Sunwize SW100C Panel Power Output (Kinda Low)

retrodog

I got a couple of these in a kit with a Morningstar MPPT charger controller. They seem to work fine with the charger, but I wanted to see how much power they'd actually supply.

They are rated at something like 23V and max of 5A so I figured that I could get at least 77W out of them with a 6.8 ohm power resistor. I decided that would be a good place to start and then lower the resistance if that worked out.

(23v*23V)/6.8ohms=77.79 Watts

So I sat up each panel by leaning one end on a lounge chair, angled toward the sun. I have the two panels so I decided to do them the same way to see if the results were similar. And they were almost identical...

Time Panel 1 Panel 2 Temperature
12:00P 67.7W 67W 112F
1:16P 56.2W 56.37W 155F

The sun was actually at it's most direct point in the second measurement so I would have expected more power, but the panels were hotter then so I guess that knocked down the efficiency.

Is that pretty normal? They are essentially down to 56% of their rated output power.

Cariboocoot

Re: Sunwize SW100C Panel Power Output (Kinda Low)

The problem is probably the numbers you're using for prediction. "23 Volts" sounds like a Voc number, not a Vmp. I don't know why you used a 6.8 Ohm resistor either. You have to run panels to a dead short to see their maximum current potential.

A panel's output is its Vmp * Imp; Voltage at maximum power times current at maximum power. The panels are 100 watt, no? So the Vmp is probably around 20, not 23, times the 5 Amp rating = 100 Watts. Panels only achieve this "perfect output" under exacting conditions. Sometimes they go higher, usually they are lower. To get an accurate reading of a panel's output at any given time you need to run the current through a digital Ammeter and likewise check the Voltage at the same time.

Then take a look at your panel temps: over 100 F? That will cost a lot of power too.

This explanation may be as unclear to you as the skies over my head right now, which are full of smoke. It's not even "magic smoke".

Hopefully Bill or Niel or someone can elaborate.

retrodog

Re: Sunwize SW100C Panel Power Output (Kinda Low)

Cariboocoot wrote: »

The problem is probably the numbers you're using for prediction. "23 Volts" sounds like a Voc number, not a Vmp. I don't know why you used a 6.8 Ohm resistor either. You have to run panels to a dead short to see their maximum current potential.

A panel's output is its Vmp * Imp; Voltage at maximum power times current at maximum power. The panels are 100 watt, no? So the Vmp is probably around 20, not 23, times the 5 Amp rating = 100 Watts. Panels only achieve this "perfect output" under exacting conditions. Sometimes they go higher, usually they are lower. To get an accurate reading of a panel's output at any given time you need to run the current through a digital Ammeter and likewise check the Voltage at the same time.

Then take a look at your panel temps: over 100 F? That will cost a lot of power too.

This explanation may be as unclear to you as the skies over my head right now, which are full of smoke. It's not even "magic smoke".

Hopefully Bill or Niel or someone can elaborate.

Vmp for these panels, according to the sellers site, is 23V. Voc is about 27V.

I don't care about max current with a short because that is at 0V and would imply 0Watts.

I used a 6.8W power resistor because it was available and would allow me to get around 77W from 23V, as a starting place. I realize that temperature plays a significant role, which is why I went to the trouble of measuring it and recording it with the data. I just didn't realize that it would drop the power level that far.

And while some of the characteristics/specifications on solar panels are new to me, I got my electrical engineering degree back in 1983 and have been designing electrical systems for military and space applications for the last 27 years. So I can figure out a lot of stuff if you just hold my hand a little bit.

reference SW100C panel specs.: http://www.sunwize.com/info_center/pdf/SW90C-100C_3_09.pdf

BB.

Re: Sunwize SW100C Panel Power Output (Kinda Low)

Yep--Assuming you have everything setup correctly, I would guess around 77% is a reasonable expectation for full power during a typical clear/sunny day.

Current is pretty much proportional to the amount of sunlight hitting the panel (and actually goes up a tiny bit with hot panels--but we usually ignore temperature effects on current).

That leaves Vmp as the first order effect of temperature. So, if your panel is rated at 23 volts and 4.35 amps then, the operating point would be around:

• 23 volts * 0.77 = 17.71 volts Vmp
• 17.71 volts * 4.35 amps = 77 watts
• 4.35 amps * 6.8 ohms = 29.58 volts (too high of resistance for full power)
• 17.71 volts Vmp / 6.8 ohms = 2.6 amps
• V^2/R = 17.71^2 / 6.8 ohms = 46 watts

So, assuming Vmp is fixed (which it is not--it is really the knee of a curve and there is about a 10% range where Vmp at Pmp is pretty flat), you could probably expect:

• (17.71 volts * 1.05)^2 / 6.8 ohms = 50.9 watts

So, with a 6.8 ohm fixed resistor and "hot panels" (from sitting in full sun for awhile)--I would expect something on the order of 51% of "rated" power--pretty close to your measured value...

The 0.77 / 0.75 derating is a conservative value (time, temperature, dirty panels, bit of haze in the air, and 5% expected losses from a charge controller--which you do not have).

