Balancing Inverter Size and Amount of Solar Panels

[JSchnee21]

Welcome Zeigh !

Did anyone actually answer your question? I'm skimmed through most of the responses, but didn't see a clear answer.

First let me ask, are you planning to do a line side tap? If you are not familiar, there are basically two different ways to connect your inverter to you house wiring. Either between the utilities (PoCo's) meter and the input breaker for your load center / panel board / breaker box (this is a "line side" tap). OR as one or more "load side" circuit breakers in your existing breaker panel (or a sub panel).

For larger installs "line side" taps are generally required and preferred, But some jurisdictions limit these. I have a line side tap for my 12kW system. While load side taps can be less expensive from an electrical wiring perspective, often installs are limited to ~7600W due to limitations in the bus bar current ratings in the panel board. Unless you have a large 400 amp panel -- which is rare.

Do you have normal USA residential wiring? 240 VAC Split-phase? 200amps?

Many / most residential jurisdictions require rapid shutdown (not sure if you're does or not) for residential roof mounted systems. Which inverter make/model are you looking at? And does it have a rapid shutdown capability, and if so with which panel level modules?

Solar Edge products (and to a lesser extent Enphase productions) have become very popular for these reasons due to panel level MPPT capabilities, panel level rapid shutdown, and enhanced monitoring and diagnostic capabilities. Other solutions exist which can be paired with conventional string inverters. But these are less common, nearly as expensive as Solar Edge, and often involve combining "dissimilar parts" from multiple manufacturers.

Different inverter MFG will "support" different degrees of over subscription before their warranties are voided. That is to say different DC to AC rations exceeding 1.0. There are a number of factors that come into play when deciding between a DC to AC ratio of 1.0 (aka no over subscription) to factors up to 1.3 or so. In most cases factors above 1.3 are not recommended, unless you have multiple roof faces, severe shading, or other factors which will strongly limit productivity or space it out through the day.

I'm not sure how you figured your 71.1% efficiency number. Perhaps here (www.solarpaneltilt.com/). But I think it will have much less of a negative impact than you are projecting.

I've never lived in AZ, but from my perspective (in NJ). It seems hot, dry, and very sunny most of the year. Given your Southern orientation, near perfect pitch, and likely solar irradiance values in excess of 1000W / Meter Squared over several hours per day . . .

https://www.nrel.gov/gis/solar.html

I would expect that your panels will meet or exceed STC output wattages for at least a few hours per day during the cooler and sunnier parts of the year. As Mike90250, Ampster, and others have pointed out, as the temperature of the array increases it's productivity will decreased depending on the thermal coefficient of the panels you have selected. That is to say, throughout the day as your panels and roof warm up in the hot AZ sun, production for a given level of solar irradiance will be less when the system is hot than when it is cool(er).

But this is usually a fairly modest effect, unless your panels have a poor coefficient and your temperatures are very, very hot. Most panel thermal coefficients range from -0.75% /

[Zeigh]

Thanks to all that responded with some great information. At this point, I believe that my original grid-tie system design should be sufficient and will include the following:

1- SolarEdge SE11400A-US Single-Phase Inverter (line-side tap)
40- Solaria PowerXT 350R-PD 350W Solar Panels (second story roof mounted, direct southern exposure)

If there are no shading issues to contend with (other than a couple of vent pipes), are optimizers even worth the trouble/money? Otherwise, let me add that my location is at 5500 feet and currently under three feet of snow, so the year-around solar panel heat factor is far from the stereotypical view of a desert state like Arizona.


Peace,
Dr. Z.

[inetdog]

Vent pipes count, and can severely reduce the panel's output even if the shadows do not cover even one full cell. They have the potential to cost you anywhere from 1/3 to the entire panel output, depending on exactly where they fall.

[solar pete]

If there are no shading issues to contend with (other than a couple of vent pipes), are optimizers even worth the trouble/money? Otherwise, let me add that my location is at 5500 feet and currently under three feet of snow, so the year-around solar panel heat factor is far from the stereotypical view of a desert state like Arizona.


Peace,
Dr. Z.

Hi Zeigh, vent pipes are bastards for causing hot spots, it takes anywhere from 3 to 8 years fior them to pop up, but they will pop up eventually. We avoid vent pipes like the plauge, if you have to put a panel where a vent pipe or chimney or anything else for that matter you need to use an optimiser, we use tigo's when you cant avoid it, cheers.

[J.P.M.]

Hi Zeigh, vent pipes are bastards for causing hot spots, it takes anywhere from 3 to 8 years fior them to pop up, but they will pop up eventually. We avoid vent pipes like the plauge, if you have to put a panel where a vent pipe ir chimney or anything else for that matter you need to use an optimiser, we use tigo's when you cant avoid it, cheers.

Hot spots ? Are you referring to placing a panel over a roof vent ? I took Inetdog's comment to refer to the loss of performance when a vent pipe shades a panel. Can you elaborate on your use of the term "hot spot" ?

