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How can I quantify expected benefits from switching from PWM to MPPT ?

WoodsieLord

New Member
Joined
Oct 4, 2019
Messages
145
Location
Buenos Aires, Argentina (230V monophasic, 50Hz)
Hello!
I got into solar in an unusual way: I wanted to use an UPS I already owned as my inverter. So I made my first purchases based on a sellers advice trying to match my budget and my technical requirements (48V. Since my UPS runs at 48V). At this point, I was aware of some of Will's videos but joined the forum slightly later.
This is what I've bought:
4x 340W mono panels: which I arranged them in Two series of two.
48V, 40A PWM charge controller.
Later on I got a small lead acid (gel) batteries that I definitely need to upgrade. (Please note that I can't get LiFePO4 batteries in Argentina, so I would stick to SLA for now!)

So! I've got the system working some way or another almost two months ago. I had ZERO experience in solar so I'm excited, happy and I already realized that this is just the beginning. I want more solar.

Recently I've aquired two additional panels (awaiting installation) and It's been a while since I've been reading about the MPPT chargers.
Apparently they're so good that almost nobody in this forum even invest in PWM charge controllers at all. So I would like to extrapolate what kind of benefit I would get if I were to substitute my PWM controller.

Most videos on the internet explain that PWM uses parallel panels while MPPT uses serial configurations so that the array voltage is higher. I take it that the MPPT algorithms "play" with this voltage headroom to squeeze more juice. The thing that puzzles me is that in my current configuration, I bet I wouldn't need to change the panel arrangement. Open voltage is currently around 98V. Maybe it's too much for the PWM controller and is wasting power.
Wiring is already done, I already got the battery bank (48V) and the inverter, so the only element I would be swapping is the charge controller.
How can I calculate what kind of benefit would I be getting?

My PWM controller was worth u$d 118. As an unfair comparison (items in stock in my country):
...a Victron Smartsolar Mppt 100v 20a 12/24v (I know, it's not 48V) is quoted at u$d 347.
...an Xtra 4415n Mppt 150v 40a 12/24/36/48v Epever is quoted at u$d 517
Other options are chinese brands that are not even mentioned or unknown

I want to know if it is worth investing in an MPPT controller or if I should buy more panels. Usually, electronics are expensive in my country but solar panels are somehow slightly more accessible.

Thanks in advance!
 
Something similar was covered recently, take a look at these two posts for some insight regarding PWM vs MPPT. Voc of the array only really matters in terms of not blowing your charger up, what you need to pay attention to in terms of power production is Vmp.

Even with Vmp in the vicinity of battery voltage a PWM controller will ditch some of the potential production. The further Vmp is above battery the worse it gets. What is the Vmp of your panels? From the Voc of your array, I'm guessing array Vmp is 78v. If so you are going to be taking a pretty bad power hit with PWM into a 48v battery.

If you post the stats of your panels, Voc, Vmp, Imp etc I can do some cacls for you.

 
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2 in series gives you a Voc of 77.72v. Imp with two sets in parallel is 17.5 amps. That yields max power of 77.72 x 17.5, 1360 watts (no surprise there, as this is 4 x the Pmax, just showing the working out). If we PWM into a 48v battery we pull the array voltage down to a little above battery voltage. Assuming the nominal voltage, Pmax from the array now becomes 48 x 17.5, 840 watts. Ouch, that's a pretty big deficit to make up compared to a MPPT controller which will largely preserve the 1360 watts position. We need to add 3 more of your 340 watt panels so for your array that means 4 in practical terms.

So there it is. Is a purchasing a MPPT controller cheaper than 4 more 340 watt panels? How about the cost of racking and cabling for the additional panels? Do you have the space to mount 4 more panels?

There's more to it than just what I've said, you don't need to use the exact same panels, exact same Vmp nor even same Imp ratings with your PWM setup. With PWM the Imp is the more important figure as this is the current the panel will produce in full sunlight when pulled down to 48v. A higher voltage panel with Imp of 10 amps will produce exactly the same power as a lower voltage panel (the panel/string must be capable of producing more than 48v for a 48v battery system) with an Imp of 10 amps when using PWM.

Panel performance changes with light exposure and temperature so that 1360 or 840 watts may well never be achieved with PWM nor MPPT, but that gives you the general idea.

*edited to add more information*
 
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Crystal clear. Thanks again. I'm convinced that I wasted money with the PWM SCC. Yet, it fueled my interest in Solar to start with, so I'll forgive myself and start saving for my next step.
Regards!
what did you buy, I notice the quote for the XTRA4415N is double for that model to what we pay in EU
 
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