SPAM: Re: Generator sizing [RE-wrenches]

[Darryl Thayer]

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.  
> 
> [Non-text portions of this message have been
> removed]
> 
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