have a 13500 on my TT and the book says it needs at
> >> least a 3500 watt generator, but it is not clear on the 7100 unit.
> >>
>  Taking the math working with the current draw of 8.2 amps gives us 1333.4w
> required to support this unit. Remember that would be a constant power
> requirement. ( E*1.414*I=W )

Power is simply E*I for resistive loads (984 watts in this case if
the AC were purely resistive) or  true power is E*I*Power factor for
non-resistive loads.  For the AC, that matters a little as we'll see

>  You need some overhead to cover lights and such plus surges. Add about
>  1k min for buffer. So safely you need a 2.5kw generator to run the
>  whole show.

This issue comes up often enough that I decided to make some
measurements.  So I whipped out my Esterline-Angus Power Master IIIB
and instrumented my RV.  This instrument measures volts, amps, KW,
KVA, KVARS, power factor, the -hour versions the above as
applicable, and inrush current.  Nifty little instrument.

I have a Coleman 15Kbtu low inrush AC on my rig.  While larger than
the unit in this thread, the numbers are representative, as is the

Here's what I measure on a nice cool 70 deg night:

volts	121
Amps	11.5
KVA	1.4
KW	1.3
PF	.94
Inrush	63 amps

Important to note that the current and power consumption goes up a
LOT in hotter weather.  The inrush doesn't go up much at all, since
it is really just the locked rotor current.  How long the power
source has to supply inrush DOES depend on thermal conditions,
however.  If it's really hot out and the unit has just recently been
run so that there's both a high absolute head pressure and a high
differential pressure across the compressor, inrush may be drawn for
as long as several seconds.  Inrush will also be drawn longer if the
power source is "soft" (voltage droops with load as with a small
generator).  My tests were performed on shore power.

To check the measurements, 121 volts * 11.5 amps = 1391 volt-amps -
close enough to 1.4KVA.  Multiplying by the PF of .94 yields 1308
watts - again, close enough to 1.3KW.  The measurement passes the
confidence check.

Now let's contemplate what these numbers tell us.  If RUNNING the AC
was the only concern, a 1500 watt generator would be more than
enough.  However, that's not the issue.  In order to RUN the AC, I
have to START it.  The issue is whether the generator can supply the
inrush KVAs required to start the compressor.  From the above, 121
volts * 63 amps = 7623 volt-amps or 7.6 KVA.  (This is worst case
since in reality, the line - even shore power - will sag a bit under
inrush conditions resulting in fewer KVA being required but worst
case is best in these analyses.)

Generators are nominally rated in kilowatts but in reality, the
rating is really in KVA, kilo-volt-amps.  If I slap a nearly purely
reactive load, say a choke or mag tape degausser) across the
generator and the load pulls the generator's full rated amps, the
generator is maximally loaded even though the watt draw, by
definition, is near zero.  I'm going to use KVA instead of kilowatts
in the rest of this discussion so that terms don't get mixed up.

Apparently it would take an 8 KVA generator to start my AC.  That
can't be true since it runs just fine on my Onan 3kw AJ generator.
How?  Each generator has a surge rating that is much higher than its
continuous rating.  For conventional generators, the instantaneous
surge rating is determined by how much iron and copper there is in
the stator, how strong the field is and how much flywheel effect
there is on the rotating parts.  Heavier = better!!  After the first
few cycles of inrush, the motor starting capability depends in large
measure on how fast the governor reacts.  For inverter generators
such as the Honda EU series, the main limiting factor is how much
current the inverter can supply before going into current-limit mode
and secondarily on how fast the engine can spool up to feed the
inverter.  A TYPICAL inrush capacity is 3 times the continuous

Taking that number, my 3 KVA onan should be able to supply 3 * 3 = 9
KVA surge.  Since it easily starts my AC, the evidence is that it
indeed can supply that much current.  One can observe the governor
slam the throttle wide open almost instantly when the AC kicks on,
followed by tapering back to about half throttle to run.

This analysis has been pretty straightforward to this point.  The
complication arises in that not all generators can supply 3X
starting current.  Lightweight and/or cheap generators usually
can't.  Iron and copper and rotating mass are minimized to reduce
cost and weight and it kills the inrush capability.  For example, I
have a 4 KVA yamaha generator that will not start my 2 HP portable
air compressor even though the inrush is less than 3X the
generator's nominal rating.  Just can't supply the umph to make it

Since very few generator manufacturers supply inrush specs (Honda on
their EU series is the major exception that I know of), it becomes
very difficult to do a paper analysis.  There is only one way to
determine if a given AC unit can run from a candidate generator and
that is to test it.  Some manufacturers have done the testing for
you.  For example, Onan specifies how many ACs of what rating their
generators can run.  And even with specs, there's no substitute for
testing.  One must find a dealer who will allow one to connect his
actual AC unit to the model being considered for purchase.  Ideally
the test should be conducted with instrumentation similar to mine so
that the hot weather "headroom" can be determined.  Peak-reading amp
clamps which will measure the inrush are fairly cheap.  If the
dealer doesn't have one, I'd certainly spend a $100 or so to verify
that the $2000 generator would do the job before buying it.  It is
also interesting to measure how much the generator speed drops
during the inrush event.  This gives an indication of how well the
governor works and how close to the edge the generator is.  My Fluke
88 DVM can measure RPM with a peak/valley hold function.  Other,
less expensive instruments can do the same.

If you don't understand all this and/or don't have the
instrumentation, then follow Pournelli's Rule which says "If you
don't know what you're doing, pay someone who does."  That payment
might be in the form of a knowledgeable and well equipped dealer who
doesn't discount or it might be a consulting engineer such as
myself.  In any event, it's cheaper to do it right rather than have
to do it over!

Back to the original question:  The way to do a paper analysis of
your situation is this.  Look in your AC owner's manual for the
specification on inrush amps.  If the manual does not contain this
spec, then take the cover off the unit and look on the compressor.
There will be a nameplate on the compressor (NOT the AC unit itself)
and one of the specs will be "LRA" or Locked Rotor Amps.  LRA is
close enough to inrush for this analysis.  I'd expect it to be in
the 50 amp range for a unit of that size.  Next, look at your
candidate generator's spec.  Get the full load amp (FLA) rating.  If
the generator is NOT an RV generator, then it likely has dual
120/240 volt outputs.  One 120 volt outlet invariably has a higher
rating than the other.  Use the high one. If you're looking at a 3
KVA unit, the FLA may be 25 amps.  Multiply the FLA by 3.  In this
example, 75 amps.  This is the generator's estimated inrush
capability.  If the inrush amps spec is significantly larger than
the LRA of the compressor, then the generator will probably run the
unit.  I say "probably" because you don't know the ACTUAL surge
capability of a given generator until you test.

Another approach is to work backwards.  Take the LRA and divide it
by 3.  If the LRA of your unit is 50 amps, dividing by 3 = 17 amps.
A generator capable of supplying 17 FLA at 120 volts is 2 KVA.  So a
nominal 2.5 KW (remember we really mean KVA here) generator will
PROBABLY start the AC.  Again, you MUST test.  Beware of discount
dealers who won't make a demonstrator available and who won't refund
your money once you gas and oil a unit you purchase.

One last bit of advice.  Resist the urge to punt all this technical
stuff and just buy too large a generator.  Conventional (non
inverter) generators are quite inefficient users of fuel at part
load and they're louder than smaller units.  A loud, gas guzzling
generator is not a fun camping companion!  The inverter generators,
and to a lesser extent, conventional diesel generators are the
exceptions to the rule.  The engine in the inverter generator is run
only as fast as necessary to supply the pravailing load.


Index Home About Blog