# Panel Voltage Decision.

Solar Expert Posts: 180 ✭✭✭
I am having 3 custom sized panels made for an RV application. The theory is to use a Morningstar Tristar TS-45 MPPT (or the 60). The question is is there any benefit, other than wiring harness losses, to using a higher voltage panel? i.e. 28V or 36V

Specs of the panels are below:

Thanks,

Rick

100W=
Peak Power(W) 100
Open Circuit Voltage (V) 32.76±1
Short Circuit Current (A) 3.82±0.2
Max. Power Voltage (V) 28±1
Max. Power Current (A) 3.57±0.25
Max. Rated System Voltage (V) 1000
Working Temperature -40c——90c
Dimension (mm) A×B×C 990×800MM
Weight 5kg

140W=
Peak Power(W) 140
Open Circuit Voltage (V) 32.76±1
Short Circuit Current (A) 5.35±0.2
Max. Power Voltage (V) 28±1
Max. Power Current (A) 5.0±0.2
Max. Rated System Voltage (V) 600
Working Temperature -40C——90?
Dimension (mm) A×B×C 1600*680MM
Weight 7kg

170W=
Peak Power(W) 140
Open Circuit Voltage (V) 32.76±1
Short Circuit Current (A) 6.5±0.2
Max. Power Voltage (V) 28±1
Max. Power Current (A) 6.07±0.25
Max. Rated System Voltage (V) 1000
Working Temperature -40C——90C
Dimension (mm) A×B×C 1956*660MM (77.02"x25.9")
Weight 8.5kg

36V

100W=
Peak Power(W) 100
Open Circuit Voltage (V) 42.12±1
Short Circuit Current (A) 2.97±0.2
Max. Power Voltage (V) 36±1
Max. Power Current (A) 2.78±0.25
Max. Rated System Voltage (V) 1000
Working Temperature -40?——90?
Dimension (mm) A×B×C 990×800MM
Weight 5kg

140W=
Peak Power(W) 140
Open Circuit Voltage (V) 42.12±1
Short Circuit Current (A) 4.16±0.2
Max. Power Voltage (V) 36±1
Max. Power Current (A) 3.89±0.2
Max. Rated System Voltage (V) 600
Working Temperature -40?——90?
Dimension (mm) A×B×C 1600*680MM
Weight 7kg

170W=
Peak Power(W) 140
Open Circuit Voltage (V) 42.12±1
Short Circuit Current (A) 4.16±0.2
Max. Power Voltage (V) 36±1
Max. Power Current (A) 3.89±0.2
Max. Rated System Voltage (V) 600
Working Temperature -40?——90?
Dimension (mm) A×B×C 1956*660MM
Weight 8.5kg

• Banned Posts: 17,615 ✭✭
Re: Panel Voltage Decision.

Using an MPPT controller there's not going to be much difference between a 28 Volt and a 36 Volt panel, assuming Wattage remains the same. There is a tiny advantage to higher Voltage for less line loss as you know, and perhaps a small gain for lower light levels or higher temperatures.

On the whole, I don't see a big advantage of one over the other. Perhaps someone else does?
• Solar Expert Posts: 180 ✭✭✭
Re: Panel Voltage Decision.

I just read in another, that for best efficiency an MPPT controller should be at 2*Battery voltage. I am using 12v (nominal) battery bank storage, wouldn't 28V panels be best?

I would like to make the entire system more efficient in lower light levels, so 36V?

Help! I only get one shot to have these panels made.

Rick
Re: Panel Voltage Decision.

I would not get too wrapped up in the optimum Vmp issue for the MPPT charge controllers. You can look in the manual--many include some version efficiency charts.

The MorningStar MPPT you are looking at is one of the most efficient--And a good choice overall.

Do you really need the custom sized panels to fit your roof (I am guessing)? How much per watt are you spending? And these are full vacuum sealed/20+ year life tempered / low iron glass panels with bypass diodes like the normal commercial panels?

As an FYI--there is one spot with Vmp in the ~50-70 volt per panel range and a 48 volt battery bank. One panel is too low of voltage to charge a 48 volt battery bank and two panels in series can be too high of Voc for the charge controller input.

I don't think this is an issue for your system (12 volt volt battery bank?).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 180 ✭✭✭
Re: Panel Voltage Decision.

Thanks for the amazingly quick reply!

Yes, the original concern was roof real estate and shadowing. When I began to explore the types of available panels there really wasn't anything usable that would fit. Also, conventional panels are quite heavy, my very small 24'"8" RV with a ~20 ft available roof area wasn't going to like all of extra weight on the roof.

These panels are semi-flexible, 3mm thick and weigh (for the 170w) about 6kg.

Seems like they are a better solution for RV roofs.

The manufacturer wants to build them as 36v panels, but will do 28v if I wish.

The cost will hover around \$3 per watt for samples. Lower for a full production run.

Rick
Re: Panel Voltage Decision.

I would pick whatever voltage that works for your system and is "standard" for their production--Fewer things to go wrong and may be easier to replace if something happens to one of your panels.

By the way, are these crystalline panels or amorphous solar panels? Typically amorphous panels need to be physically larger because they are less efficient than crystalline panels (roughly amorphous panels are 9% or less vs ~13-18% overall efficiency for crystalline panels).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 180 ✭✭✭
Re: Panel Voltage Decision.

OK 36 Volts it is!

They are mono-crystalline panels. About the same efficiency as the non-flexible types.

Rick
Re: Panel Voltage Decision.

Sounds like a great deal!

Check their warranty out--Flexible panels (many times?) don't have as long as life/warranty as glass fronted panels. Make sure they and their wiring a mounted solidly (don't want flexing wire to cause a water/air leak into the panel).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 180 ✭✭✭
Re: Panel Voltage Decision.

Bill,
The warranty is a bot shorter for full power, "only" 10 years. As for mounting, the junction boxes are custom placed for me at various positions and my longest open air wire run will be about 20". The panels themselves are designed to be affixed with a silicon adhesive spread evenly across the back. My roof is plain gelcoated fiberglass, so after prep, it should adhere quite nicely. I will then spread a bead of silicone around the edge and shape it smooth, jst in case.

Rick