Solar Panel Sizing

[JerryS]

We have three (3) solar panels on the roof of our MH and I am trying to figure out the sizing in terms of wattage capability. The panels are each 47"x21" and have 36 cells. Does anyone know how I can determine what their output should be?

 

[JimG]

The output of solar panels depend on three things; the amount of sunlight hitting the panel, the size of the panel, and the efficiency of the solar cells inside. Typical panel are about 200 watts each. And of course if you know the voltage and current output, you can calculate your actual wattage (P=I*E)

 

[JerryS]

After some research, it looks like the panels are 100watt, so, I have a total of 300watts available under full sun. Therefore, I can use a 25 amp controller/charger.

Optimum Operating Voltage (Vmp):	18.9 V
Optimum Operating Current (Imp):	5.29 A
Open - Circuit Voltage (Voc):	22.5 V
Short- Circuit Current (Isc):	5,75 A
Maximum Power at STC:	100 W
Operating Module Temperature:	-40°C to + 90°C
Maximum System Voltage:	715 VDC
Maximum Series Fuse Rating:	15A

STC: Irradiance 1000 W/m[SUP]2[/SUP], module temperature 25°C, AM=1.5;
Mechanical Specifications

Solar Cell:	Mono-crystalline 125 x 125 mm (5 inches)
No. of cells:	36 (4 x 9)
Dimensions:	47 x 21.3 x 1.4 in (1195 x 541 x 35 mm)
Weight:	16.5 lbs (7.5 kg)
Front Glass:	3.2 mm (.13 in) tempered glass
Frame:	Anodized aluminum alloy
Junction Box:	IP65 rated,
Output Cables:	4.0 mm[SUP]2[/SUP] (.006 in[SUP]2[/SUP]), 600mm(23.6inches)
Connector	MC4

 

[kaz911]

Is the label not on the back side? or inside the connector box on the back? Its hard to tell from the surface how they are wired.

But they look like mono crystaline panels (best efficiency) - if that is the case they will have a VOC (Volt Open Circuit) of about 16-18 volt. I can't tell from your picture if they are wired in parallel or serial. Most times you would wire in serial to keep current down and get voltage up - so final output would be 3 x VOC. That also means you can use thinner cable to the charge controller.

If you use a "non MPPT" charge controller - you will "loose" the power from VOC -> Battery charge voltage (so 17v-14 volt = 3 volt) will be ''lost" (very simplified). If you use a MPPT controller - then you will gain from the higher voltage.


But I would say dependent on age they are most likely 85-100 watt panels. Efficiency has gone from 12.5% on old panels to about 21% on the latest most expensive panels. Efficiency is calculated based on 1000 watt / sqm. Since your panel is about 0.64 sqm - then the calculation is simple

0.64 sqm * Efficiency in pct * 1000 = total max watt output

So with 12.5% = 0.64 * 0.125 * 1000 = 79.59 watt
And with 15 % = 0.64 * 0.150 * 1000 = 89.14 watt
And with 17.5% = 0.64 * 0.175 * 1000 = 111.43 watt.

The Voltage output is very dependent on manufacture and if they are meant to be connected in serial or only parallel. Good panels will allow up to 600 volt running through them so you can serial connect a lot of panels. Cheaper ones are <70 volt - and crap panels can't be connected in serial at all.

And remember the watt output is calculated based NOT on 12-14 volt - but at the best performance voltage of the panel - so that might be 16 to 18 volt - and again - mostly only MPPT charge controllers can use that extra "voltage" above your battery charge level.

The above is again - very simplified.

But if you want a good charge controller - then IMHO there is only one choice - MorningStar TriStar MPPT 45 It is expensive - but will give you a lot of value for the money - it is MPPT so it can use the "extra voltage" - and the current generation has Ethernet connection with little web server - so you get your "daily" production statistics. The TriStar is installed on MANY RV's in the US. Another choice is Outback Charge controllers - but I have not tried them. I use the SunSaver Duo's on my boats - but they might be just a tad to small for about 300w.

http://www.morningstarcorp.com/en/tristar mppt

 

[JerryS]

Good info, Kaz. The panels are in parallel, so, it looks like 10 ga wiring coming from them. They are mounted on roof, so, I could not see any info. On a recent sunny day, I was getting 19.9 VOC.
I am looking at a MPPT controller with PWM.
I am looking specifically at the Blue Sky Energy SB2000E 25amp, 12v operation. Do you think that I would be better off going with something like the Morningstar TS-MPPT-45 which is a 45 amp controller?

300 watts at 13.8 volts would be 21.73 amps. Probably be actually getting closer to the knee efficiency of 81 watts per panel.

Don't ask why the Zantrex Battery Monitor is connected to the solar panel output, but, there is no controller. I am still scratching my head over that one.

 

[kaz911]

Good info, Kaz. The panels are in parallel, so, it looks like 10 ga wiring coming from them. They are mounted on roof, so, I could not see any info. On a recent sunny day, I was getting 19.9 VOC.
I am looking at a MPPT controller with PWM.
I am looking specifically at the Blue Sky Energy SB2000E 25amp, 12v operation. Do you think that I would be better off going with something like the Morningstar TS-MPPT-45 which is a 45 amp controller?

300 watts at 13.8 volts would be 21.73 amps. Probably be actually getting closer to the knee efficiency of 81 watts per panel.

Don't ask why the Zantrex Battery Monitor is connected to the solar panel output, but, there is no controller. I am still scratching my head over that one.

As you can see from the specs - 5.75 amp is max output (shorted) - so without MPPT controller - max 17.25 amp output at any voltage. With MPPT you can squeeze it to maybe 20-21 amp out in optimum conditions. But do you not have a charge controller in now? Maybe hidden?

Anyway the Blue Sky does not support serial operation where the morningstar does. So once you have sun - measure voltage at panels and then at battery - if voltage drop is high - consider changing to serial operation. But the tristar is much more advanced then the blue sky - and are available on eBay for around $450 - just make certain it is the newer Ethernet model and not the old model without. A separate display is around $110






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