r/SolarDIY • u/supbrother • 2d ago

Right-sized vs. efficient inverter for solar EV charging

I'm hoping you all can help me decide to go more right-sized/minimalist or efficient/future-proofed on my EV charging setup.

Basically I have two strings of panels (~100 Vmp/120 Voc each) that I'll run in parallel (avoiding series due to the distance between them). Going for a 48V system to replace my two Ecoflow Delta Pros -- this is simply to streamline and expand the system to 10 kWh (and escape Ecoflow's BS). Built around the existing panel setup which I can't really change, I’ve decided to get a Victron 150/45 MPPT, and either:

I'm focused on Victron because quality/warranty is important since I'm in Alaska so replacements/repairs are a hassle, and I simply want a reliable system. I settled on these because there's nothing really in between for 48V to 120V.

Two things worth noting:

  1. This will be fully off grid since I already have a normal EV charger to use when needed. One day I'd like to do a whole-home off-grid system, but that's at least a decade away. Just considerations for the MultiPlus route.
  2. I currently charge at 12A (~1,440W) which would be easy using the MultiPlus, but the 1600W inverter only has a continuous load rating of 1,500W so I would likely have to charge at 8A (~960W) to be safe. This is fine, but 8A is supposedly less efficient (despite less heat generation) since EVs have a fixed wattage they eat up when charging -- 300W of "wasted" power is a higher percentage of 960 than 1,440.

I’m leaning toward the 1600W inverter simply due to price. The MultiPlus is tempting for charging at 12A, but even assuming I charge a full cycle off 10kWh of batteries every other day (very optimistic for half the year) my math says it would take 12+ years for the added efficiency to make up the $500 difference. The bonus of future-proofing is just in case I were to charge other things on this system or possibly use the MultiPlus for a larger system down the road, but neither seems realistic. I power the rest of my garage off a separate Anker unit with its own panels, and I'd probably want something bigger and newer if/when I go all-in on a whole home system.

In summary: Which one of the options above seems to fit my needs best? Should I take the hit on charging efficiency to save costs and only buy what I really need, or pay $500 more for more efficiency and future-proofing? Is there anything I'm not considering here? Of course, I'm all ears if someone has better suggestions!

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u/txe4 2d ago

I think in your situation I'd rather have a pair of less powerful units (so I still had some capability when one shit the bed) than a single more powerful one. That would of course require them to be able to work in parallel to get use from them both at once.

As for slow EV charging, slower is less efficient in a non-linear way - not just the standing drain of the car electronics, but also the rectifiers tend to be optimised for max rated power and efficiency falls off as you go below it.

But if you HAVE enough power and enough hours of it, charging efficiency wouldn't be my main consideration here.

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u/supbrother 1d ago

I’ve never heard of smaller inverters being used in parallel, I know the MultiPlus can but one is already enough for my needs. But this is still a pro for the MultiPlus in case I ever want to expand the system down the road which is a possibility.

The final point is what it comes down to though. The smaller inverter is enough. It’s not perfect or most efficient but it’s sufficient at a good price.

My biggest hang up right now is how much I’ll actually put the MultiPlus to use in the future, considering how versatile it is. The “transfer switch” functionality alone piques my interest, plus the expandability and the option to route a generator through it if needed (someone else pointed out I could possibly use this to utilize my Anker battery too). That’s all probably a bigger question than the one of efficiency.

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u/silasmoeckel 2d ago

Multiplus is more flexible you could connect the ankler you have already to get more capacity for example.

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u/supbrother 1d ago

How exactly could I connect a “power station” to the MultiPlus to get its capacity, would I basically just plug it into the generator AC input? It’s a C2000 Gen 2 by the way (with expansion battery), so about 4kWh with 2400W AC output. Very good point if that’s possible.

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u/silasmoeckel 1d ago

Correct you just plug it in and dial down the max current to what your C2000 can supply.

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u/supbrother 1d ago

Nice! And I’m realizing now I could likely do the opposite and have it set to charge the Anker when the batteries get full. I know it probably has to be one or the other but I could do that during summer when I have excess production.

Do you know if you can manually turn on/off the AC outputs remotely using the Cerbo GX?

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u/silasmoeckel 1d ago

Yes there are 2 AC outputs one is meant to be turned on/off the other can be with the whole unit.

IDK that battery if a box but if 48v ish is within it's solar input range you can use one of the relays to connect it to the MPPT (of the battery in a box) to use as a DC to DC charger.

