Mitsubishi Panel Questions (sorry, me again)

System · January 2007

Maybe it's all fluff and perfect marketing, but it does seem that from the documentation that the Mitsubishi 165w panels are a pretty good panel. Can anyone confirm or deny?

Anyway, so I'm planning on having two strings of probably 9 panels...each string wired in series, then the two strings wired in parallel to the inverter. Does this make sense or am I way off? (this is my first installation and I'll be the first to admit that the wiring part of the installation is the most confusing part...at least until the inverter, then the main panel tie-in is perfectly clear).

I'm also hazy on the math of the sizing and predicted output. (anyone care to help? I'm a little south of San Francisco). Like, 75% of STC output of panels, multiplied times this or that, etc.

So, the way I see it is that I'll have two strings...each at about 216 volts (9x24v..Vmp), both strings wired in parallel. Each string at around 6.83 amps (Imp)...bringing my voltage and amperage to about 216v/14 amps. Does this make sense?

Another (of the many) thing that confuses me is that I see that the Mitsubishi panels have the MC connectors which appear to be foolproof, but how do they connect in parallel? I'd imagine that there's some other device or connector to make this possible?

My plan is to somehow wire the panels together and into some kind of combiner or junction box under the eave of my house, then into the attic, down into the garage, then into the DC disconnect, inverter, AC disconnect (even though I believe it's no longer required by PG&E...our utility). I don't really understand how the MC cables get combined underneath the panels...and I don't really understand how they're coupled with regular THHN wire that I figured I'd run through the conduit from my junction/combiner box down through the internals of the house to the disconnect/inverter.

Let's face it, I need a lot of help, but I sincerely appreciate all of the help everyone has been providing!

BB. · January 2007

Re: Mitsubishi Panel Questions (sorry, me again)

(This is all assuming you are using a Xantrex GT inverter--if you are using something else--please ignore--other than your assumed Vmpp voltage assumption is probably only valid at 77F--not at normal operating temperatures).

On my Xantrex GT inverter, my two parallel strings where brought down as individual leads and combined right in the wiring box the the Xantrex inverter (terminals were available). The Xantrex manual is available on their website and it is pretty detailed.

I would question your configuration of the panels in two strings of 9--Xantrex's calculator shows that as not sufficient voltage for hot days (95F or higher--assuming panel model is Mitsubishi PV-MF165EB3 (165w)): 10 modules would be at 194.67 VDC at 95 degrees ambient and 11 modules would be about 214 VDC (at 95F).

http://www.xantrex.com/support/gtsizing/index.asp?lang=eng

The calculator shows that you would support a minimum of 11 panels in a string (to a maximum of 17-18 modules (assuming 32F is the coldest weather with full sun)...

So, I would either recommend that you have at least two strings of 11 panels or one string with with a maximum of 17-18 panels. Xantrex has several models of GT inverters--ranging up to 3.3 kW peak (same electronics, just firmware set to allow Xantrex different price points).

There are choices about whether you configure the panels to run near 195 vdc (unit is most efficient, current is maximum in wires so most I^2R losses) or if you choose to run the panels near 600vdc (maximum inverter voltage before damage occurs/exceed UL 600VDC rating of inverter/wiring)...

I will tell you that the Vmp voltages are pretty accurate--they are pretty close to what is predicted for my configuration (2 strings of 10 each, BP 4175 panels).

If you are set on 18 panels, I would probably recommend that set them up as one string of 18--I assume that you are on around Black Mountain (near 280 and 92). The inverter draws around 0.2 amps even when it is in standby--so I don't think you will see below 32F and 600 volts unless we have global cooling (you could always switch the Xantrex off with the AC/DC switch on the unit).

Or, if you want to be a bit more conservative, drop it to 17 panels maximum in the string.

Regarding the MIC--my installer just had a kit of terminals (male and female) and a crimping tool. The UV stable wire was brought to a junction box on the roof (MIC or Wire Nuts--don't know) and both strings where brought down separately down to my GT inverter using 10 awg THHN with a ground (or something similar) inside of metal conduit.

Other than series/parallel/number of panels--the wiring should be done using best practices for an electrician--there was not anything abnormal that I saw (I am an engineer--not an electrician).

I would recommend that, even if you don't buy a Xantrex inverter, that you play with their string calculator. Solar PV Panels are far from ideal "solar batteries" and their voltage is quite variable based on ambient temperature and solar heating effects. That nine panel string voltage could range from about 175 VDC when hot (95F) to almost 297 VDC when cold (32F) or more (if full sun, inverter disconnected and sub freezing weather).

-Bill

Roderick · January 2007

Re: Mitsubishi Panel Questions (sorry, me again)

If you're going with 2 strings of 9, there's probably another inverter with a lower input voltage requirement that will work well. I have a PVP2800-XV with that config, PV-MF165EB3, and it works.

