Solar Help

[MickD]

I have an older Class B motorhome and have been thinking about solar.
I can boondock now for 2 days before I have to recharge my 125ah battery. The only thing that draws power is the 12V fridge which runs about half the time at 3.5A and a light for an hour at most. I have limited space on the roof. My question is: Would a 150W panel like the one here
Click
keep me charged and what controller would be the minimum I should use?

Let me thank you now for all the good info I have gotten from this site.

Mick

[Salvo]

Carrying a panel inside a B gets too complicated.

Here's what OP needs:

One 150w or two 80W panels. Watch out for shade on the roof!
http://www.windturbine.ca/sun_panels.html

A $20, 20A pwm controller.
http://www.ebay.com/itm/5-30A-Solar-Charge-Controller-Regulator-12V-24V-Battery-Charging-PWM-Solar-P...

As mentioned before, two 6V batteries will make camping much easier.

Sal

[BFL13]

The OP has only a small battery bank and low daily AH usage, so IMO he could get by fine with just the one 90w panel. (as a portable so he can prop it up pointing towards the sun when parked)

Whole thing would be under $200. (Here, with no shipping, it would be $107 for a 100w panel and $29 for a 10a controller = $136 tax included)

Or you can go crazy 🙂

[HiTech]

That second panel is in the twilight zone between a 12v panel and a real 24v panel. It would not really be suitable for charging a 24v bank because of the low MPP voltage (anymore than you could charge a 12v battery well with a 13v Mpp panel).

If you used it without MPPT it would cost you more per battery charging amp ($23.40 per amp for the top one, $26.25 per amp for the bottom one using PWM controllers for both rather than MPPT for either).

Now if that is enough amps for you, the absolute cost to get going is less, but you could probably find a smaller 12v panel between the two sizes that is still cheaper per amp.

Jim

[Salvo]

That 190W panel requires a mppt controller! If you are concerned about cost, stay with 12V panels and pwm controllers.

Sal

MickD wrote:
Yes I did make a big mistake on my last post...too many adult beverages I guess.

The 190 watt single:

Vmp. 26
Imp. 7.63
Voc. 33
Isc. 7.89

$ 207.10

Do these appear to be good prices? The single 190w panel weighs 37lb the two 90w weigh 35.28lb

Mick

[Salvo]

If you're looking for max amps in a mppt controller, then nothing is simpler than:

Imax = Panel power / 12V

If you have 150W panel, then max controller current should be greater than:

I = 150W/12V = 13A

On top of that, derate by 20% to 16A.

In other words, a mppt controller connected to a 150W panel should be rated to 20A.

Sal

[MickD]

Yes I did make a big mistake on my last post...too many adult beverages I guess.
The 90 watt panels that I am looking have the following:

Vmpp 18.1
Impp. 4.96
Voc. 22.0
Isc. 5.30
Ir. 10

$ 124.20 X 2 = $ 248.40

The 190 watt single:

Vmp. 26
Imp. 7.63
Voc. 33
Isc. 7.89

$ 207.10

Do these appear to be good prices? The single 190w panel weighs 37lb the two 90w weigh 35.28lb

Mick

[BFL13]

Salvo , yes it is simplified, but the object is to just get a WAG on "expected max amps" so you can pick the amps size for the controller you will need. (You still need to get the input voltage rating business right too so you don't fry the controller with that)

[BFL13]

westend wrote:
She is fairly good looking, must run in the family.:B

Got a name?

Somebody named her Gertrude. She has been hanging around for a month visiting various sites in the campground for good places to take her naps. She goes for a swim sometimes. Should be finished moulting and go away soon. Her tag number checks out that she is a juvenile female elephant seal.

[Salvo]

That's over simplified. MPPT current is highly dependent on panel temperature and battery voltage.

The temperature coefficient of PV cells is about -0.45%/C. A 25C rise in panel temperature is not uncommon. That means a 150W panel at 50C has dropped 11% of its rated power to 133W.

PWM is more or less unaffected by heat. The panel will output the same Isc.

Sal

BFL13 wrote:

Nobody tells you your expected max amps for any given wattage of panels--except me! 🙂 here it is:-

use the same for either PWM or MPPT, they are nearly the same for this purpose:

expected amps = chosen wattage/130w x 8.2

EG, 230w/130w x 8.2 = 14.5a (jimindenver gets 15a from his with MPPT)

EG 80w/130w x 8.2= 5a (the Isc of an 80w panel and what I got with my 80w.) It works and its free! You are welcome.

