[RE-wrenches] array overloading of an inverter

Jason Szumlanski jason at floridasolardesigngroup.com
Wed Jul 13 13:34:03 PDT 2016


There are a lot of factors involved, but 20% oversizing (DC/AC ratio), is
not going to result in much clipping in most cases. Clipping only occurs
when the irradiance and temperature are right. Less than ideal conditions
may frequently occur during mid-day eliminating the chance of clipping on
those days. Morning and afternoon happens every day where upsizing module
ratings helps a lot. Widening the bell curve often has a larger effect than
clipping.

"...if conditions allow the peak output to be reached" is a big if!

In my experience, 20% is a pretty safe/low factor. Here in Florida, heat
makes modules perform below their nameplate quite a bit. Hazy skies in
summer reduce irradiance. On a personal note, I see almost no clipping on
my Enphase M215 inverters with 255W modules.

Have you ever studied Enphase's "Rightsizing" white paper? While it is
specific to their inverters, the concept applies to string inverters, too.
They also address regional differences, which are definitely a factor to
consider.

Also note that some inverters can actually produce more than the nominal
nameplate rating on the AC side.

I usually take the opposite approach that you are suggesting. Oversizing
allows you to use all of the other components to their maximum potential,
reducing overall cost per watt DC. I have also seen some pretty aggressive
tactics competitors use to show a really low cost per watt DC by pairing a
300W module with an Enphase M215. This it overkill in my opinion, despite
Enphase's suggestion that it may have benefits (and their data sheet still
recommends a max of 270W). Now that the M215 and M250 are pretty close in
price, I'm able to overcome that issue in a competitive situation. I've
been tempted to replace one of my modules with a 300W just to see what
happens. If I get around to that, I'll report back!


Jason Szumlanski
Florida Solar Design Group


On Wed, Jul 13, 2016 at 3:02 PM, Kirk Herander <vtsolar at icloud.com> wrote:

> 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 <http://www.vermontsolarnow.com>*
>
> dba Vermont Solar Engineering
>
> NABCEPTM  2003 Inaugural Certificant
>
> VT RE Incentive Program Partner
>
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>
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>
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>
>
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