[RE-wrenches] array overloading of an inverter

[Howard Arey]

I'll give a different spin and of course each situation is different and to
be judged on its own facts. Here in Texas, modules on a summer day don't
typically perform to STC ratings and a DC/AC over-ratio is entirely
appropriate. I've brought it up to 1.25 in some cases.

 

I look at the economics differently: you're putting in the same 10KW
inverters, the same 10KW appropriate wires and conduit, the same OCPD, the
same BOS costs for paperwork.so why not use all of that capacity a bit more.
Instead of looking at the lost DC clipping, think about the increased use of
inverter and beyond components.

 

With a very modest cost increase, you're using that side of the system more
(at same cost as it would be for a 10KW DC to 10KW AC system).

 

Yes, in the spring, I might see some midday clipping but a small price to
pay for increase year round performance.  Your Vermont conditions might not
play out like my Texas conditions since you live closer to STC than I do.

 

 

Howard "Scot" Arey

254-300-1228

 <mailto:scot.arey at solarcentex.com> scot.arey at solarcentex.com

 



 

 

 

From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
Behalf Of Kirk Herander
Sent: Wednesday, July 13, 2016 2:02 PM
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Subject: [RE-wrenches] array overloading of an inverter

 

Hello folks,

I am composing a response to a question a potential customer asked me. It
seems a competitor is trying to talk him into a 20% larger KW array than the
inverter AC max output rating is. The idea of course is to generate more
power on either side of peak output, but at a cost I don't feel is
justified. My opinion of this particular big-box installer I will keep to
myself. My response to the customer, trying to keep it simple:

 

"On the DC array input side, most inverters do allow an overload factor. For
instance, a 10kw AC inverter may allow for 12 kw of DC array as an input.
Whether or not this is a good idea boils down to economics and technical
reasons.

On a sunny day, the inverter generates power as a typical bell curve. Power
output rises in the morning, peaks at noon, declines in the afternoon. In my
example, the inverter can't output more than 10 kw AC. What overloading the
input will do will widen the bell curve, i.e. generating more power in the
morning and afternoon, BUT clipping the peak at 10kw on either side of noon.
So there is power to be gained in morning and afternoon, but peak power is
lost(if conditions allow the peak output to be reached), since the 12 kw
array can never be converted to more than 10 kw of AC power. Depending upon
time of year(ambient / cell temperature) and weather conditions, the peak
may be clipped at 10 kw for several hours a day. So you are both gaining and
losing power using this method. And typically the inverters are only
overloaded in this manner on large-scale farms where the economics are
favorable.

In your case, if you could actually put 200kw of DC array into 150 Kw of
inverter, the economics would never justify it. That extra 50 kw of array
would cost you $100k of more, and the dollar payback for the power that
extra 50Kw would generate will take 2 - 3x the time that the array size does
that stays inside the output limit of the inverters. This is why I'm not a
big fan of dramatically overloading the inverters, if at all,  in your case.
Any KW portion of the array which is above the nameplate kw rating of the
inverter is going to have a longer payback for these reasons. "

I feel my reasoning is sound, But I don't want to be too loose with the
facts. Comments are appreciated. Thanks.

 

Kirk Herander

Owner|Principal, VT Solar, LLC 

Celebrating our 25th Anniversary 1991-2016

www.vermontsolarnow.com

dba Vermont Solar Engineering

NABCEPTM  2003 Inaugural Certificant

VT RE Incentive Program Partner

802.863.1202

 

 

 

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