Can you simplify system with a high Watt to Vmp panel?

Surfpath · June 2012

My proposed Outback Flexmax controller has a 150 VDC maximum. I'd like to connect this to approximately 1200 watts of panel. Most of the panels I have looked at so far are in the 225-230watt, 29Vmp range (6 panels thus total ~175 vdc). Using a string calculator I'd likely need to have 2 parallel strings of 3 panels in series, as well as purchase a combiner box with fuses, additional wiring, installation etc.

Could I not just find a panel with a low enough Vmp to stay below the 150 mark, but yet still have the wattage I am looking for? Say a panel in the 200 watt, 18-20Vmp range?

Two examples of this kind of panel: SUN 210 18.3 Vmp and Evergreen 210, 18.3 Vmp (the latter are discontinued but still available perhaps).

The point is that with a total Vmp of less than 150 I could just have all 6 panels in one simple series in a circuit going directly to the CC. I am a electrical novice, and the thought of simplifying my wiring and installation (all has to be done by me) is appealing.

Here is a youtube link to a video posted by Ian from the Independant Spirit that shows exactly what I mean. He gave me the idea.

Using one of my example panels, the SUN 210, six panels would total 109 Vmp & 1260 watts. The outback could handle that.

Less is more?
-SP

ps. Here's my original system thread, FMI. I plan to repipe my PVC to ensure that the hydro makes up for the rest of my system needs. I also need to check that 1260 watts of panel can drive a 400-450 amp/hr battery bank (in case the stream dries up). Then again I have a 5.5kwr gen.;)

waynefromnscanada · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

I understand what you're saying, and hopefully others can correct me if I'm wrong here - - -
1) Most panels are now "aimed" at grid-tie customers, thus the higher voltages and probably lower price per watt because of higher production.
2) There's nothing wrong with parallel strings of PV, just the added work and expense of a combiner box.
3) Do you live in an area with cold winter temperatures that would drive up panel OC voltages? If so, that must be taken into account when deciding on panel voltages.

Surfpath · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

1) Most panels are now "aimed" at grid-tie customers, thus the higher voltages and probably lower price per watt because of higher production.
Interesting, yes, finding a couple low Vmp, high watt panels took a little while....

2) There's nothing wrong with parallel strings of PV, just the added work and expense of a combiner box.:cool:Still scares me a little.

3) Do you live in an area with cold winter temperatures that would drive up panel OC voltages? Nope, average temps here range from 19(C) at night to 28(C) during the day, all year

vtmaps · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

Surfpath,
Go with the higher voltage panels if they are cheaper or have any other features that you desire.

If you do go with higher voltage panels, do not be afraid of of a combiner box. Buy a Midnite combiner. They come with good directions and are reasonably priced at NAWS.
You can even buy them prewired: http://www.solar-electric.com/misomnsoarco3.html

I recall that you will have a 48 volt system. Unless your cable from combiner to array is very long (more than 150 ft) you will be better off with a Vmp of 75-90 volts. The reason is that the controller is more efficient when the input voltage is not too much higher than the battery voltage.

I recently discussed this topic here: http://forum.solar-electric.com/showthread.php?15907
In that discussion the battery voltage was 24 volts and it was better to have three strings of two (60 volts) than two strings of three (90 volts).

--vtMaps

vtmaps · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

waynefromnscanada wrote: »

Do you live in an area with cold winter temperatures that would drive up panel OC voltages? If so, that must be taken into account when deciding on panel voltages.

Surfpath wrote:

Nope, average temps here range from 19(C) at night to 28(C) during the day, all year

Average temps have nothing to do with it! You need to plan for the RECORD low temperature. Actually, you should plan for a few degrees colder than the record low temperature to account for radiative cooling.

--vtMaps

icarus · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

True,, but I think his is in West Indian islands,, wher record. Old might be 15C!?

I though he was in Wisc.,, until he poste pd his temp averages.

T.

solarvic · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

If you havn,t already bought the outback controler you have another option. Buy a Midnight Classic controler. They come in a 150 200 or 250 Vdc version. Most vendors sell the 150 and 200 vdc for the same price and preety comparable to the outbacks price. The 250 classic is just a little more. Depending on whar panels you buy you might be able to use the classic 200,for 1 string of 6 panels, If not then get the classic 250. :Dsolarvic:D

vtmaps · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

solarvic wrote: »

If you havn,t already bought the outback controler you have another option. Buy a Midnight Classic controler.
<snip>
Depending on what panels you buy you might be able to use the classic 200,for 1 string of 6 panels,

The Midnite Classic is an excellent controller. I would have bought one if it were available when I bought my Flexmax. But its efficiency curves are similar to the Flexmax. Therefore, I would make the same recommendations to lower your Vmp to 75-90 volts if possible. By the way, the reason to aim for higher efficiency is not because you want to get a few more watts out of your system... it is because the watts that are lost are lost in the controller, and that means the controller will run cooler if it runs more efficiently. The chief determinant of how long your equipment will last is the temperature that you run it at. Cooler is better.

