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A very simple scenario - on a budget would you go Series or Parallel and why ?

O

OnTheRoadAgain

Solar Enthusiast
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Feb 22, 2021
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You have a Dodge Minivan (just go with it for this scenario) :)

Your budget is limited and saving is important.

You want two, 100watt solar panels on your roof. The wire to connect them will be about 7 feet max.

Would you connect them in series or parallel and why? Consider the controller secondary, in other words, the way you connect your panels is all that matters and you will get a controller that matches your panel connection choice.
 
series.... higher voltage you make power sooner in the day and less voltage drop
 
GSXR1000 said:
series.... higher voltage you make power sooner in the day and less voltage drop
Click to expand...

7 feet. Voltage drop should be negligible even with 12 gauge wire

Also one small shadow on either panel and you drop to zero with series.

Shouldn't you use a MPPT to take advantage of that? Isn't MPPT generally more expensive than PWM?

Finally, what amps can you produce if in series with two 100watt panels?
 
panels have built in diodes to allow the shaded panel pass the current of the non shaded....
if you want to go pwm the yes parallel...
prolly 1 amp in 10 amps vs 5 amps
 
We're talking about installation on top of a minivan here. The total cable run is less than 7 feet.
If you go with series you have the potential for up to 42volts and could exceed the controllers rating.
So you would need to buy a more expensive MPPT controller to ensure you could handle the higher voltage.
WHY? In this case voltage drop is a non issue.

Voltage drop is not a concern with this short a cable run. You are getting no advantage by using a series connection, in fact you are going to have half the current (only 5 amps) maximum. Also in a series connection, regardless of the diode, if any shade hits either panel the output drops even further. Much more so that a parallel connecting here.

Now if you were talking about 15 feet or more than voltage drop might become a consideration...but we're talking 7 feet MAXIMUM cable run length here.

CLEARLY Parallel in this particular case is better. You get a full 10amps for charging the battery and better performance from shade passing over the panels. You can use a cheaper PWM controller. With series you can only get 5 amps for charging the battery. How is that "better" ?

People keep saying series but I think it's a knee jerk response without really considering the setup in this particular case.

I wonder if Will Prowse would recommend series in a setup on a minivan with only two panels and 7 feet the longest cable run?

This is bothering me because I cannot understand why anyone would go with a series connection in this particular case.

Perhaps I am fundamentally missing something here? If so please help me learn what I'm missing.
 
OnTheRoadAgain said:
We're talking about installation on top of a minivan here. The total cable run is less than 7 feet.
If you go with series you have the potential for up to 42volts and could exceed the controllers rating.
So you would need to buy a more expensive MPPT controller to ensure you could handle the higher voltage.
WHY? In this case voltage drop is a non issue.

Voltage drop is not a concern with this short a cable run. You are getting no advantage by using a series connection, in fact you are going to have half the current (only 5 amps) maximum. Also in a series connection, regardless of the diode, if any shade hits either panel the output drops even further. Much more so that a parallel connecting here.

Now if you were talking about 15 feet or more than voltage drop might become a consideration...but we're talking 7 feet MAXIMUM cable run length here.

CLEARLY Parallel in this particular case is better. You get a full 10amps for charging the battery and better performance from shade passing over the panels. You can use a cheaper PWM controller. With series you can only get 5 amps for charging the battery. How is that "better" ?

People keep saying series but I think it's a knee jerk response without really considering the setup in this particular case.

I wonder if Will Prowse would recommend series in a setup on a minivan with only two panels and 7 feet the longest cable run?

This is bothering me because I cannot understand why anyone would go with a series connection in this particular case.

