Inverter Testing--Continuing the thread [RE-wrenches]

Kent Sheldon kentsheldon at sbcglobal.net
Sun Apr 21 23:05:16 PDT 2002


Bill, I wholly support you and your effort to create an inverter
performance standard. Everyone thinks that efficiency is simply
Pout/Pin... WRONG. I have personally tested hundreds of inverters with
very expensive test equipment and know all the tricks. I can make an
inverter look better than 100% efficient. The single point of efficiency
listed on all the cut sheets, CEC web site, etc is totally meaningless.
The CEC number is simply a verbal claim from anyone in the represented
company, and does not have to be substantiated with any proof of
performance.

Not too long ago, I was speaking with Jeff Newmiller (Endecon/CEC)
regarding the SB2500U efficiency number on the CEC web site. He was
asking what it should be. I told him to put whatever he wanted to. He
didn't like that. So I asked him to list 103%. He didn't like that
either. So I said to just make sure it was greater than any of the other
grid tied inverters... For the record, the SB2500U is listed at 94.4 on
the CEC web site (which is truly the peak efficiency). Our literature
says greater than 93%. If anyone would like to look at actual data, send
me an email. I strongly stand behind these numbers. I have also looked
at data from some other inverters, using the same test equipment, and
have been mildly amused at the results vs. the claims.

The point of all this, is a single efficiency number doesn't mean
anything. 
Efficiency is a very difficult thing to measure. It seems simple enough
but there are many, many influencing factors. Here are a few: IGBT
temperature, AC filter temperature, DC input voltage, AC output power,
AC output voltage, AC harmonic content, instrument accuracy, AC/DC
measurement timing, measurement filtering, true RMS vs. 60Hz, etc, etc,
etc.

In my mind we need more than a standard for inverter efficiency. A
single number will always be meaningless. We need a performance index.
This is the percentage of total captured energy throughout a day, or
longer period. Who cares how efficient the inverter is, if it has a
crappy power tracker....

Bill, I would love to be part of your effort in this area. Keep in
touch.

____________________________________________
Kent Sheldon
Manager, Industrial Power Systems
SMA America
ksheldon at sma-america.com
925 513 8813 Brentwood Office (Primary)
530 273 4848 Grass Valley Office

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