timv avatar image
timv asked

7kW Solarinstallation coupled to grid without feeding to it but charging an 40kWh lithium battery


I have bought myself 7200Wp of solar panels. 24 x 300wp facing a bit of south. Incline on the roof at 50degrees.

I also have an 40kWh lithium battery. 50V min 60V max. Max charge and discharge power is 9kW. Max discharge of 18kW for 2hours. The battery will be expand every year until the first place is filled and then I have 100kWh. There is already started on making place for an second battery of another 100kWh. But that is a bit dreamy.

Now the goal is that I put the entire house offgrid, but have the grid as backup and pull in when the battery is flat. I may not feed back to the grid. In summer I think, depending on how much electric distance we drive, we can make it 4 months completely offgrid. In Winter the house will be almost offgrid and the cars will be charging most of the time at night from the grid. Offpeak time electricity is 12% cheaper.

We have a plug in hybrid. Charging is 10kWh in 3 hours or 11kWh in 8 hours. +200kWh a month. And a full EV. Charging every day 10-15KWh in 3-5 hours. +300kWh a month. Sometimes we need fast charging in between runs and then I want to be able to charge 3 phase 16A =11kW, this may be grid charging. The car can handle up to 22kW 3 phase 32A but my grid connection is only 3 phase 25A.

So the configuration needs to be a 3fase configuration with a power output of + 15KW.

What gear do I need?

Thanks in advance



offgridsolarVEConfigure 3
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3 Answers
wkirby avatar image
wkirby answered ·

Hi Tim

That is a very interesting system that you propose there.
Unfortunately, a system of that size and complexity is a bit beyond what this online community can reasonably advise upon.
It would be best to contact your local Victron dealer and ask them to design the system and specify the appropriate equipment for you.

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Fideri avatar image
Fideri answered ·

Did you ever instal system? I know you were advised to engage a solar specialist for system of that size, but that is not an option where I live. The so-called specialists disappear on the day they commission your system and get paid. You have to understand and tweak it yourself, not to mention the troubleshooting.

Anyway, I’m looking at exchanging practical information.

My planned capacity is a 42Kwh system. The only constraints I have is that I have already bought the batteries, I am about to order solar panels of 6800 watts and my «specialist» insists on using the rest of Victron equipment, which is ok with me.

Desired Set-up: 42KwH capacity, hybrid system (grid available but unreliable), no feed to grid.

Already bought: 4 LifePo4 batteries which I will wire in parallel to get nominal voltage of of 51.4 volts with 800ah at 48 volts. The batteries charge current is 100amps each of the 4 pieces. They have BMS built in.

Ordered: 20 pieces of PVs, amounting to 6800 watts (each has 340 watts, VOC of 46.4 volts and VMP of 37.4. The Maximum Power Current is 8.56 amps).

I plan to wire these in a series of two each, so that will be five series and one parallel string into one MPPT. I intend to buy the same number and type of modules and wire them the same way, so I will have 6800 watts x 2 . My specialist recommends 40 pieces of 320 watts each, so 12,800 watts.

Planned to buy: Two pieces of 250/100 MTTP chargers. My specialist recommends five which sounds like an overkill. Anyway, one will be in reserve for now. Later, I plan to connect it in Parallel with the initial one when I buy more PVs. I also plan to buy two Quattros 10,000VA which will operate in parallel. My installer recommends six! I will not be using the charger and power export functions.

Yes, I have the roof real estate and no shading issues. Plus the sun is reliable.

Any pitfalls?


P.S: Forgive me if I have used the wrong terminology!

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timv avatar image timv commented ·

My system is running. I collected all items myself and mounted and connected everything myself. And it works.

I have a youtube channel with the results. Search for Tim's powerwall.

The amount of inverter power depends on the load. So I cant confirm the 2 or 6 needed.

The amount of mppt is more then 2. 1 mppt of 250/100 can only give 100A output. 100A @ 40V (empty battery) = 4kW with 2 you can only harvest 8kW of the 12kW solar. At absolute maximum voltage 5.76kW x 2 is 11.52kW solar.

I would recommend 3 or 4 mppt for the 12800Wp solar. I have an solar aray of 3600Wp. On cold days it jumps over 4000W. So if its cloudy and the sun comes out. You trow away power. And when not, you don't run the mppt for hours at its rated maximum. Should live a lot longer with more mppts. If 1 fails, you then dont lose 50% solar.

My system is expanded. I now have aray 1 with 3600Wp solar. Aray 2 with 3600Wp solar. Aray 3 with 2655Wp solar. And have everything to finish before winter aray 4 with 2655Wp with the option of 3 more solar panels. My battery is now 56kWh and will be upgraded to 70kWh before winter.

