My Solar Install

[Socal Tom]

Over the years I’ve amassed a small collection of 50 watt solar panels. My budget couldn’t support anything bigger at the time, so now I have 3 mismatched panels. Since I was always worried they would walk away I rarely brought them. I decided with all 3 panels I should have enough power from them even mounted flat on the roof. Having already replaced the roof once due to leaks, I didn’t want to make any holes.
I discovered theses no hole RV roof mounts

https://www.amazon.com/dp/B087CMBKXM/re ... UTF8&psc=1

But since my roof doesn’t have many flat spots, and the width of my panels and the added width of the mounts wouldn’t fit atound my roof vent, I decided to have brackets on one side and use these in the middle.
The third panel is mounted with hinges, so it can be angled toward the sun if needed.

[Socal Tom]

I think I should add that I'm using a small 20 amp PWM charge controller. Based on everything that I've read, spending over $100 upgrading to an MPPT controller wouldn't give me much, and I could buy a 100Watt panel and upgrade my system for less and gain more from it.
Tom

[saltydawg]

The biggest reason to go mppt is two reasons. One you spend a lot of time with the panels partially shaded, or limited room for more panels when you need more power.

Not being in direct sun, is the reason I will go mppt when I do solar.

[Socal Tom]

The biggest reason to go mppt is two reasons. One you spend a lot of time with the panels partially shaded, or limited room for more panels when you need more power.

Not being in direct sun, is the reason I will go mppt when I do solar.

from my understanding of the differences, I wouldn't see much gain from going to MPPT, but as you stated, if you are areas with less direct sun, or if you need to get that last 10% of power out of your panels, then MPPT can get you that bit more. I'm generally camped within 20 miles of our Southern border and usually in full sun. So my understanding is that I wouldn't get that 10% increase, and I do have room to add more watts ( and at current prices it would be the cheaper route)

The conventional wisdom I've seen on the web is that PWM controllers aren't worth using, and if you don't have MPPT you have a 2nd rate system. The truth is that PWM can be just fine for many of us. The truth is that for a weekend camper like myself, I can probably go the whole weekend without charging and still be above 50% power, so I probably don't need solar at all. This system should keep my batteries in the 20 to 25% discharge range, so I can see some value there.
Tom

[TimC]

The conventional wisdom I've seen on the web is that PWM controllers aren't worth using, and if you don't have MPPT you have a 2nd rate system. The truth is that PWM can be just fine for many of us. The truth is that for a weekend camper like myself, I can probably go the whole weekend without charging and still be above 50% power, so I probably don't need solar at all. This system should keep my batteries in the 20 to 25% discharge range, so I can see some value there.
Tom

I think you are on the right track and your thinking makes sense for your use and location. I tend to read past the "2nd rate system" comments as the folks that spout that wisdom don't know the whole story behind your situation. I upgraded to MPPT because I don't live in a sunny area and there is no more room for additional panels on my roof. I tried portables for a while but didn't like the extra maintenance of putting them out and putting them away and a little anxiety about them disappearing while I'm out on a hike. My system with the MPPT gains slightly more juice from a cloudy day than the old PWM controller I had for the first three years. They are also "permanently" mounted on my roof so angle is less than optimal. MPPT will help a bit with that.

That's just my situation and I wanted the upgrade to see the difference. After my experience with both types of controllers for my son's solar system (100w, 72aHrs) I recommended a good PWM controller for his TD as he has a lighter load on his batteries and a portable suitcase.

Tim

[saltydawg]

Yeah the big reason is not in full sun. A pwm cant do any charge if the panel is putting out less voltage due to shade than the batteries voltage. Where as a mppt can take 2volts of the panel and get it up to the voltage the battery needs to charge at that moment even thou it would only be like 1 amp of charge.

Yes you would get more out of a mppt charger even in full sun. If your batteries need 14 volts at that moment and your panel is putting out 18 in full sun, that extra 4 volts is just dissipated as heat by a pwm charger. A mppt in that situation might give you another 3/4 amp of charge, if you needed it. Which comes back to needing everything you can get out of your solar if your maxed out for space.

But it sounds like you dont need it.

