Any tips on my setup?

**Mike90250** · 03-07-2017, 03:31 AM

Quality MPPT charge controllers limit their output to the rated amps, so it's own limiter will kick it at 40A all by itself
It's good to avoid parallel panels, because you need combiner boxes and circuit protection. But on a RV, it's likely you will have shade issues somewhere on the roof, so you have to go parallel, and use Combiner Box and circuit breakers.

**SunEagle** · 03-07-2017, 10:02 AM

The big issue I see is that 40Amp CC. It will limit you to about 500 watts max of pv at one time no matter how you wire them.

But maybe if you face them at different compass angles you could "cheat" and get some output from each 300w all day long. You would still have to wire them in parallel and use over current protection for each in a combiner box but at least you could get something out of all 3.

**Sunking** · 03-07-2017, 11:20 AM

Originally posted by Wrybread

- from reading this forum, I see that these 900 watts of solar panels are more like 693 watts (900 * .77).

False. Where are you getting this stuff?

Originally posted by Wrybread

- my panels are all permanently flat mounted, so their output is probably a lot less than that. Is there some formula for determining the output of a flat mounted solar panel? I'm in Northern California (latitude 38) if that's a factor.

PV Watts. It reduces you Insolation around 15%

Originally posted by Wrybread

I've read conflicting reports of what that amp rating means, but I think it means a maximum of 40 amps on either the PV or battery side?

Output. Amps = Panel Wattage / Battery voltage. 900 watts / 12 volts = 75 Amps

Originally posted by Wrybread

- since I'm connecting 3 solar panels, I can't wire them in series, correct? If so, 693 watts / 36 volts = 20 amps. Is 12 gauge wire sufficient for that?

You screwed yourself using Prime Numbers as it only leaves you with 2 options of configuring your panels of either all in series or all in parallel. You can get away with 3 panels if your controller Voc input is high enough to allow you to wire al panels in series. Your Controller wil not allow that forcing you to wire them in all in parallel loosing the some of the advantages of a MPPT Controller. NO 12 AWG is not large enough. .

Originally posted by Wrybread

- If I got a full 693 watts output from my charge controller, that would be 50 amps @ 14 volts. Should I worry about going that high? I'm thinking I'm not likely to get that much power from flat mounted solar panels, am I right?

Nope not even close. You need a 80 amp controller.

Originally posted by Wrybread

if I got a full 693 watts of output, I believe that would be a C8 charging rate (5400 watt hours / 693 watts = 7.7).

Right answer wrong units. Amp Hours, not Watt Hours.

So here is your tip: Get yourself a 80 Amp Charge Controller that wil allow you to wire your 3 panels in series.

**Wrybread** · 03-07-2017, 02:47 PM

Originally posted by Sunking

False. Where are you getting this stuff?

There's a bunch of places in the forum I found the .77% multiplier. In this post for example:

https://www.solarpaneltalk.com/forum...too-much-solar

Are you saying you think panels output 100% of their rated power?

PV Watts. It reduces you Insolation around 15%

Thanks, that's a useful stat.

Output. Amps = Panel Wattage / Battery voltage. 900 watts / 12 volts = 75 Amps

You screwed yourself using Prime Numbers as it only leaves you with 2 options of configuring your panels of either all in series or all in parallel. You can get away with 3 panels if your controller Voc input is high enough to allow you to wire al panels in series. Your Controller wil not allow that forcing you to wire them in all in parallel loosing the some of the advantages of a MPPT Controller. NO 12 AWG is not large enough. .

I'd say I didn't "screw myself" with anything, if I just installed 2 of them it wouldn't be the end of the world. But I'd really like that 3rd panel for cloudy days.

Does anyone have any comment on the plan to install the 3rd panel in a sometimes cover place, and uncover it on cloudy days? Is there a disadvantage other than the hassle?

**Wrybread** · 03-07-2017, 03:58 PM

> You would still have to .... use over current protection for each in a combiner box

Could someone possibly post a link to something that would work for that?

Or some more details?

**J.P.M.** · 03-07-2017, 04:08 PM

Originally posted by Wrybread

There's a bunch of places in the forum I found the .77% multiplier. In this post for example:

https://www.solarpaneltalk.com/forum...too-much-solar

Are you saying you think panels output 100% of their rated power?

Thanks, that's a useful stat.

I'd say I didn't "screw myself" with anything, if I just installed 2 of them it wouldn't be the end of the world. But I'd really like that 3rd panel for cloudy days.

Does anyone have any comment on the plan to install the 3rd panel in a sometimes cover place, and uncover it on cloudy days? Is there a disadvantage other than the hassle?

Since you ask for comments - a general one: Learning more about the solar resource and it's potential and limitations might serve you well before you go much further.

**Wrybread** · 03-07-2017, 04:32 PM

> Since you ask for comments - a general one: Learning more about the solar resource and it's potential and limitations might serve you well before you go much further.

That's what I'm doing by posting here and reading the various threads. But that said, I've been installing solar systems on my RVs and off-grid living situations for coming on 20 years. I'm not new to it.

