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

[BFL13]

The volts should remain the same 21ish for all 12v panels. You may be confusing 24v panels with 12v panels. Note that Isc on 24v panels is half what it would be for equal wattage 12v panels (natch! 🙂 )

You don't use PWM with 24v panels and a 12v battery system, you use MPPT (big bucks!) controllers with 24v panels to get 12v out to the batteries. Also the MPPT makes its own amps so the 24v panel amps specs mean absolutely nothing

You don't use Imp when on PWM. Use the Isc with 12v panels as expected max amps (except in edge of cloud effect when amps spike higher than Isc)

I have a pair of 100w 12v panels that get 6.3a each as Isc. Together (in parallel) they make 12.6a using the one PWM controller. If I had a single 12v 200w panel I would get the same 12.6a from the same controller.

So now you are asking, "How in heck can I figure out my expected max amps when using 24v panels where the amps spec means nothing and MPPT?"

We have reports that 235w 24v panel and MPPT gets 15amps pointed at the sun. This is about the same as you would get with 12v and PWM with 235w worth of panels. "How can that be when MPPT gets you way more amps than PWM?" you ask.

it seems that people might be seeing those half-amp specs on the 24v panel and seeing them double when using their MPPT controllers making 12v output and they think that MPPT is twice as good as PWM? 🙂 who knows? ( I think MPPT is silly for the money it costs unless you have special circumstances, but never mind me!)

[MickD]

Several have said that I should get 2 panels to total the necessary watts needed. I have no problem with that but when I look at the specs for lower watt panels the IMP goes way down also the volts. What is the effect on charging the batts? Also that means more brackets and more wire and more holes in the roof.

Mick

[pianotuna]

Hi BFL13,

You just don't camp in weather cold enough for it to matter. Or hot enough. Mena could fry his marvelous battery bank without temperature compensation.

It does matter which battery post the temperature probe is on.

[BFL13]

Some controllers have temp comp but not remote, so the temp is where the controller is. However, since the controller should be mounted close to the batteries if possible, this can make the temps "close enough" for ambient at the battery.

So far as getting the temp right for inside the battery, that is a whole other thing. Not clear that attaching the temp probe to a battery post does that either.

From what I have seen, temp comp doesn't matter while camping anyway. That would be more of a Float, storage requirement unless you are camping in winter in Regina or in summer in Death Valley 😞

[pianotuna]

Hi MickD,

Any controller that has temperature compensation at the battery will meet your needs.

[Almot]

BFL13 wrote:

What does this mean?
"• Industry’s lowest voltage per watt rating
Delivers the most cost-effective installs"

Maybe they meant low price per watt. Who cares... It's a Chineglish...

I wholeheartedly agree with suggestions to install 2*90W over one 180, wherever possible. Handling a big panel on a small roof can be a challenge. It is heavy and big. You think it fits, and then suddenly realize that you don't "fit" on the roof next to that panel.

[BFL13]

I didn't catch the price there. The Isc is good for the wattage rating and at 22v Voc vice the usual 21 Voc, so that checks out.

What does this mean?
"• Industry’s lowest voltage per watt rating
Delivers the most cost-effective installs"

The rating in watts is supposed to be Vmp x Imp so any lower voltage within the rating would make for higher amps. Except this has 22v Voc which is higher than the usual 21v and the Isc is a titch (but a tiny titch) below usual. (not like those DM 145s things with the really low Isc)

Big panel for a 12v. There are 190w 24v panels! If price is similar IMO get two 100s for easier man-handling. (where you might use it/them as a portable)

[time2roll]

With Isc close to 12 amps I recommend a 15 to 20a PWM controller.
Morningstar SunSaver 20A is close to $80 but it seems many are happy with lower cost any brand controller in the $20/$25 range.

[MickD]

I was looking at the Sunelect site and the following panel I believe I could fit. Seems like a good price.
Click
Again what controller would be adequate?

Mick

[BFL13]

You get approx. Isc with 12v and PWM as your expected max amps when pointed and the sun is up high, so about 9.5a on the 150w.

Imp and Vmp only get you the named watt size for the panel. You would only see Imp (lower amps than Isc) if the battery voltage got up way high on the IV curve--not going to happen with a controller in the 14s.

That dealer also has a 160w going for $174 not shown on his website table yet. However I would get two 80s instead for the same price (two x $87) for ease of handling, unless shipping cost makes a difference. No idea what his shipping costs are like, sorry.

Each 80w will get approx 5.1a Isc so there is a nice 10.2a package. Isc for the 100w is actually 6.4a marked on the panel, but I get 6.3 actual. But 100/130 x 8.2 is 6.3 so that works for me 🙂 (I have two of the 100s)

The site's table as a few typos in its Iscs etc he hasn't fixed, so check figures if critical. Easy way for Isc is compare with 130w at 8.2 for ratio. eg, 160/130 x 8.2 = 10.1

eg note the typo in the Imp for the 230w panel, but it is correct for the 190w given the Vmps.

[HiTech]

I saw him post that it was name plate after my post. so yes need to guess at a lower bound.

PWM current will be higher than Impp. Figure it will be approximately Isc.

Jim

[Almot]

HiTech wrote:
I take the energy audit to be a minimum of 3.5Ah*12=about 40 Ah and a maximum to be about 60 Ah (battery lasts two days).

We don't know the actual draw. I gather, the OP doesn't know either. 3.5A is taken from the nameplate. In "normal" DC fridges the actual current is anywhere from 1/3 of the current shown in compressor specs to slightly higher than compressor says. Since Norcold is not "normal" - they make their own compressors - I simply don't know. Then, there are other things besides the fridge and light - he says there aren't any, but 9 times out of 10 there are other loads, even if they are small.

Then, we don't know how low he lets his battery drop before charging it.


Very roughly, assuming there is really nothing but 40-50 Ah fridge plus 2AH light, I would say that 150W is "almost enough" on a sunny day. Since he is running a generator once a day and apparently is going to continue, "almost enough" is good enough.

10 amps for 5-6 productive hours plus shoulders should cover sunny days.

150W panel with PWM controller will not generate 10A. It says 8.7A MPP current in specs. But, given the generator, this issue becomes rather academic. Too bad. He is really close to a generator-free boondocking.

[Salvo]

The 12V, 150W panel looks good. I would use a $20 pwm controller. In addition you need a larger battery bank. If they fit, get 2 6V GC.

Sal

[pianotuna]

Hi smk,

I have no idea--but if sun does--find another person who wants some solar? Solarblvd is a good source, too.

[time2roll]

Does sunelec.com require the 4 minimum when picking up?

Here are some in Norco CA where you can buy 1 or 2:
http://www.solarblvd.com/

Bit of a road trip to save shipping.