MPPT Guidance

DIY
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1

With “regular” residential PV inverters, it’s common to oversize PV power a bit, especially when working with STC (vs NOCT) specs. I see both MPPT calculator and Click & Design providing an option to oversize the MPPT, which sounds familiar, but a MPPT is not a “regular” residential PV inverter…

Based on my panel specs I can choose between two seperate 250/70’s or a single RS 450/100 (two trackers), the latter should be my personal preference, but I can’t oversee the drawbacks (if any) of undersized MPPT’s. As the two 250/70’s are 51% undersized and the RS 450/100 even 44%. I understand both work easy within specs, but should I worry about the under-sizing at all ???

The specs of my PV array (single MPPT/Tracker) are:

SmartSolar MPPT RS 450/100-MC4

  • PV input voltage

    • MPPT max. input voltage: 450 V
    • MPPT min. input voltage @ mpp: 65 V
    • PV input voltage at min. temp.: 202.71 V
    • PV input voltage at max. temp.: 178.13 V

  • Output current

    • MPPT rated charge current: 100 A
    • MPPT max charge current at min. temp.: 44.66 A
    • MPPT max charge current at max. temp.: 39.39 A
    • PV power ratio: 44% (Undersized)

SmartSolar MPPT 250/70 Tr VE.Can

  • PV input voltage

    • MPPT max. input voltage: 250 V
    • MPPT min. input voltage @ mpp: 59 V
    • PV input voltage at min. temp.: 202.71 V
    • PV input voltage at max. temp.: 178.13 V

  • Output current

    • MPPT rated charge current: 60 A
    • MPPT max charge current at min. temp.: 46.05 A
    • MPPT max charge current at max. temp.: 40.62 A
    • PV power ratio: 51% (Undersized)
2

Undersized means you aren’t using the full potential of the mppt and could probably use a smaller one. But also its not a bad thing as in no damage can be done really.

What wattage are you panels?
A quick calculation is
No. Of panels x wattage ÷ nominal battery voltage for sized amps the charger needs to be.

E.g. 6x 500w = 3000W ÷ 48v = 62.5Amps. so a 250/ 50 or 70 would be great.

3

Undersized means you aren’t using the full potential of the mppt and could probably use a smaller one.

Understood!

But also its not a bad thing as in no damage can be done really.

Thanks for the conformation! Guess I loose (some) efficiency from the MPPT itself by under-sizing, but it’s the max. PV Voltage (>250 MPPT Model) and my “requirement” to use VE.Can which limits the choices a bit for a smaller model. So the RS is under-sized on both (max.) Voltage & Amps, the 250/70 just Amps. As the source of my question was about other considerations or best-practices, besides the laws of physics, using an under-sized MPPT. I’m now happy to order a RS 450/100 :star_struck: .

What wattage are you panels?

I’m using two strings of 5 Panels (410Wp STC / 308Wp NOCT), so that would be max. 1540Wp per string. Their pointing at 310° fixed (35° slope) in the Netherlands, so it would surprise me if they’re going to deliver anything more than 1000W-1250W on the most ideal summer day (about 26A).

As one of the two strings see some shadow during the day, I’d like to track them separately.

4

Just keep in mind that at lower lught level the 450/100 will start uo later than the 250/70 and probably shut down sooner because of the start stop voltages.

Although you can’t beat this ability. Unless you get 2 smaller controllers which then adds wiring and bit of complexity i guess.

5

what do you want to do with the harvested PV Energy? Store it in a Battery or consume most of it directly / feed it in the Net?

I ask because i wonder - i don´t se much sense in DC MPPT Converters for ESS Systems

6

@lxonline

Just keep in mind that at lower light level the 450/100 will start up later than the 250/70 and probably shut down sooner because of the start stop voltages.

Valid point… :upside_down_face:

Although you can’t beat this ability. Unless you get 2 smaller controllers which then adds wiring and bit of complexity i guess.

Yeah, only had the RS “in my basket”, didn’t apply “buy & deliver” yet… Need to do some more calculations for my final decision, at least I know I have two options now. One device is simpler indeed, but if I need the extra wires, well… But it’s also another Lynx, because with 3x MP 4k5 and two MPPT’s I’m missing a port to connect the second MPPT at my busbar :scream: . I do have a single port available at the other Lynx (T-Class) where my battery is, but I don’t think my MPPT should be connected there. Well enough to think (and worry) about, still, but thanks again!

@kr0815

What do you want to do with the harvested PV Energy? Store it in a Battery or consume most of it directly / feed it in the Net?

I already have a 2x 5 Panel 3kW AC (Enphase) setup at 130° (the other side of the roof). The MPPT’s are going to (help) charge the batteries and/or feed the in-house AC demand if needed (with the help of three MP 4k5’s for the DC/AC conversion). From 2027 it doesn’t make much sense to feed back to the net in 99% of the use-cases in the Netherlands, because you will get charged seriously for that.

I’m not going to bother you with all the stupidity of the Enphase software eco-system. Let’s just say I’m loving their Micro Inverter technology (really, they are a great piece of engineering), but everything else is amateuristic or blunt stupid… But let me skip to your next question:

I ask because i wonder - i don´t se much sense in DC MPPT Converters for ESS Systems

In my humble opinion MPPT’s are fabulous in ESS Systems, because you directly charge your DC batteries with DC current. As my Enphase system connects to the AC-Out-1 of the MP 4k5’s, they will cover the home AC loads first and charge the batteries second with any excess power.

