Re: [RE-wrenches] array overloading of an inverter

[Solar]

This thread show how this idea is really a site by site issue. Two winters ago 
we had something like 40+ days with temps below zero (-45F the coldest)  while 
in the summer temps tend to be near STC very often.  My Tigo system at home 
shows the DC side of my system (210W modules) producing 220-250W regularly 
during those cold winter months. I try take that into account when designing a 
system. 

Jesse Dahl

NABCEP PV Installation Professional
IBEW Local 292 - Electrician 
Electrical/Solar PV Instructor - HCC

Sent from my iPhone

> On Jul 13, 2016, at 3:37 PM, Kirk Herander <vtso...@icloud.com> wrote:
> 
> Of the many Enphase systems I've done, clipping is a regular issue in
> wintertime Vermont, especially with the older micros whose output wattage
> ran way below the max. panel wattage. I've seen clipping for 3 hours or more
> on super-cold,  crystal clear days, with customer complaints about not being
> able to harness the peak . The DC:AC ratio of greater than 1 may also not be
> advantageous when using module based electronics, ala Solaredge, since the
> claim of 5 -7 % greater dc to ac conversion over a string inverter appears
> to hold true in my experience. You may be already getting the gains which
> overloading will accomplish, making overloading unfeasible economically. 
> 
> -Original Message-
> From: RE-wrenches [mailto:re-wrenches-boun...@lists.re-wrenches.org] On
> Behalf Of Matt Partymiller
> Sent: Wednesday, July 13, 2016 4:16 PM
> To: RE-wrenches
> Subject: Re: [RE-wrenches] array overloading of an inverter
> 
> Kirk,
> 
> I would recommend you consider using a tool like PVsyst or Helioscope to
> model DC:AC ratios.  A quick modeling of roughly 200 kW on three Solectria
> 50 kW inverters shows about 1.3% clipping here.
> 
> It then comes down to analysis of your costs for additional modules, dc
> wiring, and inverters.
> 
> Admittedly I sit on the opposite side of the fence.  I try to provide at
> least a 1.2 ratio to minimize cost per kWh.  I am not aware of any large
> scale studies comparing PVsyst or Helioscope clipping projections to real
> world performance.  I can tell you that our arrays with monitoring and a
> ~1.2 DC:AC ratio perform as well as our older 1:1 systems.  Not that I use
> Enphase often but they have some research/whitepapers on the topic since
> their micros tend to undershoot 60 cell module power ratings.
> 
> Good luck!
> 
> 
> Matt
> 
> Matthew Partymiller
> Solar Energy Solutions LLC
> (877) 312-7456
> m...@solar-energy-solutions.com
> 
> 
>> On Wed, July 13, 2016 3:02 pm, Kirk Herander 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 
>&g

Re: [RE-wrenches] array overloading of an inverter

[Mike Kocsmiersky]

I used to think exactly like Kirk, in that I would design the DC array size to 
match the inverter size, figuring why clip potential output.  However, when you 
have 1000W/m2 shining down on the modules, a crystalline silicone module will 
heat up about 30C above ambient, and dropping your output by about 12% to 15%.  
As outlined by Chris, adding other loss factors and you will probably never 
clip in the summertime even if you are 15% greater with DC module rating vs AC 
inverter rating.  The benefits of keeping a higher DC power to AC power ratio 
allows the inverter to operate closer to peak efficiency during low light 
levels, startup and near power down conditions.  Also, as the modules degrade 
over time, the ratio will drop.  So does having the added available power 
earlier in the morning and later in the day keep the inverter on longer to 
produce more power during those times than the power lost due to noontime 
clipping in the winter?  Conventional wisdom says yes.  (Here I am trusting the 
inverter sizing software designers, and my own experience.  The only other 
drawback is additional heat buildup on the heat sink due to the extra power the 
inverter is pushing through, which the inverter should be able to handle.  
Overall I think you are best to design residential systems with a ratio of 1.12 
+/- .02 to 1 of DC array size to inverter AC rating.  For commercial systems go 
a bit higher 1.2:1 to 1.25:1.

 

Mike Kocsmiersky

Principal

Phone: 413 883-3144 

Spirit Solar

www.SpiritSolar.net

 

 

From: Chris Mason [mailto:cometenergysyst...@gmail.com] 
Sent: Thursday, July 14, 2016 8:07 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] array overloading of an inverter

 

In high temperature locations such as the Caribbean, where we are, the modules 
run at about 40C during midday. Combine the temperature derate with losses for 
dust, cabling, azimuth and tilt, it makes sense to add ten to twenty percent 
more PV.

