[RE-wrenches] Inverters Maximum Input Ratings

Tue Aug 26 16:10:45 PDT 2014

Hi Rebekah,

This is where checking simulation data versus real output data is key. Our
company has hundreds of monitored systems but very few with undersized
inverters. So, I don’t have many data points that I can study. Generally
our systems meet or beat our simulated as-built production estimates within
an amazingly close margin. I think your point about inverter heating and
derating is valid for inverters which are significantly undersized. You
seem to have a strong opinion that simulators generally underestimate
clipping. Do you have any particular sites where you learned this from?

Best, August

*From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On
Behalf Of *Rebekah Hren
*Sent:* Tuesday, August 26, 2014 10:54 AM
*To:* RE-wrenches
*Subject:* Re: [RE-wrenches] Inverters Maximum Input Ratings

One of the problems with relying on simulators (PVsyst, Helioscope,
whatever) to estimate clipping is that they rely on hourly averaged
irradiance from TMY files (with very few exceptions), which may not predict
clipping, especially for areas with variable climates.

Thinking about partly cloudy days in NC where I live and install systems,
an average hourly value predicts no or a very low percentage clipping for
systems with even a fairly high dc to ac ratio (by high I mean anything
over about 125%). Without subhourly meteo data, I would be skeptical of
simulators estimates of clipping. I believe they are generally
underestimated, which can lead to a false financial analysis of the most
cost effective dc-ac ratio.

Also, I do not think most (any?) of the simulators have the capability to
take inverter internal temperature related power derating (another form of
power clipping) into account in the simulations, which could lead to
overestimated generation. I realize hot inverter temperatures may not
coincide with the colder temps that usually create clipping, but the higher
the ratio goes the more often we see an overlap in hot days/high
temps/clipping.

Cheers,

Rebekah Hren

-- 

Tel: 336.266.8800
http://o2energies.com/
Project Engineer
NABCEP Certified PV Installation Professional™ 091209-85
NC Licensed Electrical Contractor

On Tue, Aug 26, 2014 at 11:30 AM, August Goers <august at luminalt.com> wrote:

Hi Jason and All,

Good topic. As far as I know, simple energy estimation programs like
PVWatts don’t take inverter size into account when estimating annual
production but more robust simulation programs like PVsyst do. I think it
is a matter of really being able to trust your simulator to tell the
designer how sizing the inverter will affect annual production. Maybe
others can chime in on this.

We’ve run different scenarios through PVsyst in house to see how under
sizing inverters affects the overall annual production. It is surprising
for our area (SF Bay Area) that we can really push an inverter with very
little reduction in annual simulated production. I’m talking about putting
maybe 5 kW (or even a little more) of PV on a 3.8 kW inverter or
thereabouts. Array orientation also factors in. To Issac’s point, if
keeping the inverter small reduces the initial installation cost
significantly while possibly only reducing annual production by a little
bit then I feel it is a good design.

All that said, we’re still fairly conservative with pushing our inverters.
I’ve found that it is very hard to go back to a client after the
salesperson has made the sale and try to reduce the inverter size. In other
words, we need to nail the inverter size up front during the sales process
or the client will oftentimes feel taken advantage of later. It is hard to
explain why we are only giving a client a 3.8 kW inverter when we are
calling their system 5 kW DC.

To Isaac’s questions: most of our clients wouldn’t notice about mild to
moderate clipping during peak periods. However, some would. This is why it
is important to setup proper expectations about how we designed the system
up front and do our homework to assure that the clipping is accurately
estimated and factored in to the production estimates. If we can provide
clear logical reasoning for sizing the inverter the way we did then we
shouldn’t have any problem. I have no idea how array sizing might affect
inverter longevity.

I think the general points made by others about the declining costs of the
modules themselves and increased cost of BOS components means that these
types of discussion are very valid.

Best, August

*From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On
Behalf Of *Isaac Opalinsky

*Sent:* Tuesday, August 26, 2014 6:11 AM
*To:* RE-wrenches

*Subject:* Re: [RE-wrenches] Inverters Maximum Input Ratings

Jason,

This seems to be a regular topic of discussion in our training classes not
only for microinverters, but also for string inverters.  Especially for 3.8
kW units that are optimal for backfeeding 100A service panels and 7.6 kW
units that are optimal for 200A panels.  A slightly bigger array can give a
higher total yield, maybe some power clipping, without the additional cost
of a supply-side connection.  As long as you stay below the maximum VOC and
ISC, there isn’t a safety issue.

So it really just boils down to economics and the overall value proposition
for the customer, which makes it hard to provide a blanket recommendation.
We’ve been training people for years to model PV system performance to
determine an acceptable DC/AC ratio on a project-by-project basis.

The inverter manufacturers pretty much all claim that there is no concern
about overworking or shortening the useful life of their inverters since
limiting operating power limits the operating temperature as well, but that
leaves me with two questions:

1.       Does anyone have any evidence that high DC/AC ratios does/does not
shorten the life of the inverter?

2.       If there is a small amount of power clipping (say <1% total annual
energy), are many customers likely to notice/care?

3.       If they do notice, does the customer service aspect of having to
defend a design decision outweigh the potential economic benefits of a
smaller inverter?

*Isaac Opalinsky *| Technical Trainer | *SunPower Corporation*

Desk 443-569-3476 | Cell 443-277-6286

*From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org
<re-wrenches-bounces at lists.re-wrenches.org>] *On Behalf Of *Jason Szumlanski
*Sent:* Monday, August 25, 2014 4:41 PM
*To:* RE-wrenches
*Subject:* [RE-wrenches] Inverters Maximum Input Ratings

I had a tough customer recently that grilled me on how we can put a 270W
solar module on a 215W inverter. Fortunately, Enphase has a wonderful white
paper on the subject. However, it got me thinking... Enphase has
demonstrated that higher output panels in many climates (hot SW Florida
included) can benefit from modules that far exceed the inverter rating, and
even exceed the inverter's "recommended input" rating. Enphase has shown
that 270W+ modules can show energy harvest on the M215 where it makes sense
to "oversize" the module.

I also received a similar query from a rather uninformed plan reviewer in
an area AHJ along similar lines. Fortunately I was within the "recommended
input" rating on the spec sheet of 270W with a 265W module, but I wonder
what would happen if I had paired the M215 with a 280W module on my plans,
which are becoming readily available now in 60 cell modules with 300W
modules on the near horizon. I'm pretty sure my plan would have been kicked
back for exceeding the manufacturer's recommendation.

My question, which applies to string inverters and microinverters, is how
much is too much, what would happen if you paired an array that far
exceeded the rating, and how do inverter manufacturers determine the
recommended and/or maximum rating of the connected module or array? Also,
why do some manufacturers have a simple recommendation while others have a
"maximum" rating?

Jason Szumlanski

Fafco Solar

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