--vtMaps

solarix · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

The trade-off is between going higher voltage to simplify the system and reduce wire size and wire loss, and lower voltage to improve conversion efficiency.
The more the MPPT charge controller has to take a high voltage and convert it to a lower one the less efficient it generally is. That is why you won't find many that go above 150V on their input. And when they do they get expensive. On the other hand, as you have found, low voltage arrays have to be paralleled and can really add up to some big wire and the need for a combiner. I too like the Midnite Charge Controllers as a good solution.

solarvic · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

My post was an option for the original poster. He wanted to have just 1 string in his solar array. He has a proposed outback setup so sugested this as another option. More than one way to skin a cat. You can start in the front and go to the back, or start in the back and go to the front or you can unwrap the cat sideways. For all the more it cost (no matter what cc used) I would just get the small combiner box anyway. :Dsolarvic:D

BB. · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

And a note about paralleling in general... Paralleling battery strings is what we normally warn against here. Batteries are designed to maintain "constant voltage" so when placing batteries in parallel, any slight differences in wiring (or even batteries themselves) makes for slight voltage mismatches/losses. That can "steer" current away from one battery and towards another. Causing unbalanced current flow.

With solar panels, they are, for the most part, current sources. Minor differences in voltage seen by the solar panel (wire lengths, etc.) do not change the operating current of the panel very much at all. So, current sources behave quite nicely when connecting in parallel. Everything shares its part of the total current flow (as long as the voltage seen by the panel is around Vmp or less).

However, for safety, there is the added expense of adding fuses/breakers per parallel string connection to prevent a short circuit in a panel/wiring from being powered by the rest of the array (typically, three or more parallel strings each need a series protection fuse).

-Bill

Surfpath · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

Great replies guys,
So it looks that 'Less' (voltage) with a little additional (wiring) is More. I also got a lot from vtmap's post regarding increasing his array.

A couple of related points that support what you all are saying: I designed my solar room to be ventilated, and as cool as possible. But with warm temps here, having a lower incoming voltage to the CC may help it to be more efficient. Also, I designed as low of a cable run as I could (18-20 feet from panel to CC), so voltage drop is not a big issue.

OK, where to put this combiner box: There's a nice sheltered eve on the roof where the combiner box can tuck in, at the location of the PVC conduit that I was hoping to use for the wire run from the panel to the CC (see attached photo). A small box can fit in there, perhaps 16 inches in height. The roof where the eve is located, is now installed, and the PVC just needs cutting down to size.

Attachment not found.

So if I go with 2 strings in parallel (3 panels per string), totalling six 200-235 watt panels at around 26-29 Vmp each, will the MidNite Solar MNPV2-MC4 box do the trick? Seeing that I have decent hydro potential I dont anticipate adding much solar in the future (maybe an additional couple of panels if we expand the family

).

Also....I am sure I can also figure this one out, but I am dragging today - I promised myself no more internet searches.....but what kind of AWG's will my panel to combiner run & combiner to CC run need. My guess is 10 and 8 respectively.

Thanks!
SP

vtmaps · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

Surfpath wrote: »

will the MidNite Solar MNPV2-MC4 box do the trick?

yes. the MNPV2-MC4 is prewired and is designed to accept the MC4 type connectors that are so popular these days. Make sure the panels you choose have MC4 type connectors.
--vtMaps

solarvic · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

Another sugestion to save money. I see what you propose has the high voltage fuse for up to 600 v. usually needed on high voltage strings for gt systems. You can,t shut off the power by pulling the fuse. Depending on how many circuits you want you would be better with the regular combiner boxes and then you can use breakers at less cost. Mnpv6 for 6 circuits $77.94 plus $9.30 per breaker. If you only need 3 circuits you could get the MNPV3 . which has 3 circuits for $63.64. Probably the best might be the Mnpv6 for futere growth. :Dsolarvic:D

vtmaps · June 2012

Re: Can you simplify system with a high Watt to Vmp panel?

solarvic wrote: »

you would be better with the regular combiner boxes and then you can use breakers at less cost.

Surfpath, solarvic is correct. Probably the MNPV3 would be a better choice. You will need only two circuit breakers and you could add a third if you expand. I originally mentioned the MNPV2-MC4 because it is prewired and has the MC4 connectors built in (you mentioned a fear of wiring the combiner). Look at the pictures and Midnite's wiring diagrams... there's very little wiring in them anyway. Buy an MC4 extension cord and cut it in half. Connect the MC4 end of the cord to your panels, and run the cut end of the cord into the MNPV3 box.

--vtMaps

Surfpath · July 2012

Re: Can you simplify system with a high Watt to Vmp panel?

vtmaps wrote: »

Surfpath, solarvic is correct. Probably the MNPV3 would be a better choice. You will need only two circuit breakers and you could add a third if you expand. I originally mentioned the MNPV2-MC4 because it is prewired and has the MC4 connectors built in (you mentioned a fear of wiring the combiner). Look at the pictures and Midnite's wiring diagrams... there's very little wiring in them anyway. Buy an MC4 extension cord and cut it in half. Connect the MC4 end of the cord to your panels, and run the cut end of the cord into the MNPV3 box.

--vtMaps

Hi Solarvic,
Thanks for the reply.

With my new panel configuration (3 parallel strings of three 220watt panels per string), I think I am NOW looking at a combiner like the mnpv3 or mnpv6. The small mnpv3 is appealing because it'll likely tuck in below my eve better than the 6 fuse version (11" vs 14"). The larger one would help with down the road expansion.

So, is this what I get: The box plus Three 150v 15 amp breakers?

-SP

vtmaps · July 2012

Re: Can you simplify system with a high Watt to Vmp panel?

Actually, the latest thing is to get is a combiner with an external disconnect switch. Midnite calls it their 'disco combiner'. The recently revised NEC demands that when the fire department shows up they can just use an external access switch to disconnect the panels.

I have an MNPV3 and it works just fine. If I want to disconnect my panels I must open the box and flip the individual breakers. My advice is to do what you want, unless you are subject to code. --vtMaps

Can you simplify system with a high Watt to Vmp panel?

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