-Bill

niel

Re: Sunwize SW100C Panel Power Output (Kinda Low)

you might also look at lt almost like a battery and have vmp/imp = internal resistance and this is the max power load resistance. this is of course at 25 degree c so ptc or cec figures may be better to use.

retrodog

Re: Sunwize SW100C Panel Power Output (Kinda Low)

BB. wrote: »

Yep--Assuming you have everything setup correctly, I would guess around 77% is a reasonable expectation for full power during a typical clear/sunny day.

Current is pretty much proportional to the amount of sunlight hitting the panel (and actually goes up a tiny bit with hot panels--but we usually ignore temperature effects on current).

That leaves Vmp as the first order effect of temperature. So, if your panel is rated at 23 volts and 4.35 amps then, the operating point would be around:

• 23 volts * 0.77 = 17.71 volts Vmp
• 17.71 volts * 4.35 amps = 77 watts
• 4.35 amps * 6.8 ohms = 29.58 volts (too high of resistance for full power)
• 17.71 volts Vmp / 6.8 ohms = 2.6 amps
• V^2/R = 17.71^2 / 6.8 ohms = 46 watts

So, assuming Vmp is fixed (which it is not--it is really the knee of a curve and there is about a 10% range where Vmp at Pmp is pretty flat), you could probably expect:

• (17.71 volts * 1.05)^2 / 6.8 ohms = 50.9 watts

So, with a 6.8 ohm fixed resistor and "hot panels" (from sitting in full sun for awhile)--I would expect something on the order of 51% of "rated" power--pretty close to your measured value...

The 0.77 / 0.75 derating is a conservative value (time, temperature, dirty panels, bit of haze in the air, and 5% expected losses from a charge controller--which you do not have).

-Bill

So, am I to understand then that actual panel output will never be over 56% of the rated value, even in optimum conditions of sun angle and intensity?

BB.

Re: Sunwize SW100C Panel Power Output (Kinda Low)

Just on a typical day, hot solar panel, with a 6.8 ohm power resistor.

I live in a moderate climate and have roof mounted 3.5 kW of solar panels on a GT inverter. The typical maximum is around 2.5 kW - 2.7 kW and the most I have ever seen is 3.0 kW:

• 2.5 kW / 3.5 kW = 0.71
• 2.7 kW / 3.5 kW = 0.77
• 3.0 kW / 3.5 kW = 0.89

The above is just observations as I walk by my inverter (I do not have a computer setup for logging).

Of course, there are many days when I see 2.4 kW or a bit less too. The amount of sun light is surprisingly variable (added to how hot the solar panels are).

The Grid Tied Inverters are all MPPT (maximum power point tracking) type devices and can output 100% (less efficiency) from the solar array to the utility power.

With Batteries or Load Resistors, you have many other variables that reduce actual solar production (batteries become fully charged, fixed power resistors do not let panels operate at their "optimum" power point as conditions change, etc.).

-Bill

retrodog

Re: Sunwize SW100C Panel Power Output (Kinda Low)

It starts becoming very obvious how these things are affected by location, orientation, and mounting. I've seen a lot of pictures of panels mounted horizontal atop flat rooftops. This would make me worry about heat buildup underneath. Even a moderate inclination would make a tremendous reduction in heat. I plan to experiment some more this coming weekend. The use of two reasonably matched panels makes it pretty easy to try different things and do a direct comparison.

BB.

Re: Sunwize SW100C Panel Power Output (Kinda Low)

Yep, some suggestions:

• Use PV Watts website to see how orientation, tilt, 1/2 axis tracking will affect your output over the year. (1 kW is minimum array size for program, use 0.52 derating for an off-grid battery backed AC inverter based system)
• Mount solar panels with at least 5 inches of ventilation between roof/solid surface and panels for keeping panels cooler.
• Mount with at least 5 degree tilt (even in tropics) to have "self cleaning" solar panels (dust/dirt/leaves are self shedding).

-Bill

PS: I don't think that running cool water over panels is a good idea (extra power for pump, water spotting, humidity/algae/mold growth, mineral deposits, etc.).

Opti

Re: Sunwize SW100C Panel Power Output (Kinda Low)

I bought the same setup (two 100 Watt SW100C panels and a morningstar MPPT 15A charge controller) and have been putting them through the paces and like what I've seen.

I tried it first with the panels in parallel and measured 11.52 amps into the battery array. I then configured the panels in series and saw the amps increase a little to 11.82A into the batteries with the panels showing about 164 watts together. (164w / 200w = 0.82)

As mentioned before, the panels at 100W's are only rated at 4.35A but are higher voltage than normal 12v panels. These panels really need an MPPT controller to make them perform at all and from what I've seen, they do just fine.

Just for kicks and for those who would like to see a performance comparison between a PWM and an MPPT controller, I hooked in a SunSaver SS-20 PWM charge controller up and the amps through the PWM controller to the battery only hit 9.25A. That works out to a 28% increase in amps that I'm seeing put into my batteries using the MPPT controller.