[solar pete]

Howdy, no, not over a vent pipe but close enough that the vent pipe, casts shade onto a panel in the same spot at the same time, day after day, after many years of this the panel will develop a hot spot. I had one recently, on a 10 year old install our client noticed a burn mark on a panel and it turned out to be a hot spot caused by us putting the panel too close to the vent pipe, you live and you learn, as we dont do that anymore, unless we use a tigo. P.S a shout out to Suntech who found me an old 190 watt Suntech panel and replaced it under warranty even though they didint have too, nice work Suntech.

Here is a link that goes into the technical reasons, http://www.dupont.com/content/dam/du...mitigation.pdf

[Mike90250]

When a shadow is on a PV panel. that shaded area produces no power, and in fact becomes a resistor to the power the REST of the panel is pushing. That creates a hot spot where the shade is, and also where an internal bypass diode is. Even a shadow the size of a pencil is bad.

[J.P.M.]

When a shadow is on a PV panel. that shaded area produces no power, and in fact becomes a resistor to the power the REST of the panel is pushing. That creates a hot spot where the shade is, and also where an internal bypass diode is. Even a shadow the size of a pencil is bad.

I understand what you're referring to and understand the reasons and theory but haven't been able to observe that phenomenon by measuring my array's temperatures. Long story. Refer to my description of measuring panel temps. from prior postings. In that regard ( measuring panel temps), I did similar for partial shading in afternoon shadings that occuer every sunny day for me and observed temps of shaded portions of panels to be less than the unshaded portion of the same panel generally in a way that seemed to make sense with a local energy balance on the shaded vs. the unshaded areas of the panel. That is, the shaded sections local temp. was always lower than the unshaded sections local temp. I guessed that the shading, which is minor and all late afternoon and on the west side of the array may be of short enough duration and at a time when the total POA irradiance on the unshaded portions of a panel, even on clear days is, at that time of day, getting close (enough) to the level of the diffuse irradiance (which is close to what the shaded portions see), making current in the shaded and unshaded cells in series closer together and lowers the reverse bias across the shaded cells, thus lowering the energy dissipation and so lower heating. But that's not something I've been able to figure out enough to talk intelligently about (yet). Nor have I investigated local panel temps. at or near a bypass diode.

Now you went and made more work for me.

[inetdog]

I think what Solar Pete may be referring to is the local heating that can occur when a partially shaded cell has the full current of the rest of the panel/string forced through it. This dissipates heat locally inside the semiconductor junction area of the cell and might, over the long term, result in permanent damage to the cell. Note that bypass diodes are designed to limit the forcing of current in the forward direction through under-illuminated cells, but kick in best when more than just a portion of one cell is shaded.
I suppose that a small, sharp edged shadow could also cause uneven heating of the area of a cell which will increase the thermal expansion stress within the silicon material. That might also lead to long term damage.

[NiHaoMike]

Hello,

How do I find the best balance between inverter size and amount of solar panels?

First, here are the basics of my system aimed at 100% coverage in a grid-tie setup. My average monthly usage is 2393 kWh, so I am sizing my solar electric system to a 14400 watt inverter severed by forty 350 watt solar panels. All of my solar panels will be installed upon a south-facing 6/12 roof (26.6% angle). The optimum mounting angle for solar panels in a fixed system year around in northern Arizona is 29.7% and provides an average of 71.1% efficiency.

So, that means that my intended system would only be pumping out 9954 watts of the 14000 watts maximum. 56 solar panels would be needed to make up for this loss and serve the maximum capacity of the inverter flexible solar panels. Of course all these numbers are averages, so how do I calculate the best balance of solar panels to include? There is allot of solar electric research information out there, but apparently I can't seem to hone in on this particular detail (or give my contact information to be spammed to death).


Peace,
Dr. Z.

We got our system turned on two days back. It is supposed to be 12 KW, and 30 panels. The park we see is about 10kw. Initially, we figured we would wait a few days, but also thought will check the inverters. of the two inverters one is rated at 7.6 and other is 3.8. That is total of 11.4 - less than 12 that we paid for or what the panels would theoretically generate. Did they shortchange us? Do we need to address / escalate the issue. Also, I thought there was a way by which Thru our app, we should be able to see which panels are producing what. We have three sets - two sets with 7 each, and one with 16) No power wall. Thoughts ?

[SunEagle]

We got our system turned on two days back. It is supposed to be 12 KW, and 30 panels. The park we see is about 10kw. Initially, we figured we would wait a few days, but also thought will check the inverters. of the two inverters one is rated at 7.6 and other is 3.8. That is total of 11.4 - less than 12 that we paid for or what the panels would theoretically generate. Did they shortchange us? Do we need to address / escalate the issue. Also, I thought there was a way by which Thru our app, we should be able to see which panels are producing what. We have three sets - two sets with 7 each, and one with 16) No power wall. Thoughts ?