Perhaps I am fundamentally missing something here? If so please help me learn what I'm missing.
Click to expand...
an mppt scc will get much more "power" out of a series set of panels as it can extract maxiumum power even considering the buck/boost losses(you can get 20-30% increase in power which is even more important with small solar array). As the working voltage drops, an mppt does not help.

voltage drop is always a concern based on your cable power loss; its NOT the distance, its the resistive losses (and 7ft distance means 14ft of cable); if you vehicle is wired with 14awg wire, even 10amps would be measurable losses.

your confusion is probably coming from your starting goals which you have not really formalized:
1) do you want to make the most efficient use of the limit solar panel area you have
2) do you want the cheapest solution possible
3) you have not yet provided what the "system load" is, which is a MAJOR design factor? If you are only looking at charging a cell phone, efficiency means nothing; if you want to run a cooler it means everything!

your current question is similar to the statement:
" how do I attach together two 8ft long 2"x4" pieces of lumber?"
and people are split between telling you to just tape them end-to-end, and others say screw them together side-to-side...

and then you do not tell anyone that your goal is either to lift a car..or a butterfly.
One solution can handle both scenarios, the other solution can only handle one scenario.
 
OnTheRoadAgain said:
CLEARLY Parallel in this particular case is better. You get a full 10amps for charging the battery and better performance from shade passing over the panels. You can use a cheaper PWM controller. With series you can only get 5 amps for charging the battery. How is that "better" ?
Click to expand...
CLEARLY, you seem to have decided before even asking the question.

I am looking at adding a DC coupled array to charge my home storage battery. While doing all the math, I would want to series as much as possible. Yes, it is less shade tolerant, but that is the ONLY drawback. In your case with just 2 x 100 watt panels, making about 20 volts at 5 amps, it seems that getting as much power as possible would be important. If you went with a PWM controller, you will never make the most power. Yes, you might see 10 amps of charge current if you happen to park with the van tilted perfectly into the sun on a perfectly clear cool day. At 13.8 volts that 10 amps is a charging power of 138 watts out of your 200 watts of panels. Now put the panels in series, the PWM controllers gets 40 volts at just 5 amps going in (Same perfect panel position) and the MPPT tracks the ideal load and uses a DC-Dc converter to push the power into the same 13.8 volts at the battery. Figure only 95% efficient to make it fair, so that 200 watts becomes 190 watts / 13.8 volts = 13.77 amps. More than 35% more charge power. And when the van is flat, and the sun is not directly overhead, which is 99% of the time, the MPPT will always convert far more of the power to the battery voltage. At very poor lighting, the series panels will make twice the voltage and therefore get above the battery and allow charging where the parallel panels will basically just shut down. Over the length of an entire day, the MPPT and series panels will far outperform the parallel panels. Just don't park in the shade.
 
Thanks for taking the time to reply and explain. It is appreciated.

Here is a forum thread on this exact topic with what appear to be very knowledgable people that better explain why MPPT for this scenario is not necessarily better.
They explain it FAR better than I can and they seem to mostly support my assertion and then back it up with exactly why.

https://forum.solar-electric.com/di...rge-controllers-vs-pwm-for-very-small-systems

If this topic is of interest, and you are not closed to other information and POV's then it's a good read.

Perhaps this topic is more subjective than I thought. However, I will admit right off the bat I had some misconceptions but I am more than willing to learn more.

Let's all try to remember this is a process of learning......for everyone. No hostility is required or welcome.
When I said "clearly".....that may have come across as arrogant, but in reality it may have simply been more that I am ignorant of certain facts.

Anyway, as far as that thread I mentioned, they say
One reason stated is that panel efficiency in reality drops when hot. That voltage drop affects MPPT controllers more than PWM controllers.
Another is that in reality, with such a small system, the only real gain is with large differences between the incoming voltage and the target battery voltage.

At the end of the day, I'm now not sure there is an absolute best choice. So many factors.
So clearly, I wasn't correct in saying "clearly"
 
Last edited:
OnTheRoadAgain said:
Anyway, as far as that thread I mentioned, they say
One reason stated is that panel efficiency in reality drops when hot. That voltage drop affects MPPT controllers more than PWM controllers.
Another is that in reality, with such a small system, the only real gain is with large differences between the incoming voltage and the target battery voltage.
Click to expand...
This is an interesting one, so I tried to find a bit of data on that. And from reading what they are saying, the MPPT will drop more from it's peak output when it gets hot compared to a PWM from it's peak. But here is the problem with that argument. The peak power of the MPPT is quite a bit higher, so as it falls off in the heat, it just comes down and reduces it's lead a bit over the PWM. But I can not find any situation where the PWM will out perform an MPPT in actual watts out. The one good argument for a PWM is with the cost, especially on a very small system. For the cost difference, you could buy another solar panel, and then the PWM system with 3 panels does make more power than an MPPT system with only 2 panels that cost about the same. So sure, if you have the room for another panel, the PWM could win, but if you are limited to a given amount of solar panel, the MPPT will make better use of the panels and extract more power.

