Hello everyone…
I want to build an off-grid system with an autonomy of at least 3-5 days. I already have 2x575W panels, but I have questions about the MPPT (which one?) and the battery (how many and what Ah). Could you tell me what I need? Can anyone help me choose equipment?
Hi @dimitris, welcome to our forum 
To start with, tell us what loads you’re planning to connect to this system. Five days of autonomy is a relatively long period of time, especially if you’re planning to replenish the energy using only those two PV panels.
A simple car radio (10A, 12V), a washing machine (used once a week), and a refrigerator (total max 16A, 240V), an all-in-one computer and router (70W, 240V), LED lighting (12V). A 24V compressor for opening/closing the front door. I have a 24V/240V Step-Up Inverter (2000W, Pick 4000W) and a 24V/12V Step-Down Inverter, 40A. I anticipate using 12V (radio, light), 24V (battery monitoring, compressor), and 240V (PC, refrigerator, washing machine). Nothing more. The kitchen and heating are powered by gas. If I will have any problems, I have a 6KW petrol generator available. I plan to use this generator to charge the batteries on a sunless day.
You need to work out what you need in kwh, then from this you can estimate the ah of storage required.
You state a simple car radio at 10a/12v, this seems excessive unless you have radio blasting out at very high volumes, more actual load requirements are needed, radio might need a 10a fuse, but might only use 0.1a at very low volume.
For the estimation , duration of given load is multiplied by load to give kwh, then add up all loads over whatever duration you need back up for to give what size battery you need.
Then you can work out what size pv panel array you need to charge battery if required.
Feel you may be underestimating the cost and size of system for minimum of 3-5 days backup, without actually working out whats requited.
I was asked what I would be connecting to the load. I replied: a radio (it has a 10A fuse), a PC with a router (totaling 70W), a refrigerator, a washing machine (once a week), and LED lighting. Nothing more. I also wrote down what additional equipment I had and what I planned to use it for. Now it turns out I need to complete at least an electrical engineering course to answer my own question, because the technicians who work in this field daily are unwilling or unable to answer. I don’t understand.
I strongly advise you to find an installer/ electrician who has extensive experience with Victron gear, (or a similar brand).
I’m a electrician with long experience but ESS was (and still is) for most part unknown to me, only after reading extensively have acquired some knowledge about ESS.
Does this mean that technicians who work with Victron equipment aren’t commenting on the VIctron community forum? So what is this? HydePark? American Freestyle? And Victron is branding it with their logo? So how can I get answers to my questions and concerns? How can I trust this company’s products and spend hundreds (if not thousands) of euros on equipment I’m not sure will meet my system’s expectations? Let us all get back to the main topic instead of chattering about nothing specific.
You’ve provided instantaneous figures, now you need to determine the duration that those appliances will be used per day, this will enable you to produce a figure in kWh … no one knows how long you will be using your appliances, only you.
Once an estimate of your daily consumption has been made a sensible decision can then be made on battery capacity.
Once the size of your battery has been determined, based on your location, your sun hours, your ability to provide alternatives energy sources, an assesment can be made for the amount of PV and associated hardware required.
You will likely not find anyone here that will be willinging to design your system for free, however you will find more than enough self help education resources here and at the Victron Professional site.
In the first and third post it was defined… I give up. I see that stupid AI (or certainly the first salesperson off the street) has more to say about Victron equipment than the members of the Victron Community. I no longer care why the Community is unwilling or unable to provide the information requested by an ordinary mortal willing to pay hundreds of euros for a brand recognized worldwide as trustworthy and reliable. I have this in… Where—you can guess… The topic is outdated and, as I myself note, not worth the investment of money or effort in launching such a system based on Victron equipment. I bid you a cold farewell.
The most accurate description that you have provided for your usage is “once a week” … usage is the product of instantaneous consumption and time, I’m assuming that you will not be using your washing machine for 24 hours ? … if you require accurate answers you will need to provide accurate data.
Sorry Dimitris, but you haven’t given us nearly enough data to magically predict your system requirements. Somehow we’re supposed to know how long you run that 4 kW inverter, or how long your lighting is on.
So I’ll dust off my crystal ball and assume you need about 8 kWh per day.
That means a system providing 5 days of autonomy would — shockingly enough — require about 40 kWh of storage. If we further assume you want to build this on a 12 V system (because why not), you’re looking at over 3000 Ah of batteries.
Meanwhile, your PV panels — assuming they’re perfectly positioned and always producing at maximum output — will manage roughly 1000 kWh per year.
You don’t need to be a math genius to see that this simply won’t work.
So either you provide your actual energy consumption, or you can stay offended all you want — either way, none of us will be able to help you.
A clear sign of professionalism is when a professional asks the right question to an idiot who (apparently not very well) knows they want to do something but isn’t quite sure how or what (especially when Victron’s offerings are truly extensive). I provided answers that were as good as the questions. It’s obvious that none of these basic devices (or rather, there are a minimal number of them, without any special features) will (in any case) operate 24 hours a day. And the data I presented clearly indicates the expected daily power requirement. However, this wasn’t obvious (e.g., none of you know or are not very aware that a car radio in its standard configuration and without an amplifier does NOT draw more than 60W because that’s how its output stages are protected against burnout). Furthermore, none of you asked what the LED lighting wattage is, or even realizes that it draws up to 14W/m. Even the title itself says something (though it’s quite overrated). I also revealed what additional equipment I have and what I plan to use it for (supporting, in this case a 6kW generator). Apparently, however, only I am/was required to read with comprehension. Your “crystal balls” are not only dusty, but their operating systems are also seriously damaged. I repeat: Ask the right question and you will receive the right answer. However, this “system” unfortunately works both ways. I agree, I asked a very vague question, but I expected the professionals to clarify the details. And that did NOT happen. Consequently, my assessment of your professionalism (as represented by Victron) is mediocre at best so far. However, I see that there is a glimmer (albeit only a glimmer) of hope that this rating will improve.