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dayandnight avatar image dayandnight timv commented ·

Hello @TimV,

I just looked at your video. Impressive how many cells you have handled. Good job! I was wondering about the layout with 5 rows, each with 12 cells. You wired the middle row with a much longer fuse-wire as the other cells. You don't worry that there is much higher voltage drop on this innermost cells as on the outer cells?

Did I understand right that each package with 60 cells has a total capacity of 10 Ah? So each row in your shelf, it was 12 packages isn't it, has about 120 Ah which gives you at 60V per row about 7,2 kWh. Or did I miscalculate it? How many package you can store until Sten brakes? ;-) I like your unconventional wooden shelf! Great Idea!

I am afraid I didn't understand what are you specially asking about in the beginning of the thread, because your system is already running with 3-fase victron?

Are you looking for a special wallbox designed to your needs which communicate in the idea of Victrons ESS and supply the neccessary power during the night to your EVs out of TIMs powerwall?

Kind regards from Munich


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timv avatar image timv dayandnight commented ·

The question was asked 13 months ago. 2 months later I orderd all the stuff I tought I needed. And planced and installed it myself. I self tought myself learning from the Victron information sites how I could make my system work. There is the limit of power. When the EV is charging with 11 kW. There is not much inverter power left for other stuff. I can only charge the EV with 11kW when no other consumptions are running. Only lights/tv/fridge/freezer is allowed then... I only need to buy 1 big fuse for the battery. Every pack is 120P. The first 2 rows are 14.5kWh each. 3th and 4th are a little less. 4 rows is total 56kWh. But using a lower upper limit (only 4V) and a higher lower limit(3.2V) as pack limits. This means a bare minimum Voltage of 48V (cuts out slighty before that. Bms tells Victron to stop disscharging battery when the first pack hits 3.2V. High Voltage is 60V. Currently working on an adjustable charger for the EV. I learned that 9A is its lowest charging current. So I can charge 3 fase with 6kW. Thats the goal for next year. This year, 9 more solar panels on the new garage that will be build by me right after the hot summer. Also 1 more row for the rack. The rack will handle 7 rows. And I hope to get 100kWh with it but it looks it will be just a little below that... Average mAh/cell is a little to low. And might not be possible to get that target in the next 5400 cells.

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dayandnight avatar image dayandnight timv commented ·

"... The question was asked 13 months ago..."

Hello Tim!

Oh my God! How stupid! Time passes .... ;-)

Perhaps I can help now for your ideas for the EV. I constructed my own wallbox by using a simply EVSE module and a Siemens Logo! I can adjust down to 6 A for charging. 5A doesn't work. But I think it is at the end depending on the car, too. But with my nissan eNV and my BMW i3 it works with 6A. So with 3 phase 6A you come down to 3,9kW or with one phase to 1,3 kW.

But I do more as simply charging. I can run the wallbox in an only-solar-mode. The Siemens Logo! gets from then energy meter how much energy would be feed back to the grid and it adjusts the wallbox evse-module to charge only with the corresponding energy. So the grid meter is 0. It works similar as the Victron ESS. I have to change now the programming of the Siemens Logo!, so it cooperates with the shortly installed Victron ESS system. Never ending story! ;-)

Idea for you: I think it would be easy to do similar to the only-solar-mode a different power-adjust-mode, which checks with the energy meter when charging the car for totally used energy. In case the value goes up a certain border, it reduces automatically the charging current down until to 6A and than to 0A. It can switch from charging with three phase to charging with one phase, too. Which is somehow more complex to realize. This system will protect your main fuse and gives you the freedom to cook during EV charging, too or what ever is needed.

By the way Siemens Logo programming is easy and I am sure it will not cause you quite a headache.


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Fideri avatar image
Fideri answered ·

I also watched the videos. So many cells! Impressive setup you’ve go there.
Thanks for the tips. My maths was obviously off. Up to this day, I don’t know how I got a credit in school. I assumed that 100amps capacity of the 250/100 MPPT was ideal. I would be lucky to get a sustained 50amps out of it. I thought the charge current of 100amps of the batteries was if you wanted to “quick charge” them. I thought the effects of one charger might be say from 4 hours to 8 hours. I certainly didn’t think the batteries would discharge say to 40 volts which is a good point. I will do the maths again and maybe get 250/100 x 4, two now plus two later. Unfortunately, the Victron calculators recommended nothing for my panels or voltage.
I left the complicated power needs calculations to the experts. Overall, I will need about about 42030 watts per day.
I will start with 6800 watts of solar panels and double the capacity later.

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