Solar Installation FAQ

[BradS7535]

So I've been reading through all the solar install threads and I still have a few questions I'd like to get everyone's input on.

Series vs Parallel

So from my reading, I understand the basics of the differences between the two.

• Series works better if you have mismatched panels
• Panel voltages add in series, current remains the same
• Panel current add in parallel, voltage remains the same
• Series more susceptible to shading degradation
• Series = MPPT Parallel = PWM or MPPT

Obviously you can have a combination of series and parallel to fit your needs, but my basic question is what is the more popular method here? I am sure there are specific circumstances where one method is better than another, but from an "overall" perspective which is the more robust/efficient way? Do you get much more out of a parallel system with MPPT over using a PWM? If your camping habits are a mix of full sun and partial shade.

Also, it seems most of the installs here have the batteries located in the back compartment, which makes for a nice rear shell install. What about if your batteries are on the tongue? Where do you install the controller (assuming not weather-proof)? Do you prefer front or rear shell with batteries on tongue?

I would like to contribute a nice technical FAQ/Install guide for the Tech Forum combining all the information in all the different install threads and this one to be a 1-Stop shop for future solar installs, instead of reading through the various threads.

My main question and point of internal debate is which of the following 3 options is the best overall assuming cost isn't a huge driving factor (<$200 cost difference).

1) Series w/ MPPT
2) Parallel w/ MPPT
3) Parallel w/PWM


Note: I still haven't quite figured out the inverter part of the equation yet, I'll get to that later, so lets leave that out of this discussion (unless adding an inverter is a large factor of which design to go with)

[Padgett]

Can treat inverter entirely separate, mine is in the cabin storage compartment under the bed so a very short run of AWG #2 through a 100A circuit breaker to the batteries (my travel Keurig pulls 64A for about 3 minutes. I have a 1KW continuous inverter.

Nice thing about panels on the rear module is that they shade the bed area.

I have 200W of mono through a 30A PWM controller that worked just fine at Sun'n'Fun this year.

Was building on what I had (including the WFO) and if building from scratch would have a much more integrated installation and 300W of panels.

From a location standpoint if had front batteries I'd be very tempted to move it all to the rear, the two compartments under the bed are just so handy and share a common bulkhead. The run to the converter is also short.

The other consideration is whether you need/want AC while dry camping. I consider that essential but not all care.

[LoveToCamp]

On my 2417 Sport, the batteries were on the tongue. I installed the panel on the rear panel, as it was easier to adjust for the added weight. I ran the #10 wire to the PWM controller, which I mounted on the front of the lower box. This was protected from weather by the shell when closed, and protected by the open shell when camping. Pics are in this thread, post #37.

When we got our 2014 2417, I installed one 100w panel on the back shell, as that is where it fit best. My battery is under the dinette bench, which is in the front slide. The PWM controller was installed at the TM factory, and is under my sink, by the entry door. About 10' of wire run to get to the battery, and 18' of wire to get to the solar panel.

I camp dry all the time, and have not run out of power, even in winter when I run the furnace through the evening, and overnight. I am almost fully charged every evening, even when camping amongst trees (Rocky Mountain tree cover is not as dense as coastal/eastern cover). I added a second panel and a second battery last year, and am very happy with my parallel panel installation.

In the thread linked above, there is discussion about MPPT vs PWM. Most conclude MPPT is not worth the money for the systems we can install in our TMs. Larger trailers where you can mount 400+ watts of solar are worth the MPPT installation. We don't have enough room on our roof, according to those who have done the research, to warrant the expense of MPPT. But, others will recommend MPPT, as they have a different measure of benefit.

[BradS7535]

Padgett/LoveToCamp,

Thanks for your input.

One thing I am trying to determine as well in regards to MPPT vs PWM is the effect on overall system cost/value.

I read the post from rickst29 about the difference in efficiency between the two, but I'm a bit confused as to how to calculate the benefit of a MPPT in terms of output current.

Assume perfect/ideal conditions, same configuration (parallel), same solar panels (say 2x100W) (Vmp = 18.9, Imp = 5.29) and what you get out of MPPT vs PWM.

PWM will pulse to get down to the charging voltage of 14.5V with all the amps (5.29*2) so P=V*I, 14.5 * 5.29 * 2 = 153.4W. So I'm paying for 200W + PWM controller and I am actually "getting" 153.4W with 10.58A going to the batteries.

I'm trying to figure out how to calculate the same with an MPPT controller. With the same setup the panels will put out 18.9V (higher than the needed 14.5V) with a total current of 10.58A. Am I correct in assuming that with the MPPT controller I'll "get" the full 200W, which means P/V=I, 200/14.5 = 13.8A? The excess voltage above 14.5 is getting converted to extra amps. So I'm paying for 200W+MPPT controller and I'm actually "getting" 200W with 13.8A going to the batteries?