While it is true that at a small difference in voltage, the MPPT can't step up the current, it will still virtually match the PWM. And with the panels in series, the voltage will go above the battery with less light on the panels than if they were in parallel.

An MPPT controller does have a processor inside to run an algorithm to track the voltage and current and make constant adjustments to optimize the power conversion. Just like any small computing device, that processor will use some of the power you are collecting. So in a very low power situation, the energy needed to run that processor may become a fair part of the load. So obviously, for a 3 watt solar cell running a security light, an MPPT control would be silly.

So it does always come back to how much power do you need? How much space do you have? What is the budget?

If you have the room and want to save money, a PWM may work well. If you need the most power in a limited space, and the budget can handle it, an MPPT will make more power.
 
GXMnow said:
This is an interesting one, so I tried to find a bit of data on that. And from reading what they are saying, the MPPT will drop more from it's peak output when it gets hot compared to a PWM from it's peak. But here is the problem with that argument. The peak power of the MPPT is quite a bit higher, so as it falls off in the heat, it just comes down and reduces it's lead a bit over the PWM. But I can not find any situation where the PWM will out perform an MPPT in actual watts out. The one good argument for a PWM is with the cost, especially on a very small system. For the cost difference, you could buy another solar panel, and then the PWM system with 3 panels does make more power than an MPPT system with only 2 panels that cost about the same. So sure, if you have the room for another panel, the PWM could win, but if you are limited to a given amount of solar panel, the MPPT will make better use of the panels and extract more power.

While it is true that at a small difference in voltage, the MPPT can't step up the current, it will still virtually match the PWM. And with the panels in series, the voltage will go above the battery with less light on the panels than if they were in parallel.

An MPPT controller does have a processor inside to run an algorithm to track the voltage and current and make constant adjustments to optimize the power conversion. Just like any small computing device, that processor will use some of the power you are collecting. So in a very low power situation, the energy needed to run that processor may become a fair part of the load. So obviously, for a 3 watt solar cell running a security light, an MPPT control would be silly.

So it does always come back to how much power do you need? How much space do you have? What is the budget?

If you have the room and want to save money, a PWM may work well. If you need the most power in a limited space, and the budget can handle it, an MPPT will make more power.
Click to expand...

Good stuff. Again, appreciated.
Can we generalize and say that "all" MPPT controllers will produce more power than "all" PWM controllers under the same conditions?

Another source.....
https://solarcraft.net/resources/articles/pwm-vs-mppt-solar-charge-controllers

PWM vs MPPT Solar Charge Controllers​

► Comparing the Two​

If maximizing charging capacity were the only factor considered when specifying a solar controller, everyone would use a MPPT controller. But the two technologies are different, each with it’s own advantages. The decision depends on site conditions, system components, size of array and load, and finally the cost for a particular solar power system.

► Temperature Conditions​

An MPPT controller is better suited for colder conditions. As solar module operating temperature goes down, the Vmp1 increases. That’s because the voltage of the solar panels operating at their peak power point at Standard Testing Conditions (STC is 25C°) is about 17V while the battery voltage is about 13.5V. The MPPT controller is able to capture the excess module voltage to charge the batteries. As a result, a MPPT controller in cool conditions can produce up to 20 – 25% more charging than a PWM controller.

In comparison, a PWM controller is unable to capture excess voltage because the pulse width modulation technology charges at the same voltage as the battery. However, when solar panels are deployed in warm or hot climates, their Vmp decreases, and the peak power point operates at a voltage that is closer to the voltage of a 12V battery. There is no excess voltage to be transferred to the battery making the MPPT controller unnecessary and negating the advantage of an MPPT over a PWM.