[BFL13]

Flat vs pointed results depend on how high the sun is at the time. The higher the sun the closer flat results get to pointed.

You have the daily sun movement up and down and the annual change in declination change of sun's height, which goes with your latitude.

Soooo---it gets complicated. there are websites that try to show the diff for location and calendar.

I did a comparison last year in May for here using my 130w panel (8.2a Isc) but that would not come out the same in California --being farther south, the diff would be less since the sun is higher down there.

(the dips in tracking amps from 8.2 to 7.7 was due to only moving the panel three times a day instead of following the sun all day)

Time, Flat , Tilted, Tracking- Amps

0700- 1.0, 0.6, 3.7
0800- 2.0, 1.7, 5.7
0900- 3.6, 3.6, 7.3
1000- 5.1, 6.2, 8.1
1100- 6.3, 6.8, 8.2
1200- 6.9, 7.7, 7.7
1300- 7.2, 8.3, 8.3
1400- 7.1, 7.7, 7.7
1500- 6.4, 6.9, 8.1
1600- 5.0, 6.6, 8.1
1700- 3.9, 5.4, 7.3
1800- 2.0, 3.3, 5.7
1900- 1.0, 1.7, 3.7

[mena661]

How much would you get with those panels flat?

[BFL13]

The OP found these 190w 12v panels earlier in the thread.

http://www.sunelec.com/Specs/Evergreen/ES-A-190_195_fa2.pdf

Isc is 11.8a

Using my formula, 190/130 x 8.2 = 11.98a so that checks out given that the Voc is 22 instead of 21v.

Now we get another 190w panel but this one is a 24v

http://www.windturbine.ca/sun_panels.html

Isc is 5.91 and Voc is 44v (note how Isc is half the 12v version's and Voc is twice)

AFAIK (based on jimindenver) you would still get about 12a with the 24v panel and MPPT (required for the 24 to get it back to 12v and to get the proper amps per watt)

Using just the watts 190 divided by battery voltage of say 13v and rising, you would expect to get 190/13 = 14.6a or a 2.6 amp gain over the PWM version. 2.6/12 = 21% which is the sort of claim you see in MPPT advertising. BUT:

jimindenver gets 15a from his 230w panel with MPPT using a 20a controller (so it does not clip the amps)

Others with 230w panels and the 15L 15amp controller also get 15a but that controller does clip the amps to its rated 15, so it could be they are missing an amp or two. jimindenver example shows they are not. Nobody else has posted "actuals" that would change this story (so far! 🙂 )

The OP has a typo in the amps for his latest find so it is hard to follow. A 90w panel will have an Isc of approx 5.7a so with two, you would get about 11.4a.

[Almot]

The panel with VMP 26 is what is called "24V nominal". Mostly, 24V nominal panels have VMP around 30, so 26V is an odd number. For your purposes consider it a "24V panel", i.e. the one that requires expensive MPPT controller.

Using PWM controller with this panel will result in too much loss of power. It will simply clip the voltage down to 14V that battery requires, and because the Imp of this panel is already low, you will lose the power. OTH, MPPT controller will convert the excessive volts into more amps, making 14.8A @ 14V out of 7.9A @26V. With panels that are not 12V nominal, you have no choice but use MPPT.

I have to retract my statement about 190W 12V nominal panel - the biggest panel that is still 12V nominal, is probably 150W-160W.

[time2roll]

I = current (amps)
V = Voltage
mp = Maximum Power.
oc = Open Circuit
sc = Short Circuit

You really want to watch Isc or maximum short circuit amps so you get the proper rated controller.

Generally Imp x Vmp = panel rating in watts.

1.16 & 17.4 seems like a 20w panel not 90w.

Basically you are buying amps. The I (amps) is what you will get into the battery. The controller basically clips voltage down to what is needed to properly charge and maintain the battery.

[MickD]

I was looking at the Sun Electric site at the 190watt panel and the IMP was 7.89a & VMP 26.2. On the same page was a 90watt panel and the IMPP was 1.16a & VMPP 17.4.
What is the difference of IMP and IMPP and VMP and VMPP?

Mick