Any tips on my setup?

[J.P.M.]

One thing to understand about PVWatts is that it is calculating hourly averages.

PVWatts does not calculate hourly averages.

[Wrybread]

> It had some weblinks that our software sent it to review for approval and made it unviewable until a MOD said it was ok.

Aha, makes sense, thanks.

[SunEagle]

Hmm, what happened to the reply I posted?

It had some weblinks that our software sent it to review for approval and made it unviewable until a MOD said it was ok.

[Wrybread]

I posted a reply here but it said "unapproved", and then it was deleted. Mod, what's up?

[sensij]

> Use PVWatts

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

http://pvwatts.nrel.gov/

Yes, that is the one. A very good grid tie system can be represented reasonably well with "premium" panels and an 8% loss factor. For off grid panels, I would use those same assumptions to set an upper estimate to the DC production you might see. You can apply the efficiency curve from the charge controller manual to calculate how much of that is lost converting down to the battery voltage.

One thing to understand about PVWatts is that it is calculating hourly averages. Instantaneous power spikes of a few seconds to minutes are possible, especially with reflections from the edges of clouds. As long as your charge controller is operating properly, it will handle it. 3 panels in parallel meet the Voc and Isc requirements for the Tracer 4210RN, and it will let you know if it sees values outside of its limits. Despite Sunking's misgivings, the 40 A CC ought to work, although you'll be leaving energy on the table relative to what you'd produce with a 60 A CC.

You would probably be better off with a 4th panel and moving to a 24 V setup... the panels, CC, and batteries would support it, but you'll have to see if your inverter is compatible. You'd have to keep an eye on the charge rate through, maybe cover part of the panels equally on both strings during the times of the year in which you'd be hitting the batteries too hard.

[Wrybread]

Hmm, what happened to the reply I posted?

[Wrybread]

What multiplier? There is no multiplier and causing you to make poor decisions and opinions based on Junk Science.

As I explained a few times, by "multiplier" I'm referring to predicting an estimate of actual PV output versus stated ratings. Thanks to this website I can see what I was looking for is the PTC rating, which I can look up here:

http://gosolarcalifornia.com/equipment/pv_modules.php

And I can look up how it varies throughout the year in any given region here:

http://pvwatts.nrel.gov/pvwatts.php

Furthermore my panels are flat mounted, so the multiplier (or percentage of their output if you prefer) is going to be even less.

MPPT Output Current = Panel Wattage / Battery Voltage.

Your 40 amp MPPT Charger maximum Input is 520 watts. Its maximum Voc Input = 92 Volts. Read the manual.

Your Controller is incompatible with your panels unless you are just using 1 panel. With 2 panels you are screwed twice over. You cannot wire them in series because you will exceed the maximum Voc input. 3rd panel is out of the question, and screws you a third time. 3 is a prime number which means you only have 2 possible panel configurations of all in series or all in parallel. Something expensive happens when you put 3 or more panels in parallel. It requires you to use fuses, 3 times the amount of much larger more expensive wire.

Where are you getting your info from? Here's the manual:

http://www.aasolar.co.nz/Instructions%20-%20A%20Series.pdf

It clearly says the maximum Voc input is 100 volts. But even if it was 92 volts, that's still perfect for two panels in series (each of which is 46 volts open circuit). But since its the roof of an RV and there's shade issues, I'll probably go with parallel wiring anyway.

And for the 3 panels, the plan I'm half considering at the moment is to only expose (or connect) the 3rd panel (all of which are wired in parallel) when its cloudy. So it's essentially two 300 watt panels flat mounted, wired in parallel. Or I might just go with the 2 panels and keep the 3rd panel for another project. Seems a shame though, since here on the Northern California coast it's cloudy for weeks at a time, especially in summer and winter, and it would be nice to have a way to occasionally boost the power when needed.

Thinking out loud now, but like most modern MPPT charge controllers, my controller outputs its data through a serial interface. There's no reason a Raspberry Pi or similar couldn't trip a relay that connects and disconnects a 3rd panel depending on the current charge conditions. Or I can just manually flip a switch of course.

[Sunking]

Hmm, interesting, could be. What do you think the multiplier would be when using MPPT?

What multiplier? There is no multiplier and causing you to make poor decisions and opinions based on Junk Science.

MPPT Output Current = Panel Wattage / Battery Voltage.

Your 40 amp MPPT Charger maximum Input is 520 watts. Its maximum Voc Input = 92 Volts. Read the manual.

Your Controller is incompatible with your panels unless you are just using 1 panel. With 2 panels you are screwed twice over. You cannot wire them in series because you will exceed the maximum Voc input. 3rd panel is out of the question, and screws you a third time. 3 is a prime number which means you only have 2 possible panel configurations of all in series or all in parallel. Something expensive happens when you put 3 or more panels in parallel. It requires you to use fuses, 3 times the amount of much larger more expensive wire.

With either 2 or 3 of those panels you need a MPPT Controller with at least 150 Voc input. Either a 60 amp controller using two panels, or 80 amps with 3 panels and a 200 Voc input. Then you get to wire all the panels in series with small wire and just one set. You pinching pennies is going to cost you a fortune. Your way no matter what you do is a 500 watt system at best from 900 watts of panels.

[Wrybread]

> Use PVWatts

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

http://pvwatts.nrel.gov/

[sensij]

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.

[littleharbor]

[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.

[Wrybread]

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.

[SunEagle]

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]

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]

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..