BB. · January 2007

Re: Mitsubishi Panel Questions (sorry, me again)

Yes, the 2800 watt version of the PV operates down to 170 VDC (500 VDC max):

http://store.solar-electric.com/pvpogrtiein.html
http://www.pvpowered.com/

The larger ones require from 205 vdc to 240 vdc (depending on model).

It all depends...

-Bill

System2 · January 2007

Re: Mitsubishi Panel Questions (sorry, me again)

Hi, thank you for your responses.

It seems like there's just so much to learn, and other than forums and overly-general books, there's no place to really learn this stuff.

For example, how do you decide which starting voltage your inverter selection should have? What I can tell you about my location is that over winter it slips below freezing maybe 15 nights per year, but barely, always warms up to 45F+, and in the summer it rarely goes over 100 and is usually in the 70s and 80s, dropping into the 60s at night.

I'm playing with the Xantrex sizing calculator but I'm not even sure how to read this thing. It seems like too many of the wattages exceed the various max's.

I have the Mitsubishi 165w panel, Xantrex GT3.3 240v inverter, 32f low, 104f high (I assume these are supposed to be relatively true all-time high temps? Or averages? I dont get it). It says min # of modules is 10, max is 18...and max of 2 strings. The rest of it doesn't make any sense to me. Has anyone ever seen a good explanation of the math of sizing inverters and deciding how to wire them? I don't know why I'm not grasping this!

mike95490 · January 2007

Re: Mitsubishi Panel Questions (sorry, me again)

re: Xantrex sizing calculator

It seems overly aggressive to me, I've never hit the max it calls out, I could easily added more panels and still be safe. If your panels will not be optimally aimed, you will fall short of the calculator's values.

BB. · January 2007

Re: Mitsubishi Panel Questions (sorry, me again)

The Xantrex numbers are pretty accurate from my experience... Notice that you have STC rating (solar panel mfg. ratings that are really only realistically valid in quite cold weather). And you have PTC rating (California's requirement for rebate--more reasonable--but still a bit on the high side); and finally CEC rating (PTC rating * efficiency of inverter--value for which California will calculate the actual rebate).

From my experience (in San Mateo), my system is rated at CEC of 2,933 watts. I have seen 2,999 watts for a few minutes a couple of times--but mostly 2,500-2,600 watts max (or less) day to day (manual readings, not via computer interface).

So, choose how many panels you want to pay for. Choose the inverter(s) for the power you want. Check the inverter's voltage and current requirements and (for best inverter efficiency) pick a string size/configuration that gives you the lowest voltage (at highest temperature for your area--although, if you are worried about shading, raising the minimum voltage with keep the inverter from dropping off-line with a bit of shade).

The high voltage calculation is the worst case (cold temp, full sun, inverter either just manually switched on or in anti-islanding mode). Design your system so that you would not exceed your inverter's (and NEC) maximum voltage and there will be no chance of damage to your inverter--some controllers actually store the maximum voltage for warranty claims--Outback MX60 does this).

My system runs around 285-320 VDC (from calculator--32F-95F, 285 VDC minimum Vmpp, 480 VDC max--open circuit voltage).

I believe that different inverters behave differently when supplied with more power than they can converter... Some will shut down or substantially cut back on power output, others will simply limit power to their rated maximum--assuming the inverters do not over-temp (Xantrex GT, IIRC, simply just limits to its nameplate rating). And California will not give rebates based on only solar PV panel ratings--they will only pay for the maximum for panels*inverter-efficiency, up to the inverter's rating.

Depending on the physics of the inverter/panel combinations, you will find that there are certain combinations that cannot be supported... For example using your:Mitsubishi PV-MF165EB3 panels and a GT 3.0 inverter, 32F to 95F:

Single String:
1-9 panels (minimum voltage not supported for temperature range)
10 panels (minimum voltage below inverter requirement of 195 VDC at 95F+)
11-16 panels (everything within range, less than inverter's maximum wattage rating)
17 panels (on 32F day, MPPT feature not supported--you may not get optimum power until panels warm up a bit)
18 panels (same as 17, but now pretty close to maximum safe voltage--probably not a problem--but drop panel to 17 if worried)
19 or more (not recommended or safe).

Two Strings:
10+10=20 panels (minimum voltage not supported for temperature range)
11+11=22 panels (close to maximum CEC rebate value--if not 54 watts over)
12+12=24 panels (exceeds CEC rebate levels for inverter capacity by ~300 watts)
13+13=26 panels (exceeds CEC rebate levels for inverter capacity by ~600 watts
etc..

So, you realistic number of panels that can be supported is 11-17 (or 18), or 22 panels for the GT 3.0 inverter... You can support up to 24 (or even reasonably 26 panels if you toss about 300 watts or $750 in CEC rebates) modules if you use the GT 3.3 inverter.

I would probably feel very comfortable with 12-17 modules, or get the 3.3 kW inverter and run 24-26 panels.

The inverter will run fine with 11 (or 22) panels--but if you are having issues with shadows, I would not use those configurations as they would probably be more likely to cut out generation on warm days during the period of shadows shadows (the extra panel(s) will give you more overhead to generate power if one panel is shadowed--without the extra panel, the inverter will shut down until the shadow has passed).

