Instant benefit from replacing Solar PWM with MPPT controller?

[SV Simple Life]

Hi there,

I'm close to buying a 37 footer with 2 x 100w panels, and a PWM controller.

Can anyone quantify the likely increase in output, if I replace the controller with an MPPT one?

Thanks in advance,
Eamonn

[sailorchic34]

With two 100 watt panels (12v) you would get an approximate increase of 19 watts an hour, in real life.

You could wire the panels in series and gain some efficiency, but there are always shading issues on a boat on the hook, swinging with the tide and wind.

That's why I consider adding a third 100 watt solar panel to a PWM system which gives a average 65 watts per hour output. I say 65 Watts as you never get rated output from a solar panel on a boat. Not for long anyway.

[john61ct]

Lots more benefits if the PWM is an old or too-cheap unit, adjustability to ensure actually getting proper charging will extend life of your banks over time.

[SV Simple Life]

Thank you both....I think I'll look for new panels too - there must be higher-capacity panels available now in the 60" x 24" or similar format. I'm tied to roughly those dimensions, without a re-design of the bimini.

[chris95040]

Originally Posted by sailorchic34

With two 100 watt panels (12v) you would get an approximate increase of 19 watts an hour, in real life.

You could wire the panels in series and gain some efficiency, but there are always shading issues on a boat on the hook, swinging with the tide and wind.

That's why I consider adding a third 100 watt solar panel to a PWM system which gives a average 65 watts per hour output. I say 65 Watts as you never get rated output from a solar panel on a boat. Not for long anyway.

Watt is a measure of power (energy per unit time). 'watt per hour' is a nonsense unit for this discussion.

[john61ct]

Common enough mistake, I think most would figure out 19W (= wH / hr) was meant.

[StuM]

Even stating "19 Watts" is nonsense. That's a very precise number. At what time of day? For how many hours? Under what conditions?

How many extra Watt hours per day you would get changing from PWM to MPPT with 200W of panels depends on a range of factors.

What is the Voc of the panels?
What are climatic conditions like (ambient air temperature, insolation hours and latitude)
What is the size of your battery bank?
What is the state of charge of your batteries in the morning?
What sort of loads do you typically draw during the day?
Are the panels in series or parallel?

The answer is that he may get anywhere from very little to 20 (?) % but without knowing a lot more, it is impossible to say.

[guyrj33]

Originally Posted by chris95040

Watt is a measure of power (energy per unit time). 'watt per hour' is a nonsense unit for this discussion.

Sorry Chris you got that one wrong, how about trying to stay with the generally positive attitude on this list. If you see something wrong offer a polite corrections or clarification.

Watt = Voltage * Current or W=V*I
Watt is an instantaneous measure of power, to do any useful work you need power over time or Watt Hours.

[sailorchic34]

Originally Posted by StuM

Even stating "19 Watts" is nonsense. That's a very precise number. At what time of day? For how many hours? Under what conditions?

Oh good lord. People, watts per hour is just another way of saying watt hour. It helps the folks, who are not engineers understand a bit easier.

I said "approximate" increase of 19 watts per hour. That was based on 65% of rated panel output and a 15% increase with a MPPT controller, which for a 12v panel is generous.

Why 65% of rated output? because that's what I've measured in Northern California and southern Florida with 100 watt 12v panels on sailboats at anchor. Could be 60%, probably not more then 70%.

OP said two 100 watt panels. So they were nominal 12V, with Voc of 19v or higher. Really its the Vmp that is more important as Voc is voltage at zero current flow. We really want current flow.

I assuming 14v with a PWM controller and 17v for a MPPT controler. Of course these numbers change all the time, but lets just use this for kicks and giggles.

A 100 watt panel will output about 65 watts during peak sunlight with a pwm controller. Lots of variables, but this is just an example, It's close in any case. Assuming a 15% increase in output efficiency with a MPPT controller the panel would increase output by 9.75 watts per hour. My real world data from Florida, showed a 12% increase.

In any case 9.75 watts times two panels is 19.5 watts. I rounded this down to 19 as I knew my assumptions were a tad high. I'm an engineer I can do that.

Of course I mean during the peak 5 hours of sun, when the panels are working the best. I probably did not mean at midnight.

By saying approximate, I know that any increase could be as little as 12 watts or as much as 25 watts, for two typical (perhaps older) 100 watt panels. Yes he could wire the two 12v panels in series and in theory increase peak solar output under ideal conditions. BUT in a sailboat with shading issues, odds are more times then not one panel would be shaded.

By shading, I of course mean with as little as one cell shaded more then 50 percent. As the panel cells are wired in series, shading one cell will cut panel output lots. Lots could mean up to 80 percent reduction, give or take a bit. Trying to keep it general ish.

BTW in southern Florida, ages ago, I saw a real world increase of 8 watts per hour or Watt hour, with a single 12v 100 watt panel and a MPPT controller (blue sky), during peak daylight. It was that piddling increase that made me decide to use PWM with an extra panel as it gave more bang for the buck.

Of course I used the word "Approximate" so folks on CF would not jump on my case, for trying to be to precise. Yeah, that worked out sooo well.

[SV Simple Life]

Oh my gosh, what did I start?!

Thanks Sailorchic, I now have far more knowledge than I deserve! As the OP, can I now declare this this thread closed? My thanks to you all! :-)

[Paul Elliott]

Originally Posted by sailorchic34

Oh good lord. People, watts per hour is just another way of saying watt hour. It helps the folks, who are not engineers understand a bit easier.

And I will politely disagree with you on this point. "Watt hours" is correct and reasonably easy to understand. "Watts per hour" is incorrect and confusing, since it actually means something quite different.

I do agree with you on the "approximate" thing.

[Paul Elliott]

Originally Posted by MV Content

Oh my gosh, what did I start?!

Thanks Sailorchic, I now have far more knowledge than I deserve! As the OP, can I now declare this this thread closed? My thanks to you all! :-)

No, no, no! This thread is not closed until we have at least a dozen pages of argument about terminology. Again.

I would guess that whatever you're getting now, with MPPT you will get very approximately 10% more. And this depends on a lot of factors.

[john61ct]

Watts per hour is commonly used to define watt-hours, but I agree is actually confusing, since watt-hours per unit of time is more precisely correct.

However using such nitpicking to put others down is indeed uncalled for.

[48d north]

As we are also putting new 100w panels on our boat this season we appreciate all the treads that have been cropping up lately with this topic. And we especially appreciate sailorchic's responses in lay-person's terms as I struggle with putting solar on a sailboat. (We use alternative energy at home, much easier). In speaking with 2 well known manufacturers of solar kits it was strongly recommended that we stay with a PWM controller because the cost of a MPPT did not start to payback till we were well over 400w and to just add another 100w panel to the set up if we needed. How does that sound sailorchic? Thanks OP for your questions to get this going.

[GTom]

This one was measured and posted by Maine Sail