[RE-wrenches] array overloading of an inverter

Wed Jul 13 13:37:35 PDT 2016

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-bounces at lists.re-wrenches.org] On
Behalf Of Matt Partymiller
Sent: Wednesday, July 13, 2016 4:16 PM
To: RE-wrenches <re-wrenches at 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
matt at 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
>
>
>
>
>
>
>
>
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