Generator sizing [RE-wrenches]

[Ray Walters]

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HI;
On larger systems (over 10 KW ) Its good to also look at the fuel 
consumption at different loads. For instance compare a 10 Kw unit at 
full load vs a 20 Kw unit at 1/2 load. This data isn't readily available 
for all generators, but reputable Manus like Kohler have it availalble.
Another factor to consider with conventional portable generators is 
power quality under load. I just sold a Yamaha EF6600 watt generator 
(definitely good quality) that under full load had a harmonic spike in 
the waveform that was hitting 415 VAC peak. ( peak on a clean 240 VAC 
rms should be about 340) When I looked at it with a Silly scope , you 
could see the spike on the leading edge of each wave. I've seen this 
with most gennys under load. Only Inverter-generators, and highly 
regulated units like Kohler have a clean wave under load.
The point is: you have to be familiar with the genny's waveform to 
properly size it, and you have to know what your charger will do with a 
variable wave form. Last week I measured an old IBE charger that when 
VAC dropped from 120 to 115, the DC output went to half current. More 
puzzling, the waveform of the genny was looking good still (165 V peak).
We are about to try a Northern Star generator (10.5 KW propane) with a 
Honda motor. It claims < 5% THD on the waveform; we'll see!
Its still tough IMHO to nail down genny sizing with a simple equation 
only. Just when you think you have a combo that works (ie; Kohler genny 
into a SW) the manus change the product line and you have to start all 
over from scratch. At least the better inverters (Outback) allow you to 
program the amps AC in, and have generator assist so you can avoid 
disaster by not overloading a mis sized generator.

R. Walters
SolarRay

  Darryl Thayer wrote:

>Thank you Allan
>I think you have it about nailed.  My customer wanted
>a second opinion, and I thank you for the logic behind
>your method.
>Darryl
>
>--- Allan Sindelar <allan at positiveenergysolar.com>
>wrote:
>
>  
>
>>Darryl,
>>Starting with a better quality unit, that can be
>>counted on to deliver continuous output, figure
>>inverter maximum (or desired) AC input, divided by
>>(deration for elevation above sea level times
>>deration for how close to maximum rated output we
>>want to run it). 
>>
>>For elevation deration we use mfgr's specs, or 3.5%
>>per 1,000 ft above sea level if no mfgr's specs.
>>
>>Example: An Outback VFX can take a maximum of 19A AC
>>in, I believe. At 6,000 feet that's 3.5 X 6 = 21%;
>>100 - 21 = 795, or 0.79. If I want to run the gennie
>>at no more than 85% of maximum output, then:
>>
>>____19___
>>0.79 X 0.85  =  28.3A @ 120V, or at least 3,400
>>rated watts.  
>>
>>Then adjust for other factors, like 120V output
>>availability; if 120/240 only you need a step-down
>>transformer, with its attendant losses; reputation
>>of the generator for hangin' in there when you work
>>it that hard; wire losses, if distance is involved,
>>and power factor. Inverter efficiency doesn't play
>>directly into this equation, as it more affects DC
>>output. 
>>
>>On a big off-grid system we are doing, two VFX3648s,
>>using "Outback stacking" with the Hub, is plenty to
>>run the house and barn. However, we are using a Quad
>>Stack in order to get the charging C/ rate up high
>>enough; the other two inverters are just for
>>charging. At 6,500 feet, we need 18kW including
>>derations, and are using an Onan RS20000 unit.
>>
>>Hope this helps.
>>Allan @+E
>>
>>----- Original Message ----- 
>>From: "Darryl Thayer" <daryl_solar at yahoo.com>
>>
>>    
>>
>>>What logic process do you use to size a generator?
>>>      
>>>
>> If
>>    
>>
>>>the generator is oversized and runs at part load
>>>      
>>>
>>its
>>    
>>
>>>effiecency is affected, also my customer is afarid
>>>      
>>>
>>of
>>    
>>
>>>diesels in cold minnesota climate and would perfer
>>>      
>>>
>>a
>>    
>>
>>>propane generator.  
>>>      
>>>
>>    
>>
>  
>

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