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

[mena661]

HiTech wrote:
I double dare ya 🙂

You're not helping!! LOL!

BFL13 wrote:

HiTech wrote:
I double dare ya 🙂

Ok.--mena's sister the mermaid in her birthday suit (seen asking about solar panels--on topic!)

OMG dude! LMAO! That's it, I'm sending this to my sister.

EDIT: Almot's the only one on topic. LOL!

[westend]

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

Got a name?

[pianotuna]

Hi BFL,

What! No shade? *grin*

[HiTech]

Great spot BFL!!

Jim

[Almot]

MickD wrote:
Several have said that I should get 2 panels to total the necessary watts needed.

I think they (including me too) said that you should get 2 panels with same total watts as one big panel, say up to 180W. You still can go with one 180W 12V nominal panel without getting into costly MPPT controllers (read below), but 180W panel is big and heavy.

Yes, there will be 8 brackets VS 4.

There won't be "more wire" with 2 panels. You will use the panel #10 wire to the roof junction box, and then a thicker wire into the rig. With 2 panels you run #10 wire from each panel to the same junction box. You already have those #10, they come with panels unless the panel is really cheap and small, and you will need some sort of roof junction bus anyway.

when I look at the specs for lower watt panels the IMP goes way down also the volts.

Panels up to 190-200W are - usually - 12V nominal. Vmp is around 17, give or take a little, and Imp is lower when wattage is lower. 2*90W wired in parallel will provide same Imp as one 180W.

BUT... if you look into some really big panels, like 200W or 240W, you'll notice that they have Vmp around 30. This means you can't use PWM controller anymore, and need to buy more expensive MPPT one.

[BFL13]

HiTech wrote:
I double dare ya 🙂

Ok.--mena's sister the mermaid in her birthday suit (seen asking about solar panels--on topic!)

[HiTech]

I double dare ya 🙂

[BFL13]

mena661 wrote:

BFL13 wrote:
You are welcome.

LOL! You are on a roll today buddy!

Be careful or I will post a picture of your mermaid sister skinny-dipping

[time2roll]

MickD wrote:
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

As long as they are nominal 12v panels they will be fine. Just make sure total Isc does not exceed controller rating.

Isc is short circuit current or max amps.

[mena661]

BFL13 wrote:
You are welcome.

LOL! You are on a roll today buddy!

[BFL13]

We should note that a "17v" panel is so named for its Vmp (?) and that its Voc will be 21 or 22v, but that is also called a "12v" panel since it is for "12v" batteries.

So the same panel can be called a 12v, a 17v, or a 22v panel.

Not to be confused with higher voltage panels like 36v ones that are called "24v" panels except if they are higher voltage to go with 36v or 48v battery systems.

controller sizing depends on not going over their rated input voltage spec from the panel(s), so you need to pay attention what voltage you are talking about. also note that voltage will rise when it gets cold, so allow for that extra amount too when sizing the controller for voltage input.

controllers also have an amps rating, so you do need to have some idea of your expected max amps and allow another 20% margin on top of that.

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.

[HiTech]

X2 though I have a 15v panel. It is designed to be used without a controller.

Jim

[pianotuna]

Hi BFL13,

My point is that 17 volt panels are better than 22 volt when using a PWM technology charge controller, so he has nothing to worry about if say the panels were 16.5 volts.

It is true the output voltage is controlled by the battery on pwm.

It is true that lower voltage so long as it is around 17 is better when using pwm.

It is true that with 36 volt panels one would be nuts to use pwm. MPPT would be the way to go then.

I've never seen a 15 volt panel. That would be a rare bird indeed.

[BFL13]

PT is confusing matters by not explaining. Panel voltage is the same as battery voltage with a 12v system. As battery voltage rises the spread between battery voltage and panel Voc spec voltage shrinks, reducing amps.

With lower battery voltage (and so panel voltage) the spread is large and amps can flow.

However, you don't want those lower voltage panels they had for a while with only 32 cells that were "15v" instead of "17v" like 36 cell panels. those lower voltage panels could not charge RV batteries to 14v because the spread was too small between 14 and 15 so they went to 17 to get more spread so battery voltage could rise to where it needs to go.

BTW, with a B and roof real estate "issues" you might be better off with 24v panels and MPPT for the amps you can get in that space, but it will co$t you for that MPPT. OTOH a B can't carry many batteries, so you don't need that many amps perhaps.

[pianotuna]

Hi MickD,

If you are doing a PWM controller volts being down on the panels is a good thing.