Undoing overpanelling by adding an MPPT

I need some advice on if it will be worth adding another MPPT to my system. I have 16 450W panels which in theory is 7.2kw on a victron 250/100 mppt. I can see every day that I am hitting my limit with the charge controller quite early. I thought about moving one string to a new 250/70 and leaving 3 strings on the 250/100. That way I should always get the max out of my panels. I dont really have good space to add more panels - so I thought I could maximize what I already have one my roof.

Is this a good idea? Or is is just a waste of effort and money.

Well, you can probably do the math on that one. The solar power curve is half a sine-wave if I am not mistaken. I reason, because power is proportional to cos(theta), where theta is the incoming angle of insolation.

You want the area under the curve, so you have to integrate over ∫cos(x) dx, which is sin(x), but you have to imagine a little theta1 and theta2 below and above that integral (the starting and and ending angles where solar starts/stops clipping in the morning and afternoon).

So the portion of energy available over the clipping period will be sin(theta2) - sin(theta1) where theta2 is the angle in the afternoon where it stops clipping, and theta1 is the angle in the morning where it starts clipping. Use that to scale from what you are actually making to what you could be making, multiply by 0.8 (efficiency) and price of electricity, and calculate sunny days to pay for the MPPT.

Measuring the angle of the insolation is left as an exercise for the reader. I imagine you could do it by holding a stick in such a way that it throws no shadow, and then measure that with a protractor, or employ some more trig :slight_smile:

I forgot a lot of this calculus, but something like this. You can calculate the area of that slice between the two angles (as a fraction), and then work out how much larger the slice is than the triangle under the limited line.

It is also possible that I have my angles swapped…

My rough estimate if I look at my graph on vrm (not using any math) would be that there is about a 4-5kwh possible gain. Lets say it is use-able for 1/3 of the year (when I actually need it and the sun shines). It equates to nearly R2000 a year in additional savings, so probably a payback of about 7 years. Not great - but also not completely a waste…

Any other caveats to adding a second MPPT ? It looks like a fairly easy modification. Some new DC cables, fuses, and networking the two MPPTS via victronconnect.

In some cases you should not do this. For example, if you are running an ESS system (you have a Multi/Quattro in the same system), or if you have a CAN-bus battery, then the battery or the inverter/charger controls the MPPTs via the GX device, and they should not be networked.

Currently my smartsolar is connected to the cerbo via the VE direct port. Should I change it to VE.CAN and then daisy chain the second or is there a different way?

That’s what I would do, yes. Put it on the CAN-bus and daisy-chain it. Otherwise you need another VE.Direct cable.

Faced with a similar dilemma, this is what I would do. ( Actually, I have done it).
I assume you have a Victron inverter.
I have arrays facing North, East and West.
I couple the East and West arrays through the MPPT. This arrangement separates the individual arrays’ power peaks at different times of the day. The East array peaks before noon, and the West array peaks after noon.
The combined E-W power peak is still noon but considerably reduced.
The north-facing arrays produce a power peak at noon. Instead of using another MPPT, I have reconfigured these panels in an HV series array and used a PV inverter ac-coupled downstream of the Victron inverter.
(PV inverters like Solis have built-in high voltage MPPT(s). They can be less expensive than a Victron MPPT and increase the available AC generation to boot.

Read this chat, jumped up to check if my 150/35 was CANbus (less wires!) , but it is not.
The 250/85 is, so why not use the CANBus port, then I have a VE.Direct cable spare …

If both MPPTs were CANbus, yes connect, no don’t … :rofl:

So my question is, for clarification, if one MPPT is Canbus, and the 2nd one is not, can one be one on CANbus, the other on the VE.Direct cable in an ESS system?

Because the context of the term “network” doesn’t mean “connect”.

You can “network” the MPPTs together using VictronConnect. They then act together, go to float together, etc etc… in systems where you don’t have external control via the GX device.

If the GX device does external control, then don’t “network” the MPPTs with VictronConnect. Otherwise it interferes with the entirely different “networking” done by the GX device.

Do yourself a favour, start VictronConnect, connect to an MPPT, and tap around a bit, you will see the option to create a network there. That network interferes with other kinds of “networks”. That was the point :slight_smile:

AHA!! Thanks.

Jip, maybe I did, or maybe I did not, :slight_smile: “browse around a bit” with that one day, or not.

Decided nope, too much can go offline at the most inopportune moment vs wires … wires are ALLWAYS better.

How do you measure these profiles?
I’m thinking that having an ammeter on each string would be handy…

I favour the flexibility of a clip-on ammeter. Once the whole array is decided upon and connected the combined current is read in VRM.
I must admit though, I have had it easy as the apex of the roof I have done this on is almost true N-S.
So I can mirror the E-W panels at the same pitch on either side of the apex.
Practically I have guessed the optimum number, then installed the panels, measured at noon in the summer, added some more panels, and so on. until I am within 5A of the mppt’s max output.
I have never noticed clipping, but it could be there slightly.
With my next build, I want to explore if there is an acceptable amount of clipping.
I am prepared to clip, say 5% at noon, say for 5% of the time, if it means I am increasing the utilization factor of the MPPT by a far higher percentage.
Or in a car analogy, (favoured on this forum) I am willing to have the governor kick in now and again at 100km/h, if I can increase my average speed from 60km/h to 80 km/h.

Edit: I don’t have an optimum clipping factor. My thinking is not mathematically defined, just a gut feeling.


How do you know if the MPPT is clipping?

It manifests as a flat-topped production curve.

I presume you always keep the number of panels on the E and W the same??

I do, certainly, the series voltage of the string has to be identical.
However, I could see instances where different pitches for E & W respectively may take advantage of asymmetry.
Shade issues and Summer weather patterns would also be factors.

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