[Tom&Shelly]

Yeah the big reason is not in full sun. A pwm cant do any charge if the panel is putting out less voltage due to shade than the batteries voltage. Where as a mppt can take 2volts of the panel and get it up to the voltage the battery needs to charge at that moment even thou it would only be like 1 amp of charge.

So if your panel is in full Sun, but not pointed optimally, is it the voltage output that drops? I'm designing a solar power system for our non-electrified garage to charge and maintain the teardrop battery, just did some calculations of the solar angle throughout the year, and now I'm wondering if I made a mistake ordering a PWM controller.

Tom

[Socal Tom]

An MPPT can turns amps into volts or turns volts into amps which ever makes sense. With PWM, once volts get above the cutoff, starts sending amps.

[tony.latham]

I've got a 100-watt panel on the roof and a 30-watt panel on the tongue box. The panels are controlled with a PWM controller.

I went with a PWM because MPPTs are big and more expensive.



We use 8-10 amps per night. (Mostly from my CPAP).

Even in the camp above with a canopy and a broken cloud cover the battery was charged by noon. If we're in the open, it's charged up by 11:00 at the latest ––depending on how high the ridges are to the east.



Sure, I could change the controller out with an MPPT .... but why do I need a more efficient system?

Tony

[saltydawg]

So if your panel is in full Sun, but not pointed optimally, is it the voltage output that drops? I'm designing a solar power system for our non-electrified garage to charge and maintain the teardrop battery, just did some calculations of the solar angle throughout the year, and now I'm wondering if I made a mistake ordering a PWM controller.

Tom

There is a couple of things at play, one is just simple trig. The sun put out X amount of watts per sq foot. If you panel is at an angle that it only is half the size when viewed from the point of view of the sun there is only half the energy to be converted. But modern panels have a 90 degree ( 45 past perfectly perpendicular each way ) where they dont lose much. See the pic below, this is major factor in loss. Another is reflectance, at larger angles the sun reflects more of its energy off the glass on the front of the panel. This is why flexible panels can be better, they are less reflective.

Most panels are also rated at a much higher voltage than what is used. Most "12 volt" panels will put out around 20 volts open circuit, under an actual load they will do 16 to 18 volts. This is done for a few reason, the big one in manufactures minds is warranty, they say the panel will put out "12" volts, but they put out 16, after a 10 years they might only be putting out 13 volts, but that still above the 12 volt warranty. Another is dirt and shade, a dirty panel can lose a few % points and still put out what they say it will, same with cloudy days. it covers their butt.

Where pwm falls on its face is shade, you drop a leaf on a "100" watt panel, it might only put out 50 watts where in full sun it was making 70 watts. but remember watts is amps times volts. Yes the voltage drops, as the shaded cell becomes a resistor. Once the whole cell become dark it kills it all. Now shaded does not mean its dark, it means it not full sun, example is 50% of the energy is being blocked ( over the whole panel it can still produce 50% of the energy. see video below.

So yes if your battery is at 13.6 volts so almost fully charged, and its get cloudy enough that the panel can only put out 13.5 volts at say 5 amps then charging stops. Where a mppt will bump the 13.5 to the 13.7 needed to push power into the battery but it might only be at 4 amps. The other side is the higher voltage wasted by pwm chargers, they just block the extra voltage.

The reason most people say you will get 20 to 30 % out of a mppt charger is everything I wrote above, sun angle, cloudy, shading etc. During the peak direct sun they may put out with in a few % of each other, but at large angles, cloudy etc the mppt will put out more when the pwm would just stop charging. So if you tested it every day for a year, you might get 20% more over the whole year with a mppt, remember its an average, not a perfect day test.

Again if you in full sun and have plenty of spare power, like tony does pwm if fine.

I dont know your install so I cant say what charger you need. But hopefully the above info can help answer that for you.

https://youtu.be/Cwvf6lS7P9s

[saltydawg]

another good test video

https://youtu.be/-WGU5j7Rh1g

direct comparison video

https://youtu.be/fSsrEzDcGf8

this video he does not show the actual power out put from pwm. He does show the weather and the output from the mppt charger. As he says take it with a grain of salt. He also mis speaks a few times but does correct himself. Snowy weather he is getting 150 to 200 % more power on the mppt chargers, from what he says.

