Inverter Power Ratings [RE-wrenches]

jberdner at sma-america.com jberdner at sma-america.com
Tue Apr 30 09:43:49 PDT 2002


William:

I think the whole idea of trying to maximizing the inverter output does
not make sense (economically).
What we really are trying to do in a PV system is to maximize the energy
we capture and convert from the PV array.

The cost of the inverter is typically 20 to 30% of the module cost and
maybe 10% of total installed system cost.
Even if you ran the inverter at 75% of it's rating you are only
"wasting" 25% of 10% of the total system cost.
At 75% of rated power (very conservative) the inverter is more efficient
so we are really talking about less than 2.5% of total system cost.
This must me weighed against the economic impact of trying to run the
inverter al full power all the time.
IMHO, trying to get a few more Watt Hours out of the inverter by adding
expensive modules does not add up.
For the price of the 20% more modules you could almost pay for a second
inverter. 
Then  both inverters would operate at lower power, higher efficiency,
and cooler temperatures.
Lower temperatures = longer life = happier customers.

While there are lots of variables, most of the power in a "typical" grid
tied system is generated during the middle of the day during in Summer.
If you add modules that you can't use during this mid day peak period
most of the output of the additional modules is wasted.
Yes, you will get a little more on the slopes of the bell curve but at
what cost ?
The inverter will probably flat top the curve and limit power during the
peak part of the day.
When you add up the Watt hours gained and lost I think the net result
will be very small.
In addition, you have added complexity to the system which will reduce
the long term reliability.

UL imposes limitations, and rightly so, on inverter input, output, and
temperature.
If you run into one of these UL limits and the inverter reduces it's
output power then you are going backwards (economically speaking).
We (SMA) consider some derating to be normal in Spring and Fall but too
much or too often is costing you money.
Array sizing is always a trade off between Winter/Summer performance
when array output is lower and Spring/Fall when it is highest.

If you have any questions, or if I can be of any further assistance,
please do not hesitate to contact me.

Best Regards,

John Berdner

SMA America, Inc.
20830 Red Dog Road
Grass Valley, CA  95945
530.273.4895 (voice)
530.274.7271 (fax) 



-----Original Message-----
From: William Miller [mailto:wrmiller at slonet.org] 
Sent: Monday, April 29, 2002 10:20 PM
To: RE-wrenches at topica.com
Subject: Re: Inverter Power Ratings [RE-wrenches]


Friends:

I've toyed with this idea:  For the hours before and after solar noon,
the grid tie inverter fed by fixed arrays is operating at reduced input
due to off-angle irriadiance.  What if one where to feed an inverter
with, say, 20% more PV input than the inverter is rated for?

!. The inverter self regulates during the hour before and after solar
noon.

2. During non-peak hours, overcast or low seasonal sun angle times, the
inverter runs at a higher output.

You've answered the question in that the inverter life span will be
reduced.  Depending on how severe this reduction is, however, this still
may be a worthwhile scenario for some users, i.e. replace the inverter
in 5 to 10 years but enjoy higher off peak output for all of those
years.

Comments???

William Miller




At 01:35 PM 4/29/02 -0700, you wrote:
>Hi Don and all.
>
>Been gone and just returned to this thread.
>
>I agree with Don and kent.  Makes no sense to push the inverter, any 
>inverter. I mean I wouldn't recommend someone to run a SW4024 at 4kw 
>for all day.
>
>Peace,
>jay
>
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