[RE-wrenches] Inverter oversizing

Wed Jan 19 16:03:07 PST 2011

It makes sense to never drive your car at top speed for the tires, engine etc. 
and this generally applies to virtually any type of technology we own.

There can be an argument made to say the engineers have put in a safety factor, 
so don't worry, but I'd like my inverter to have a longer service life.

When you factor in heat on components and the cycling they experience, plus the 
added edge of cloud effects, it seems prudent to lean on the side of being 
conservative.

Seems like if we don't we will be replacing lots of inverters in the coming 
years. We really don't know how mods will perform over time. The whole + - what 
their nameplate is, is anyones guess.

Out here in the tropics, we already need to contend with humidity and salt with 
the heat, so being mindful of the variables is important.

However, many integrators in a sales type of environment could encourage putting 
more mods on an inverter to sell more horsepower.

Keith

________________________________
From: Dave Palumbo <dave at independentpowerllc.com>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Sent: Wed, January 19, 2011 1:55:19 PM
Subject: Re: [RE-wrenches] Inverter oversizing

Bill,

In addition to power production losses due to arrays that are oversized for the 
inverter (although within manufacturers specs) how concerned should we be with 
shorter inverter service life because the inverter is working at its maximum?

David Palumbo
Independent Power LLC 
462 Solar Way Drive
Hyde Park, VT 05655
www.independentpowerllc.com 
NABCEP Certified PV Installer
Vermont Solar Partner
23 Years Experience, (802) 888-7194 

From:re-wrenches-bounces at lists.re-wrenches.org 
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Bill Brooks
Sent: Wednesday, January 19, 2011 6:20 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Inverter oversizing

Kent,

The article that David Brearley cited is a very good discussion on this subject. 
It clearly shows the under-prediction of losses due to hitting the inverter peak 
power capability when using longer-term data. This study was done by the 
Fraunhofer Institute in southern Germany. They get a lot more clouds there so 
the results might be comparable to some of the more cloudy regions of the United 
States. I would expect the results to be worse in much of the U.S. that gets 
clouds and higher irradiation than Freiburg. However, the results will 
definitely be less for much of California since clouds don’t happen for sections 
of the year. 

It all matters where you are.

Great discussion.

Bill.

From:re-wrenches-bounces at lists.re-wrenches.org 
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kent Osterberg
Sent: Wednesday, January 19, 2011 1:41 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Inverter oversizing

Bill,

The data are 15 minute averages so I'm sure that some edge of cloud events are 
washed out in the average. The energy they contributed was in the total 
measurement, but wouldn't have been included in my sum of clipped output.  
Observing the data, you can see some edge of cloud effects despite the 15 minute 
averaging. And on many days I suspect there is edge of cloud effect that 
wouldn't have hit the clipping limit too.  Obviously, missing some short 
interval events biases the results a little but probably not by very much.  For 
the sake of argument assume that there was a 3 minute, spike in output 100-watts 
above my threshold setting.  In the 15-minute average that still would have been 
a 20-watt bump and with 10-watt data resolution, it probably would have showed 
up, but say it didn't show up or that it end up just below the threshold.  Say 
this happened once  every week, not likely, but if it did the missed data would 
amount to 0.25 kWhr or about 5% as much as the total observed with the data 
clipped at 800 watts.  So you have to really stretch the brief edge of cloud 
argument to integrate enough energy to throw my graph off by very much.

Kent Osterberg
Blue Mountain Solar, Inc.

Bill Brooks 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.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. 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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