[RE-wrenches] Inverters Maximum Input Ratings

Wed Aug 27 05:08:21 PDT 2014

Rebekah,

My understanding was that PVsyst modeled inverter nominal power not
maximum power.  I believe this helps to compensate for inverter
temperature limitations.  For instance, depending on inverter location and
power ratio, I would suppose that there may be instances where the
simulation could clip potential max power production.  This may balance
those instances where inverter temp derating would result in production
loss.

I would be curious to see how subhourly data could affect a simulation. 
Admittedly not something I have thought about.

Matt

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

On Tue, August 26, 2014 7:10 pm, August Goers wrote:
> 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 donâ€™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 Professionalâ„¢ 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 donâ€™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.
>
>
>
> Weâ€™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. Iâ€™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 Issacâ€™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, weâ€™re still fairly conservative with pushing our
> inverters. Iâ€™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 Isaacâ€™s questions: most of our clients wouldnâ€™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 shouldnâ€™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 isnâ€™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. Weâ€™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
>
>
> â€‹Fafco Solarâ€‹
>
>
>
>
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