In a series configuration with MPPT I'm not sure how to calculate the actual current out. In rickst29's example the MPPT had a maximum current output of 15A which led to 217W. The panels were only 200W, so how would I calculate the current output from that system (I'm assuming its less than 15W).

I'd like to try to calculate $/Watt and $/Amp to see the normalized cost difference between the two.

[LoveToCamp]

There are some solar/electric experts, which I am certainly not, on the forum. A good thread is this one, if you have not been there, yet. The experts discuss much more than I care to think about. I know my setup works, and it cost about $250, plus whatever TrailManor charged me for my PWM controller (which I am quite happy with).

My batteries (I added a second one, as one was dead the day before I was leaving on a trip. Got a replacement, and shop told my my old battery was fine, just discharged - phantom loads, or parasitic drain) have plenty of juice to do the job every day, even when we camped in Utah with overcast skies and rain most of the week. Single panel/single battery worked fine on an earlier trip, with panel in full shade all day, and much of many days being overcast. I can't justify an MPPT controller, based on my experience.

[scrubjaysnest]

In simple terms, the cost per watt break even point is about 400 watts of solar.
Below that MPPT isn't worth it. A quality Morningstar MPPT for 400 watts is about $400, for a PWM it's about $150.
Series panels require MPPT.
Under real world conditions it will be hard to tell the difference in MPPT vs PWM at 400 watts of parallel panels.

[BradS7535]

So I talked to Renogy tech support and they said that a MPPT is about 97% efficient at converting excess voltage to extra amps.

So a simple "ideal" comparison for my example above would be:

PWM (Parallel):
5.29*2 = 10.58 charging amps

MPPT (Parallel):
(5.29*2) + .97*(18.9V-14.5V) = 14.85 charging amps

MPPT (Series):
5.29 + .97*(37.8V-14.5V) = 27.89 charging amps

You can see that the MPPT is much more useful in series rather than in parallel, so in a parallel application it doesn't make much sense cost-wise to go to MPPT.

However, I've been looking at different panels/controller combinations and found an interesting option.

Single 250W 24V panel w/ MPPT controller, Voc=37.5V, Isc = 8.87A, Vmp=30.1V, Imp=8.32. Using a single 250W panel with a MPPT controller will give me: 8.32 + .97*(30.1V-14.5V) = 23.45 charging amps.

Unfortunately the charge controller has a maximum output current of 20A, but with cable losses and everything else realistically I'll probably be getting slightly less than 20A anyways.

The cost between a 2x100W PWM system and 1x250W MPPT system is about $130, but I am getting more charging amps out of the 250W system. The $/amp is cheaper on the 250W system.

Let me know your opinions or if I've got anything incorrect here.

[Dchisenh]

One of the biggest things to consider is what type of batteries you have or are planning to get. A lot of the cheaper MPPT controllers out there can't adequately charge the popular Trojan T-105's at 14.8 like the manufacturer recommends, so while you might be getting more amps, it's not as 'healthy' for your batteries at the lower volts. Once you get into the MPPT controllers that are user adjustable, you're talking a pretty penny that might put the setup out of your budget.

You're doing your homework and getting into the nitty-gritty, so kudos for that!

From what I've seen, solar setups usually fall into one of two categories: 1) the minimum to keep batteries and systems working while camping for a few days or 2) the maximum with a large/unlimited budget and the best/greatest/newest components out there. If you're in the first group (like most folks are), a few panels, short wire runs to minimize line loss with parallel arrays, and a PWM controller are more cost effective. If you've got the money to blow or are going boondocking for a very long time/indefinitely using a large battery bank, large and/or high voltage array setup in series (negating line loss that way), an MPPT controller is the way to go.

If I won the lottery, I'd get some lithium batteries, a Morningstar Tristar 60 MPPT controller and as many panels as I could realistically fit!

So, all that said, at this early stage of planning, I'd suggest you first figure out what your loads will be, get enough batteries to support that and then figure out what panel/controller combo will allow you to feed them taking into consideration roof-space/wire runs/budget.

-Dan

[scrubjaysnest]

Don't forget that is based on ideal conditions of full sun and panel temp of 77 deg to 80 degs where they test them in a lab. Real world with full sun and and temps in the 40's you will go well over the 20 amps of the controller. Length of the wire impacts voltage not current with and end result of lower wattage. Real world expect 77 to 80 % for the controller efficiency.
Real world you need about 140 watts of solar per 100 Ah of battery and while GC-2's prefer about 22 amps at 14.8 volts always figure your solar size at 14.5 volts as a rule of thumb.

[BradS7535]

Thanks for the info. The inner nerd in me always wants to get the best or at least make the best out of what I have. Hate knowing I paid money for 200W but I'm only getting less than that with a PWM controller. But then again gotta think practically and economically.

Out of the installs here: PWM or MPPT and where did you install your controller (if not in the rear storage compartment)?