Generator sizing [RE-wrenches]

[Allan Sindelar]

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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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