Does the above make sense? Again, I am not a Xantrex sales person--it is just the unit I ended up with and know most about.

-Bill

System2 · January 2007

Re: Mitsubishi Panel Questions (sorry, me again)

It's going to take me a while to digest all of that, but one question right off the bat that I have is that it seems like there's a max on the rebates? The sizing calculator shows astericks where the wattage exceeds the rebate, or something.

I'm so damn confused it's actually starting to irritate me. Obviously I should hire someone to do all of this for me, but I also believe that if you want something done right (like drilling holes into my roof), you have to do it yourself!

System2 · January 2007

Re: Mitsubishi Panel Questions (sorry, me again)

I'm shooting for 18 panels. So, as I see it, 165w X 18 = 2970w. Does this mean there's no reason to really go for an inverter over 3kw? I'd probably want to oversize it a bit in case I add more panels later. So in the sizing tool I selected the Xantrex GT3.3 (240v).

Am I reading this correctly (this page is extremely hard for a newbie to understand), where for 18 panels my options are either 1 string of 18 panels (which shows 594.53 max volts/max temp...getting close to the inverter max of 600Vdc), or add more panels and make it two strings of at least 11 panels each (in order to get the min Vmp above the 195Vdc inverter minimum)? In this scenario, is there any way that the 18 panels could realistically produce more than 594.53 watts if wired in one string?

I guess this is starting to make sense. I still don't understand the relationship between temperatures and voltage/current, etc.

BB. · January 2007

Re: Mitsubishi Panel Questions (sorry, me again)

Yes, you are starting to understand the various issues (voltage, current, inverter, etc.)...

There is a limit on the CEC rebate (something like 10kW for a home--don't remember the exact rules)... But the *** Limit you are seeing is that they will not pay you for (example) 4kWatts of panels on a 3kWatt maximum inverter (they will only pay you for the 3,000 watts CEC). They will also not pay you for a 4kW inverter with only 2,000 watts of panels (they will only pay you for the 2,000 watts PTC * inverter efficiency).

The CEC requirements for payment is, basically, the PTC rating of the panels * inverter efficiency (say 0.95) and that value cannot exceed the nameplate output of the inverter...

Regarding the 594 VDC max @ 32F -- probably not in our area (would have to be full sun on a <32F day)... However, I would ask the question of Solar Guppy--he is one of the electrical engineering designers of the Xantrex GT series inverters. If we get snow forecasted with clear weather (once about every ten-twenty years), we can always turn off the master disconnect until the panels/weather warm up.

--never mind, looks like you have just asked the question... Interesting to see what his answer will be.

The temperature issue is that solar PV panel's voltage is inversely proportional to the temperature. Hot panel, low voltage. Very cold panel, much higher voltage. So, the first issue is that you want the panels to (ideally) supply power on both cold and hot days for your area (panel voltage range and inverter/solar controller voltage ranges overlap).

Realistically, on cold/clear winter days and fresh snow reflecting light on the panels too (and sun low on the horizon), you can see as much (or more) peak power generated than on a very hot summer day with the panel facing the sun directly. Of course, you will generate more energy during the summer because of longer daylight and (usually) better weather.

In the end, if you have the money, I would probably install the panels, wiring, and inverter with the final capacity now (if that is 22 panels) rather than play around later to change from 18 panels in series to two times 11 panels in series parallel. The pain of adding the extra down to the inverter and panels/mounts on the roof plus the extra labor just seems not be worth the temporary savings (plus, it is possible that there will not be any, or as large, CEC rebates later down the road).

-Bill

Roderick · January 2007

Re: Mitsubishi Panel Questions (sorry, me again)

It's clear to me from the spec sheet that the GT3.3 will tolerate 594V on the input, but if I were taking this path, I would want someone to assure me that the inverter will also put out power under such conditions (that is, it won't turn off for "voltage out of range").

In the mild winters we have here, I have never seen the Mitsubishi 165W panels put out even the 30.4V open-circuit voltage, (which is supposed to be at 25 C), even in the mornings during the cold snap a few weeks ago, when we had nights in the 20's (F). With 18 in series, I would expect I would expect the highest open circuit voltage to actually be about 530 volts, and you'd see about 440 volts as an operating point on cold mornings, down to about 400 or so when it gets warmer. Of course, you must design to spec, but what I'm trying to say is there should be no fear of actually exceeding 600 volts in practice, and that the operating point for your array would be well within bounds for your inverter.

The 18 panels in my setup level off at 2500 watts AC around noon on a clear day. They will briefly go to nearly 2900 watts when the sun has just peeked out from behind a cloud. So they will not come close to filling the capacity of your inverter.

And, as you were probably thinking, if you get more panels later, you could reconfigure for 2 strings of 11. Hey, maybe panels will be cheaper in a few years, beating a degradation in rebate.

Mitsubishi Panel Questions (sorry, me again)

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