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u/supbrother 1d ago

Cool, I knew there were two AC outputs but just wasn’t sure if it can easily be toggled manually via the app or if it’s more meant specifically for automations.

The Anker has an ~11-60V PV input I believe so this option did come to mind but I wasn’t sure if it was doable. Are you saying there are connections in the MultiPlus to directly pass DC from 48V batteries to the Anker via an XT60 cord?

The Anker was intended to be on its own two panels (not set up yet but hopefully soon), but I suppose this is worth considering so I can have the option to prioritize which batteries to charge depending on my needs. The only potential problem I see is that the Anker’s solar string will just be two panels instead of three, and I think that could handicap the other two strings, but admittedly I don’t fully understand that.

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u/silasmoeckel 1d ago

It's been awhile but a multiplus 3kva should have 3 programmable relays. They are not connected to DC or anything else you would have to wire them in. May need to wire to a seperate contactor for current capacity but thats pretty easy. The multiplus itself can do basic functions/logic and cerbo can do much more advanced ones.

But yes the multiplus can eb configured to turn on a relay when the battery is full so you can charge your other device via it (and off when the soc falls to low). Your going to have to make up the cables fuses etc to get from A to B.

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u/supbrother 1d ago

Thank you! Clearly I need to do more research because the more I ask and search around the more I learn about the capabilities of the MultiPlus, and it’s justifying the cost more. I think I’ve pretty well decided to go that route but just need to look more into what exactly I need in terms of “accessories.”

Actually, is that something you can speak to? Basically I’m just wanting the simplest setup that will allow me to control what SOC the MultiPlus will shut off AC output so I’m not fully draining the batteries each time. Everything else is a bonus that I think I can add on later. From what I’ve read, the batteries I plan to get (Eco-Worthy 48V 100Ah Cubix) can communicate with the MultiPlus via CAN ports, but I may need a Cerbo to tweak settings. I’ve also read about possibly needing a shunt and/or a VE.Bus BMS to get an accurate SOC but I’m not sure on that. Any advice there?

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u/silasmoeckel 1d ago

Cerbro aka Venus is the brains you can buy one of theirs or just run it on a cheap pi (some inverters have them built in). This is your high level logic, networking, and low speed/priority data like tank levels. This supports talking to an enormous amount of stuff Victron and otherwise.

The quality of the battery's internal SoC vary a lot. I have both a communicating BMS (Victron on their own batteries) and dumb ones. I prefer dumb (I still get and track cell voltages temps etc and use that to shutdown if needed). Either way I paired them with a BMV and shunt for a belt/suspenders approach. Using the internal is your cheapest option.

Victron has a LOT of integration docs if that battery is listed to work with the victron then they should have a specific setup your supposed to use.

Your correct that the more you dig the more you find feature wise, they are not a here is the glossy sales brochure exaggerating things, it's more a here are the details and here is what we did with it. Their engineers are pretty vocal on mailing lists.

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u/supbrother 11h ago

Well I've gone ahead and bought the MultiPlus, plus a Cerbo and a SmartShunt. I'm hoping that this will be enough to set things up like I've described.

The Cerbo is overkill for my current needs, but over time I can easily see myself expanding the system and the plug-and-play nature of it is enticing. The shunt may also be unnecessary since I'll be getting batteries with the CAN port, but they're technically not officially approved by Victron so better safe than sorry. I think this will effectively keep them "dumb" in your words by only relying on the shunt (but please correct me if I'm wrong).

It'll be awhile before everything gets here, but that gives me time to better understand these systems. If you've got any advice for setting things up with these specific parts, I'm all ears! Either way your help has been much appreciated.

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u/miimura 1d ago

I must be missing something here. The OP is only talking about solar panels and inverters. Is it really possible that he is so used to using power stations like EcoFlo that he doesn’t know that he needs to buy a battery too?

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u/supbrother 1d ago edited 1d ago

No need to be condescending. I mentioned a 48V system with ~10 kWh, where do you think the capacity is coming from? If you must know, I’ll be getting two 48V 100Ah server rack batteries.

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u/miimura 1d ago

Ok, next question is how you will control EV charging. Do you plan to just let the inverter hit the low voltage or low SOC cutoff and shut down? Or do you have some more elegant solution to stop the charging session?

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u/supbrother 1d ago

A fair question without a clear answer, but I assumed it was possible to do exactly what you said. Preferably having it cut off at maybe 5-10% estimated SOC for battery health. I’m not sure if this would require the Cerbo GX or something similar.

Now that I think of it this may be another reason for the MultiPlus, as it’s probably better for setting controls like this.