► Array to Load Ratio​


In a scenario where the solar array is large relative to the power draw from the batteries by the load, the batteries will stay close to a full state of charge. A PWM controller is capable of efficiently maintaining the system without the added expense of an MPPT controller.

► Size of the System​


Low power systems are better suited to a PWM controller because:
  • A PWM controller operates at a relatively constant harvesting efficiency regardless of the size of the array
  • A PWM controller is less expensive that a MPPT, so is a more economical choice for a small system
  • A MPPT controller is much less efficient in low power applications. Systems 170W or higher tickle the MPPT’s sweet spot
 
I dont think a 200W system is “small” in the MPPT vs PWM context. Modest, perhaps, but not small.

IMO, when real estate is extremely limited, it’s far more important to get the most out of limited square-feet as possible so a MPPT combined with good panels with bypass diodes would be in order.
 
Freqz said:
I dont think a 200W system is “small” in the MPPT vs PWM context. Modest, perhaps, but not small.

IMO, when real estate is extremely limited, it’s far more important to get the most out of limited square-feet as possible so a MPPT combined with good panels with bypass diodes would be in order.
Click to expand...

A 200watt, 100watt x 2, 2 panel system will not give you 200watts of usable power

► Size of the System​

Low power systems are better suited to a PWM controller because:
  • A PWM controller operates at a relatively constant harvesting efficiency regardless of the size of the array
  • A PWM controller is less expensive that a MPPT, so is a more economical choice for a small system
  • A MPPT controller is much less efficient in low power applications. Systems 170W or higher tickle the MPPT’s sweet spot
 
OnTheRoadAgain said:
A 200watt, 100watt x 2, 2 panel system will not give you 200watts of usable power

► Size of the System​

Low power systems are better suited to a PWM controller because:
  • A PWM controller operates at a relatively constant harvesting efficiency regardless of the size of the array
  • A PWM controller is less expensive that a MPPT, so is a more economical choice for a small system
  • A MPPT controller is much less efficient in low power applications. Systems 170W or higher tickle the MPPT’s sweet spot
Click to expand...
Right and all the more reason to squeeze every watt possible from the system. But do whatever you like: if I were spending my money on a van setup, I’d spring the extra $40 or whatever on a budget MPPT— cheaper than a third panel that you don’t have room for anyway.
 
Freqz said:
Right and all the more reason to squeeze every watt possible from the system. But do whatever you like: if I were spending my money on a van setup, I’d spring the extra $40 or whatever on a budget MPPT— cheaper than a third panel that you don’t have room for anyway.
Click to expand...

why would I do that? My 2 100watt panels make a realistc 130watts combined as do most dual 100watt panel setups.

https://solarcraft.net/resources/articles/pwm-vs-mppt-solar-charge-controllers
"A MPPT controller is much less efficient in low power applications."
 
Diysolar123 said:
your current question is similar to the statement:
" how do I attach together two 8ft long 2"x4" pieces of lumber?"
and people are split between telling you to just tape them end-to-end, and others say screw them together side-to-side...
Click to expand...
Or...... take them both back to the store and buy a 16 foot length of 2"x 4" instead. ;)
Sorry, it's over 100 degrees outside here today, so I'm housebound and bored hence the stupid comments.
 
Budget is king in small systems. Infinite budget, small roof space MPPT. Very limited budget and economics per watt comes into play. You may be better off getting a pwm controller (if on a very bare bones budget) and using money saved to put towards an extra panel or briefcase that can be ground deployed (or just pocket it). The 30 percent quoted is typically overblown, renogy for example states a max 15 percent increase in output (usually at extremely cold temps at that). On average, in parallel, an mppt might add 5 watts per 100 watt panel. I like MPPTs, but I also have a pwm on a small system and it has performed well enough for my needs.

Or pivot completely. Buy a single used or leftover residential panel that will fit your space and an economical mppt (30a Duoracer is my current small system favorite as it can also maintain your starter battery if you keep the house battery 12v). The combo might be comparable in cost to two small panels plus mounting and connectors with a PWM.
 
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