Given that the string design is never perfect, I would rather go a little over 
than way under. When we were installing 240W modules on European Sunny Tripower 
20KW units using 1,000V PV designs, we could either go with three strings = 
18KW or 4 strings = 24KW. The three strings would have seriously underpowered 
the inverter. 

 

On Wed, Jul 13, 2016 at 3:02 PM, Kirk Herander <vtso...@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

dba Vermont Solar Engineering

NABCEPTM  2003 Inaugural Certificant

VT RE Incentive Program Partner

802.863.1202

 

 

 


___
List spo

Re: [RE-wrenches] array overloading of an inverter

[Chris Mason]

In high temperature locations such as the Caribbean, where we are, the
modules run at about 40C during midday. Combine the temperature derate with
losses for dust, cabling, azimuth and tilt, it makes sense to add ten to
twenty percent more PV.
Given that the string design is never perfect, I would rather go a little
over than way under. When we were installing 240W modules on European Sunny
Tripower 20KW units using 1,000V PV designs, we could either go with three
strings = 18KW or 4 strings = 24KW. The three strings would have seriously
underpowered the inverter.

On Wed, Jul 13, 2016 at 3:02 PM, Kirk Herander  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 *
>
> dba Vermont Solar Engineering
>
> NABCEPTM  2003 Inaugural Certificant
>
> VT RE Incentive Program Partner
>
> 802.863.1202
>
>
>
>
>
>
>
> ___
> List sponsored by Redwood Alliance
>
> List Address: RE-wrenches@lists.re-wrenches.org
>
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>
>


-- 
Chris Mason
NABCEP Certified Solar PV Installer™
Solar Design Engineer
Generac Generators Industrial technician

www.cometsolar.com 
264.235.5670
869.662.5670
Skype: netconcepts
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Re: [RE-wrenches] array overloading of an inverter

[Kirk Herander]

Of the many Enphase systems I've done, clipping is a regular issue in
wintertime Vermont, especially with the older micros whose output wattage
ran way below the max. panel wattage. I've seen clipping for 3 hours or more
on super-cold,  crystal clear days, with customer complaints about not being
able to harness the peak . The DC:AC ratio of greater than 1 may also not be
advantageous when using module based electronics, ala Solaredge, since the
claim of 5 -7 % greater dc to ac conversion over a string inverter appears
to hold true in my experience. You may be already getting the gains which
overloading will accomplish, making overloading unfeasible economically. 

-Original Message-
From: RE-wrenches [mailto:re-wrenches-boun...@lists.re-wrenches.org] On
Behalf Of Matt Partymiller
Sent: Wednesday, July 13, 2016 4:16 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] array overloading of an inverter

Kirk,

I would recommend you consider using a tool like PVsyst or Helioscope to
model DC:AC ratios.  A quick modeling of roughly 200 kW on three Solectria
50 kW inverters shows about 1.3% clipping here.

It then comes down to analysis of your costs for additional modules, dc
wiring, and inverters.

Admittedly I sit on the opposite side of the fence.  I try to provide at
least a 1.2 ratio to minimize cost per kWh.  I am not aware of any large
scale studies comparing PVsyst or Helioscope clipping projections to real
world performance.  I can tell you that our arrays with monitoring and a
~1.2 DC:AC ratio perform as well as our older 1:1 systems.  Not that I use
Enphase often but they have some research/whitepapers on the topic since
their micros tend to undershoot 60 cell module power ratings.

Good luck!


Matt

Matthew Partymiller
Solar Energy Solutions LLC
(877) 312-7456
m...@solar-energy-solutions.com


On Wed, July 13, 2016 3:02 pm, Kirk Herander 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
>
> dba Vermont Solar Engineering
>
> NABCEPTM  2003 Inaugural Certificant
>
>
> VT RE Incentive Program Partner
>
>
> 802.863.1202
>
>
>
>
>
>
>
>
> ___
> List sponsored by Redwood Alliance
>
>
> List Address: RE-wrenches@lists.re-wrenches.org
>
>
> Cha

Re: [RE-wrenches] array overloading of an inverter

[Daniel Young]

I'm with Matt in terms of Design. I use 1.2-1.35 as a general rule. With
current inverter sizes, you almost always can get a good fit within this
range.