I fully know that I'm pushing it a bit with the possibility of installing 3 panels of this size. And I'm not necessarily going to do it, but I'm wondering what the disadvantage of installing a 3rd panel that's default covered (i.e. not in use).

**SunEagle** · 03-07-2017, 04:38 PM

Originally posted by Wrybread

There's a bunch of places in the forum I found the .77% multiplier. In this post for example:

https://www.solarpaneltalk.com/forum...too-much-solar

Are you saying you think panels output 100% of their rated power?

Thanks, that's a useful stat.

I'd say I didn't "screw myself" with anything, if I just installed 2 of them it wouldn't be the end of the world. But I'd really like that 3rd panel for cloudy days.

Does anyone have any comment on the plan to install the 3rd panel in a sometimes cover place, and uncover it on cloudy days? Is there a disadvantage other than the hassle?

I think you may be confusing that 77% multiplier with one that is used (actually it is 67%) when someone uses a PWM type CC instead of an MPPT type.

A PWM will turn a 400 watt system into a 268watt one which is like losing 33% because of how is converts the watts to charging amps..

**Wrybread** · 03-07-2017, 04:45 PM

I think you may be confusing that 77% multiplier with one that is used (actually it is 67%) when someone uses a PWM type CC instead of an MPPT type.

A PWM will turn a 400 watt system into a 268watt one which is like losing 33% because of how is converts the watts to charging amps..

Hmm, interesting, could be. What do you think the multiplier would be when using MPPT? In my experience it's not 100%, but I'm wondering if other people have found otherwise?

And the linked quote where I got that figure fyi:

"Solar professionals usually assume that considering that the panels rarely have perfect sun to put out full power and that a charge controller isn't perfect either combined with a batteries internal resistance an average system will actually put about 77% of the panels rated power into the batteries.. Personally I think it's closer to 80% but many pros use 77% as the number when designing a system. Your panels can put out more on cold sunny days."

That sounds about right to me, but I'm wondering if the number has changed.

**SunEagle** · 03-07-2017, 04:55 PM

Originally posted by Wrybread

Hmm, interesting, could be. What do you think the multiplier would be when using MPPT? In my experience it's not 100%, but I'm wondering if other people have found otherwise?

And the linked quote where I got that figure fyi:

That sounds about right to me, but I'm wondering if the number has changed.

Well nothing is 100% efficient. There are losses that result from the weather to the how you convert sunlight to DC electrical to AC electrical to mechanical work, etc. The more you put into the equation the more you lose.

It is always best to start with the highest efficient equipment and to keep the equation as simple as possible to minimize the losses.

As for what multiplier to use with an MPPT, I would say which one? The cheap one from China or the high quality one rated 5 stars?
How do you plan on using it?
Where do you plan on using it?
What type of panels do you plan on using?
How do you plan on wiring those panels?
What type of batteries do you plan on using?
How much of those batteries do you plan on discharging daily?
I can go on and on but I think you get the picture.

**Wrybread** · 03-07-2017, 05:09 PM

Thanks, very reasonable obviously.

I've still never seen a panel go anywhere near 100% of its rating, even with an Outback MPPT charge controller and a pro installation, which my neighbor has. But its certainly closer than my setup, with my panels flat mounted... And of course being in Northern California I'm not exactly on the equator.

As far as the Chinese cheapy charge controllers, I've been really impressed with the Tracer. I gather that there's some MPPT controllers sold on eBay that aren't actually MPPT, which is hilarious, but if you do a little research you can find true MPPT charge controllers, which this is. That said, I'm sure the Outback is better, and if I was designing this system for an off-grid house I'd certainly get it.

**littleharbor** · 03-07-2017, 05:48 PM

[QUOTE=Wrybread;n344593].

I've still never seen a panel go anywhere near 100% of its rating, even with an Outback MPPT charge controller

Panels will put out MORE than 100% of their rating. High altitude installations on clear, cold sunny days do it all the time. That being said, in hot. arid deserts, while quite sunny your output will be considerably less that the STC rating. Add to this all previously mentioned factors and your net result is what it is.

**sensij** · 03-07-2017, 06:44 PM

Use PVWatts, as others have suggested. The hourly output option gives nice information about what a setup might produce over the course of a typical year. For Stockton, CA, 900 W of panels laying horizontal on the roof models out to a peak of about 750 W for a few hours a year, 144 hours over 700 W. A 60 A charge controller should be OK. As Mike said, some charge controllers control the panel operating point well enough that you can overload the solar and they will keep the current in check by moving away from the mpp. You'll lose out on production you might have had with a bigger charge controller, but the system still operates.

The manual for your 40 A tracer warns against allowing your Isc to exceed 40A, but with only 3 panels in parallel, you should be safe there.

**Wrybread** · 03-07-2017, 07:08 PM

> Use PVWatts

Thanks, very interesting. You're talking about this website, right?

PVWatts Calculator

http://pvwatts.nrel.gov/

Estimates the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations

Any tips on my setup?

Any tips on my setup?

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