I would think that your contract would include the specific hardware that is supposed to be installed. A contractor should not change anything unless they get written approval from their customer. So I would check the proposal and see if you are really getting a 12kw system or something else.

[J.P.M.]

We got our system turned on two days back. It is supposed to be 12 KW, and 30 panels. The park we see is about 10kw. Initially, we figured we would wait a few days, but also thought will check the inverters. of the two inverters one is rated at 7.6 and other is 3.8. That is total of 11.4 - less than 12 that we paid for or what the panels would theoretically generate. Did they shortchange us? Do we need to address / escalate the issue. Also, I thought there was a way by which Thru our app, we should be able to see which panels are producing what. We have three sets - two sets with 7 each, and one with 16) No power wall. Thoughts ?

Without more info it reads to me like the setup is a bit different - not much different and not necessarily bad - just not what's commonly seen.

Assuming a typo ("peak" ?), as for a system maximum instantaneous output of 10kW, depending on the system orientation, season, time of day and other things, the common max. instantaneous output a system will produce is probably something like about 0.80 to maybe 0.86 or so of the system's STC rating.

10 kW/12 STC kW = 0.88 kW/STC kW. That doesn't look too bad to me.

Before you escalate the issue you think you may have, I'd respectfully suggest you educate yourself on the basics of what you bought.
Otherwise, you may well embarrass yourself and waste the installer's time.

Take what you want of the above. Scrap the rest.

[bcroe]

It is supposed to be 12 KW, and 30 panels. The peak we see is about 10kw. of the two inverters one is rated at 7.6 and other is 3.8. That is total of 11.4 - less than 12 that we paid for or what the panels would theoretically generate. We have three sets - two sets with 7 each, and one with 16)

My 2 inverter system is on somewhat the same scale. When first operated,
I observed certain times when one inverter was saturated, in clipping, but
the other was not. Available energy was not being captured. After some
study I was able to re arrange some wiring, reconfigure so both inverters
would hit clipping at the same time.

You may have a similar problem. I calculate the capacity of your 3 sets
are 2.8KW, 2.8KW, and 6.4KW. I do not see a way to connect those to
your inverters with an equal percentage of loading. If so, one of the
inverters is likely going into clipping early and losing energy, while the
other is less loaded. Perhaps the panels could be better arranged to
match up, depending on the inverter inputs. Otherwise increasing the
size of one inverter might be best, the 3.8 might better be a 5.5. Ill
even wager the 3.8 was some surplus they were trying to use up.
good luck, Bruce Roe

[OCJ]

We got our system turned on two days back. It is supposed to be 12 KW, and 30 panels. The park we see is about 10kw. Initially, we figured we would wait a few days, but also thought will check the inverters. of the two inverters one is rated at 7.6 and other is 3.8. That is total of 11.4 - less than 12 that we paid for or what the panels would theoretically generate. Did they shortchange us? Do we need to address / escalate the issue. Also, I thought there was a way by which Thru our app, we should be able to see which panels are producing what. We have three sets - two sets with 7 each, and one with 16) No power wall. Thoughts ?

You have a Tesla or SolarEdge system? Before you go opening frivolous tickets perhaps you need to understand the concept of DC/AC ratios and clipping. And if you wanted to see per panel production then you definitely should have done more pre-installation research because you're on string inverter and without optimizers you're not getting panel level production numbers.

The DC / AC ratio is DC divided by AC, so on your overall ratio is 12 / 11.4 or 1.05 ratio, which is fine unless you're sitting on the equator with your panels facing straight up, and even then it's probably mostly fine. Now, how they are wired into each inverter may be a different ratio, but how each ratio effects clipping is going to be a function of your location and panel direction. That is, SoCal through the Southern states to Florida if your panels are South facing then they will clip at a lower DC to AC ratio.

PVWatts can be real helpful here for figuring out what potential clipping losses may be. If you're Tesla you may also need your Plan Set so you can see how many panels are on each inverter.

Clipping is when your panels produce more than your inverter can convert. It may start as early as Feb if you have a high DC/AC ratio and your panels are perfectly South and the weather is cool. As the year goes on and we pass summer solstice and panels heat up, clipping will eventually go away. Keep in mind while it looks ugly and creates a flat top on your solar curve, the actual amount of kWh up there may be only a few percent when annualized. So the trade-off here is the cost of additional inverter vs that lost production, also keeping in mind panels degrade as much as 2% in the first year and 0.25% - 0.5% in subsequent years.

I realized I just threw a lot of info here at you but these are the things you need to understand to speak intelligently to your installer about being shortchanged and requesting escalations.

[Ampster]

I agree with OCJ that a DC to AC ratio of 1.05 is fairly conservative. Also the philosophy of bcroe is worth mentioning. Optimal inverter utilization is often of greater economic benefit that having a perfect match of panel size to inverter capacity because panels rarely put out their rated output and they degrade over time.