[RE-wrenches] Inverter oversizing

Thu Jan 20 06:23:55 PST 2011

On a couple of Enphase systems I monitor 
regularly, the modules are oversized compared to 
the inverters.  One system has Sharp 224 W 
modules and M-190s (capable of producing 199 
W).  The array is a ground mount at a 30 degree tilt at 39+ degrees latitude.

When the system was new in the autumn, I noticed 
two days of some clipping.  Since then, none.  In 
the summer, when the sun angle is better, the 
heat will keep the wattage down so clipping will 
very seldom occur.  The modules will not be close 
to STC, so the inverters should not be pushed to their maximum.

The same is true of string inverters.  Since real 
power of the modules in generally significantly 
below the nameplate, inverters will generally not 
run at maximum power.   When we take in all the 
factors affecting the actual array power, I think 
that 120% over sizing from nameplate wattage is generally safe.

Drake

At 09:14 PM 1/19/2011, you wrote:
>David,
>
>Thanks for sharing that paper.  The labeling the 
>graphs for the 10-second and 1-minute data in 
>Freiburg appears to be reversed - the one minute 
>averaging seems to have more data in all of the 
>bins above 1000 W/sq m.  Basically, these graphs 
>show that irradiance observations above 1100 
>watts per square meter are fleeting and 
>disappear in hourly averages.   Such occurrences 
>are also masked to a small extent by 1-minute averages.
>
>Kent Osterberg
>Blue Mountain Solar, Inc.
>
>
>
>David Brearley wrote:
>>This reminds me of a scholarly article I came 
>>across about a year ago while doing some 
>>research. Here’s a link to it if anyone is interested:
>>
>><http://www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther.pdf>www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther.pdf
>>
>>David Brearley, Senior Technical Editor
>>SolarPro magazine
>>NABCEP Certified PV Installer ™
>><david.brearley at solarprofessional.htm>david.brearley at solarprofessional.com
>>Direct: 541.261.6545
>>
>>On 1/19/11 12:29 PM, "Bill Brooks" 
>><<billbrooks7 at yahoo.htm>billbrooks7 at yahoo.com> wrote:
>>
>>Kent,
>>
>>How often were your data records? To capture 
>>edge of cloud effects, you need one-second 
>>data. Not many people gather that fast or that 
>>much data on inverters. I don’t think there is 
>>that much energy in these spikes, but they are 
>>real and make some difference. 15-minute 
>>average data will completely wash out this data.
>>
>>This is also a deficiency in modeling software 
>>since most models are using hourly data.
>>
>>Bill.
>>
>>
>>From: 
>><re-wrenches-bounces at lists.re-wrenches.htm>re-wrenches-bounces at lists.re-wrenches.org 
>>[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kent Osterberg
>>Sent: Tuesday, January 18, 2011 1:18 PM
>>To: Wrenches; Marco Mangelsdorf
>>Subject: [RE-wrenches] Inverter oversizing
>>
>>
>>Attached is a graph that I produced to document 
>>the effect of various ratios between the PV 
>>array size and the inverter size.  I extracted 
>>output power data for a 1020-watt system 
>>located in NE Oregon that is on the Sunny 
>>Portal 
>><<http://www.sunnyportal.com/Templates/PublicPageOverview.aspx?page=85820a73-a347-48fb-b8d1-92e5f9b78ab3&plant=608681a7-ef60-4edb-84ff-07110db0ab6a&splang=en-US>http://www.sunnyportal.com/Templates/PublicPageOverview.aspx?page=85820a73-a347-48fb-b8d1-92e5f9b78ab3&plant=608681a7-ef60-4edb-84ff-07110db0ab6a&splang=en-US> 
>>. The data are publicly accessible so feel free 
>>to run your own analysis.  Better yet, analyze 
>>the data for a system near you.
>>
>>Using 2009 data, I looked at how much energy 
>>would have been lost if the output was clipped 
>>at 800W, 810W, .... 1020W.  I used 2009 data 
>>because there was a period in 2010 when the 
>>Sunny Webbox didn't have internet access.  At 
>>800 watts, power clipping would have happened 
>>on about 25% of the days.  Yet the energy that 
>>would have been lost was only 0.38% of the annual total.
>>
>>The results shown on this graph aren't 
>>universal, results would be a little different 
>>in 2010, it would be different in some other 
>>climate, it would have been different at 
>>another elevation, it would be different with a 
>>different array angle, ..., and the module 
>>tolerance and inverter efficiency also effect 
>>the results.  Modules in this system are 
>>Suntech 170-watt +/-3%.  The inverter is 
>>Sunnyboy 1800 that  should be operating at close to 93% efficiency.
>>
>>Kent Osterberg
>>Blue Mountain Solar. Inc.
>>
>>
>>
>>
>>
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