Be aware that the source of the AC power from the Enphase system started as DC (converted to AC by each Micro Inverter), so charging my DC batteries from that source will get another AC/DC conversion for battery storage and again a DC/AC conversion when consumed. I’m not a native speaker, but I believe the saying is “Third time’s a charm…”, well, that isn’t true for power conversion :innocent: .

Another thing is controlling the charge loads with 100% SoC. When connected to the grid the Enphase will feed back in to the net, which will cost me money (see above), when off-grid the MP 4k5’s (because they are in charge of the main power now) are capable to “frequency shift” the AC feed by manipulating (slightly ramp up) the standard 50Hz AC frequency. That will work for limiting the AC inverter, but it has all kind of side effects I don’t want. The simplest example would be that your digital clocks will be a bit faster than normal :nerd_face: , but I have some other concerns/workloads for not wanting that type of PV Inverter throttling.

Did I already mentioned the stupidity of the Enphase software eco-system :laughing: ? Now they do provide a ModBus connection for throttling with their local monitoring device (Enphase Envoy), but mine is not capable at doing that (simple version). I don’t have the required firmware (on-purpose) and even than you need a “special” support request hoping someone will respond and allows (sigh) you to use that functionality.

But back to the MPPT’s, these devices can be throttled/managed by your GX and if my Enphase covers my house load and/or the battery charge demands, the MPPT’s are dismissed for a while :). If in any situation my batteries are loaded, the MPPT’s are shut off and I still have excess PV power, I’m going to start a anti-salmonella run with my heat pump or just heat the water in the boiler towards 70°C :wink: . But that scenario would be a real outlier, my Solar Power is quite balanced for my house loads + ESS.

TL;DR

In my humble opinion MPPT’s are fabulous in ESS Systems…:innocent:

7

I agree for battery charging directly (if you are cycling) DC mppts are the way to go.

For running loads directly AC coupled is excellent. But using AC pv to charge batteries is harder on the victron inverters and less efficient as now there are two (actually 3)conversion paths.

8

For running loads directly AC coupled is excellent. But using AC pv to charge batteries is harder on the victron inverters and less efficient as now there are two (actually 3)conversion paths.

True. The challenge starts when being grid connected and you don’t have a (supported) way of throttling down your AC PV system, like my situation explained above. When having excess AC PV power on top of your house load, this needs to go somewhere. That could be feeding back to the grid, which get charged significantly due to net congestion or use it to charge your batteries for later use. Inefficient? Sure! That’s why I made my “Third time’s a charm…” remark earlier.

But from a “Victron” point-of-view, when you’re grid connected, things like “Frequency Shifting” will not work because your MP is not in charge of the AC power (Grid Code), the grid is. So in that case, although inefficient is the ‘best’ (economical) way to charge with your excess PV AC instead of the way more efficient MPPT route. Of course this is totally stupid, but mainly the result of failed government policy by endorsing PV systems for citizens (among others… :smiling_face_with_sunglasses: ), but don’t scale up the grid capacity at the same time…

9

Sure, making use of what you already have is economically sound as well.

10

Generally speaking 2/3 DC-PV (MPPT) and 1/3 AC-PV (Fronius, Enphase, Hoymiles…) is the optimal solution for most users with Victron MP2 coupled battery.
If you can manage loads significantly (EV at home during day) or have loads with high correlation to sun, e.g. air conditioning more AC-PV might be better.
If you have high share of night loads, a higher share of MPPTs would be better.

11

sorry, but here i disagree

all the consumption we have in the end is 230V powered, except the Cerbo no 52V users

Today we had 120 kWh from Sun - i don´t care much about efficiency to load the 30kWh Batteries - that were still 70% full

Problem is the winter - not much sun, cloudy sky, fog

we have 25kWp of PV (Hoymiles Microinverters) , but still far away to be enough for the HeatPump

My goal is to heat the house as much as possible with energy from the sun

I don´t think it is really clever to push this energy through MPPT / MP2, better to use them directly

12

As always, it depends…

If you can start from scratch, have a few hundred square meters of roof facing east, south and west (Northern Hemisphere) and start designing your PV system, you probably will find an excellent balance between direct AC and DC (MPPT) connected panels.

Now the reality, like my situation: An existing AC PV (facing 130°) system that produces more than I can directly consume with limited to no control. Not enough excess power to help me through the night. Now it makes perfect sense to not extend this system with even more AC PV (facing 310°), but use MPPT’s here. I do have full control over them, so I can “adapt” to the uncontrollable AC PV power, and extend my solar input from (South) East (130°) to (North) West (°). Best of both worlds IMHO.

Now every situation, house load and PV setup is different, so is the “perfect” setup.

13

I agree, with the special case to optimze for winter operation and heat pump with a factor 5-10 between summer and winter production, a share of AC-PV for direct soncumption for the heat pump might be better.

That’s why I stated “generally speaking”. For special cases and partial optimization e.g. winter production in norther hemispahere for heat pump operation optimal setup might diviate significantly. I have 16 of my 20 panels on Hoymiles micro grid inverters due to heavy shading and Victron MPPT not being compatible with optimziers like TIGO.
Nevertheless, the MPPT 150/45 with 2p2s panels is more efficient thanan HMS-2000-4T with some panels in same direction while not shdaed. If it get partially shaded, the HMS ist more efficient as it has four MPPTs, one per panel.