Mppt vs pwm

[doughpat]

Hi all, I have a very good price on brand new PWM Zamp solar controller, and the salesman is claiming that for a relatively small set up, say underneath 600 W, wired in parallel, there is no advantage to using MPPT. I plan to use 2 to 3 Solar panels.

[uncle_bob]

Hi

Ummm..... errrr .... why would that be true?

I'd say the only likely reason is that he wants to move out the inferior controllers he has sitting in stock.

Max power point works at any power level. It improves the performance of *any* solar setup.

Bob

[doughpat]

I think the idea was that when wires in parallel, voltage stays relatively low. If I understand correctly mppts are able to utilize excess voltage, which doesn’t tend to occur with relatively small systems wired in parallel.

[AirMiles]

Originally Posted by doughpat

I think the idea was that when wires in parallel, voltage stays relatively low. If I understand correctly mppts are able to utilize excess voltage, which doesn’t tend to occur with relatively small systems wired in parallel.

At what voltage do your panels operate? Mine operate at 17.9V. https://www.renogy.com/100-watt-12-v...ompact-design/ With a PWM controller, you would throw away 3.5V when charging at 14.4V. Why would you throw away 3.5V/17.9V to save save a few bucks on a controller? That's a nearly 20% waste of your panels. I would only buy an MPPT controller. I want to convert every volt my solar panels can produce into amps so my batteries stay fully charged. Roof space is limited on all Airstreams, so there's no room to waste 20% of panel output.

[uncle_bob]

Hi

A MPPT controller changes its operating point to match whatever the cells are putting out. Depending on how much sun you have, that number could be just about anything. An old style controller makes a single guess about what should be right. It set of sun conditions it will be right. In all the rest, it will be wrong. This is true if the panels are in parallel or if they are in series.

Bob

[GMFL]

Originally Posted by uncle_bob

Hi

A MPPT controller changes its operating point to match whatever the cells are putting out. Depending on how much sun you have, that number could be just about anything. An old style controller makes a single guess about what should be right. It set of sun conditions it will be right. In all the rest, it will be wrong. This is true if the panels are in parallel or if they are in series.

Bob

Correct. MPPT (Maximum Power Point Tracking) adjust its charge profile to match what it is given to maximize its efficiency. It can do MORE with LESS.
PWM does not have the ability to track change in incoming voltage.

[waninae39]

Mppt is the modern and more efficient. it can optimize power usage from the cells

pwm is old and outdated. it works, but you will not get the most from your cells

[gator.bigfoot]

It all depends on where you camp and what type of panels you have. PWM is great and reliable and works well with the lower voltage panels. But MPPT will operate like a PWM if the voltage becomes too low. So in reality it really depends on what you are paying for the MPPT. With the cost of MPPT becoming very low the cost benefit factor that PWM enjoyed is eroded. In the excellent article from Victron you can see the difference for yourself. You can also see that MPPT works best with the higher voltage panels (greater than 24V) https://www.victronenergy.com/upload...WM-or-MPPT.pdf you can also see that PWM and MPPT performance is only equal at 75C excerpt. Conclusion: at Tcell=75°C and Vbat=13V the difference in performance between the two controllers is negligible. Also at lower temperatures where the battery voltage will increase you will have issues with the lower voltage panels. MPPT was made for the higher voltage panels so unless you wire the panels in series in these situations you will not get the most out of these low cheap low voltage panels. But if you wire in series you also have more effects in regard to shading. To offset the shading effects you need more panels. Ideally we would have room for the 60 cell panels and we could use optimizers but with an AS your space is limited.

Where I camp (under trees) I rarely get full sun on one panel for more than an hour. Most of the time all of them are shaded (and heavily). I run the PWM, but if I had to do it again I would run one PWM for each panel. Mind you if I had to do it again I would buy the higher voltage 24V panels and get the MPPT.

When we were designing and building our own solar factories (and for others) we only concentrated on one thing. Maximum power. No one cared about 12V panels that were compact and efficient. There wasn't any money in that. Heck the Amorphous panels we built were 3x3 meters. Try putting that on your roof! The smallest crystalline panels we made were 60 panel ones, but the 72 was the norm. We also didn't design the panels in regard to shading. Yes they were strung in series and parallel with a couple of bypass diodes thrown in, but not like we use in the RV world. I wish I had gotten into the RV solar in those days, I would have liked to build the "ultimate" RV panel with lots of bypass diodes for shading.

My advice, in this day and age the MPPT is only slightly more than the PWM so go with the MPPT. But stick with quality products like Morning Star and Victron. And while you're at it, upgrade the wiring from the roof and to you batteries and don't forget the temperature compensating probe if you batteries are located outside of the unit and away from the charge controller.