Climate plays a big role on this. As do the costs of your materials and
labor. What we use as guidance is this:

[If I save 5% on a systems cost by going with a smaller inverter, and the
"clipping" losses are only 2% of my yearly energy, then the tradeoff is
worth it. I produce more kWh for the dollars spent.]

The above is a simplistic view not using real numbers (don't have time right
now to run costs on different options in you climate :) ) You can also think
about how the inverter life is affected by operating closer to its limits
for a longer amount of time each year, but that is a hard variable to
quantify.

I ran a set of simulations just for kicks on what I believe to be one of
your Jobs Kirk. @ cypress semiconductor. Hopefully to get a better idea of
what these DC:AC ratios mean in your climate.

I put 6.16kW DC of Solar modules on the roof @15degrees (22 x 280W modules).
I ran the simulation with a SB4000TLUS-22, a SB5000TLUS-22, and
SB6000TLUS-22. And I got 3.8%, 0.3%, and 0.0% clipping losses respectively.

Based on the above, my gut says that the 5kW is certainly worth it. And even
the 4k inverter might, but I'm not comfortable going to a 1.5:1 DC:AC ratio
for so many other reasons.

With Regards,

Daniel Young, 
NABCEP Certified PV Installation ProfessionalTM: Cert #031508-90

-Original Message-
From: RE-wrenches [mailto:re-wrenches-boun...@lists.re-wrenches.org] On
Behalf Of Matt Partymiller
Sent: Wednesday, July 13, 2016 4:16 PM
To: RE-wrenches <re-wrenches@lists.re-wrenches.org>
Subject: Re: [RE-wrenches] array overloading of an inverter

Kirk,

I would recommend you consider using a tool like PVsyst or Helioscope to
model DC:AC ratios.  A quick modeling of roughly 200 kW on three Solectria
50 kW inverters shows about 1.3% clipping here.

It then comes down to analysis of your costs for additional modules, dc
wiring, and inverters.

Admittedly I sit on the opposite side of the fence.  I try to provide at
least a 1.2 ratio to minimize cost per kWh.  I am not aware of any large
scale studies comparing PVsyst or Helioscope clipping projections to real
world performance.  I can tell you that our arrays with monitoring and a
~1.2 DC:AC ratio perform as well as our older 1:1 systems.  Not that I use
Enphase often but they have some research/whitepapers on the topic since
their micros tend to undershoot 60 cell module power ratings.

Good luck!


Matt

Matthew Partymiller
Solar Energy Solutions LLC
(877) 312-7456
m...@solar-energy-solutions.com


On Wed, July 13, 2016 3:02 pm, Kirk Herander 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 ca

Re: [RE-wrenches] array overloading of an inverter

[Jason Szumlanski]

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  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 *
>
> dba Vermont Solar Engineering
>
> NABCEPTM  2003 Inaugural Certificant
>
> VT RE Incentive Program Partner
>
> 802.863.1202
>
>
>
>
>
>
>
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Re: [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.a...@solarcentex.com> scot.a...@solarcentex.com

 



 

 

 

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

[Matt Partymiller]

Kirk,

I would recommend you consider using a tool like PVsyst or Helioscope to
model DC:AC ratios.  A quick modeling of roughly 200 kW on three Solectria
50 kW inverters shows about 1.3% clipping here.

It then comes down to analysis of your costs for additional modules, dc
wiring, and inverters.

Admittedly I sit on the opposite side of the fence.  I try to provide at
least a 1.2 ratio to minimize cost per kWh.  I am not aware of any large
scale studies comparing PVsyst or Helioscope clipping projections to real
world performance.  I can tell you that our arrays with monitoring and a
~1.2 DC:AC ratio perform as well as our older 1:1 systems.  Not that I use
Enphase often but they have some research/whitepapers on the topic since
their micros tend to undershoot 60 cell module power ratings.

Good luck!


Matt

Matthew Partymiller
Solar Energy Solutions LLC
(877) 312-7456
m...@solar-energy-solutions.com


On Wed, July 13, 2016 3:02 pm, Kirk Herander 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
>
> dba Vermont Solar Engineering
>
> NABCEPTM  2003 Inaugural Certificant
>
>
> VT RE Incentive Program Partner
>
>
> 802.863.1202
>
>
>
>
>
>
>
>
> ___
> List sponsored by Redwood Alliance
>
>
> List Address: RE-wrenches@lists.re-wrenches.org
>
>
> Change listserver email address & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
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> ml
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[RE-wrenches] array overloading of an inverter

[Kirk Herander]

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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