Again it depends on your install if you should go mppt. On a shed roof throwing another 80 buck 100 watt panel might give you twice the bump of a mppt charge for the same price.

[Tom&Shelly]

I've got a 100-watt panel on the roof and a 30-watt panel on the tongue box. The panels are controlled with a PWM controller.

I went with a PWM because MPPTs are big and more expensive.

We use 8-10 amps per night. (Mostly from my CPAP).

Even in the camp above with a canopy and a broken cloud cover the battery was charged by noon. If we're in the open, it's charged up by 11:00 at the latest ––depending on how high the ridges are to the east.

Sure, I could change the controller out with an MPPT .... but why do I need a more efficient system?

Tony

Pretty good data. And a system similar to Socal Tom's.

Not to hijack the thread, but I bought a cheap 100 W panel and the identical (I think) PWM controller to Tony's, with the intention of installing the controller in the 'drop and that particular panel on either the roof or SW facing wall of the garage. Spent some time today trying to calculate the best angle for pointing the panel. (Later, I'll buy a different panel for use in the field, or perhaps for mounting on our teardrop.)

Since both of your panels are basically horizontal, if the voltage out of the panels were strictly proportional to the (cosine of the--but never mind that detail) angle of the Sun to the panel, a PWM controller would almost never work--the voltage would never reach 12 volts. Since Tony's system works (well), that's obviously not how the panels work! Most of the power drop due to imperfect pointing must be the result of current (amperage) out of the panels.

Later, I plan to install more panels on the garage for lighting and small power tools. For that, I'll buy a (larger and heavier) MPPT controller, along with an inverter, and a battery bank independent of the teardrop's.

Oh, and the reasons I bought the PWM controller: A. I always do what Tony does, except when I don't. 2. That particular controller was designed to mount on a panel and hide the wires behind it. Only other solution I found were some controllers with optional (and spendy) remote controls and/or blue tooth modules.

Again, sorry for the hijack Tom. I like your mounting! We may steal the idea.

Tom

[saltydawg]

Since both of your panels are basically horizontal, if the voltage out of the panels were strictly proportional to the (cosine of the--but never mind that detail) angle of the Sun to the panel, a PWM controller would almost never work--the voltage would never reach 12 volts. Since Tony's system works (well), that's obviously not how the panels work! Most of the power drop due to imperfect pointing must be the result of current (amperage) out of the panels.

Tom

another reason why panels put out more, if you look at the chart I posted, you are right it does not look like a direct proportional math problem. Each cell on the panel is going to put out x number of volts, at y amps, until it does not put out anything usable. I used the 50 % number just as an example.I would bet the amperage drops faster, but the volt are going to drop too. Vs a battery where as the volts drop, the amerage used goes higher.

[Tom&Shelly]

I dont know your install so I cant say what charger you need. But hopefully the above info can help answer that for you.

Thank you Scott! Yes, you've mentioned several things I hadn't thought about. And thank you for the plot.

We should have the panel itself in a few days, and I'll temporarily hook it to some sort of load and measure the voltage and current while pointing it at various angles wrt the Sun. Our system is similar enough to Tony's (except this particular panel will be mounted on a garage), that I think we'll have similar results.

Tom

[Tom&Shelly]

Since both of your panels are basically horizontal, if the voltage out of the panels were strictly proportional to the (cosine of the--but never mind that detail) angle of the Sun to the panel, a PWM controller would almost never work--the voltage would never reach 12 volts. Since Tony's system works (well), that's obviously not how the panels work! Most of the power drop due to imperfect pointing must be the result of current (amperage) out of the panels.

Tom

another reason why panels put out more, if you look at the chart I posted, you are right it does not look like a direct proportional math problem. Each cell on the panel is going to put out x number of volts, at y amps, until it does not put out anything usable. I used the 50 % number just as an example.I would bet the amperage drops faster, but the volt are going to drop too. Vs a battery where as the volts drop, the amerage used goes higher.

I believe you are right. In fact, I think I just found the relationship (for a single cell in the panel). Under "equivalent circuit of a solar cell":

https://en.wikipedia.org/wiki/Theory_of_solar_cells#Equivalent_circuit_of_a_solar_cell

Those equations under the diagram are about as un-simple as it can get!

Tom