Just like 1234enough pointed out at the very beginning, we need to estimate your energy demand in kWh. And we can only do that if you tell us exactly what power your loads have and how long they run during the day. And no — Stating “Radio 12V / 10A” isn’t a perfectly sufficient power rating. What’s still missing is how long that radio actually operates. And yes, as you already noticed yourself, the 12V / 10A label on the radio does not mean it will be drawing a constant 120 W.
So, in case you missed it, I’ll ask again — very directly this time:
How much energy, in kWh, do you consume per day?
This is the most important question you should be answering in this thread. Instead, you keep complaining that none of us has asked you the “right” question yet. Sorry, but you clearly don’t understand how this works.
It also doesn’t help at all that you mentioned having 14 W/m LED strip, because you never told us how many meters of it you have, nor how many hours per day it’s on.
On top of that, in the thread title you wrote that you want to build an energy storage system with a capacity of 3 kW. The problem is that kW is not a unit of capacity — it’s a unit of power. So what exactly did you mean?
Do you want a storage system with a maximum power of 3 kW, or did you actually mean a capacity of 3 kWh?
Finally, a specific question… And I won’t go into detail now. However, I will provide the most precise and specific answers possible. Here’s the first: I need about 1.5 kWh per day (maximum 2 kWh). Apart from the refrigerator (obviously) and the washing machine (once a week), all other appliances don’t run longer than 5 hours. Neither individually nor together. And it doesn’t matter how many meters the LEDs are, because “requirement for about 1.5 to a maximum of 2 kWh per day” is my known daily requirement and nothing more (!!). In an extreme case, which I don’t foresee but expect, I have a generator 6 kW available….. “I want a storage system with a maximum power of 3 kW.”(!!)……. is a Maximum.
If I may suggest this, since you’re from Poland (Poznań, Poland), perhaps we can speak Polish. Then there will be no doubt as to whether I’m referring to a unit of power or capacity.
Możemy, tylko nikt więcej nas nie zrozumie w tym przypadku.
To może przez Viber? +35799794415……. emot rodem z jednej z płyt Pink Floyd
Nie kolego, zostańmy tu na forum. Będzie dla potomnych.
Jeśli potrzebujesz do 2kWh/dzień, to dla maksymalnego zakładanego czasu podtrzymania tj 5 dni potrzeba minimum 10kWh. Do tego należy doliczyć przynajmniej 20% strat na przetwarzanie. Mówię o wszystkich skumulowanych stratach od ładowania do pełnego zużycia energii z magazynowaniem włącznie. Uwzględniając to lądujemy z akumulatorem o pojemności użytkowej 12kWh.
Idziemy dalej. Pytanie w jakiej technologi ten akumulator miałby być wykonany? Jeśli chcesz zbudować coś we własnym zakresie - to pewnie chcesz użyć LFP/LiFePo4. Zakładam też, że chcesz zachować możliwie jak największą żywotność ogniw, więc będziesz chciał je używać w zakresie maksymalnie 80% dostępnej pojemności. Finalnie potrzeba więc okolice 15kWh i to jest dobry punkt odniesienia dla dalszych rozważań. To oczywiście wszystko z założeniem, że nie będzie żadnego doładowania w czasie tych 5 dni.
Stąd jest już prosto. Jeśli system ma mieć napięcie bazowe 12V, to wychodzi nam około 1200Ah pojemności akumulatora. Dla wyższych napięć odpowiednio mniej. Dla 24V 600Ah, dla 48V 300Ah.
Wybór MPPT będzie zależał od wybranego napięcia akumulatora. Jeśli trzymasz się okolic 12/24V to wystarczy Smart Solar 100/50. Jeśli zdecydujesz się na wyższe napięcie, to możesz użyć dużo tańszego Smart Solar 100/20. To jest najmniejszy (i jedyny w rodzinie 100V) kontroler ładowania, który działa z systemem 48V.
Bardzo podoba mi się taka dobra i konkretna odpowiedź. A więc: Założenie 5 dni jest bardzo optymistyczne. Wystarczą nawet 3 dni. Przypominam, że jestem w posiadaniu generatora 6kW (na wszelki wypadek) Technologia akku? Skłaniam się do żelu - ale się przy nim nie upieram. System ma bazować na 24V (bo niektóre odbiorniki pracują na 24V - kompresor sterowany przekaźnikiem) a i tak muszę stosować step down do 12V (oświetlenie). Dodatkowo chcę zaznaczyć, że mieszkam w regionie o przeciętnej 300 dni pełnego słońca w roku. Stąd zejście z 5 do 3 dni podtrzymania. Zależy mi jednak na MPPT z możliwością podłączenia generatora (gdyby jednak ten musiał okazać się przydatny do podładowania akku). Jakaś sugestia w kwestii MPPT z możliwością podłączenia zewnętrznego (ewentualnego) generatora?
Jeśli masz zamiar używać akumulatorów żelowych, to trzeba radykalnie zwiększyć ich pojemność ze względu na znacznie mniejsze dostępne bezpieczne poziomy rozładowania. Nawet jeśli użyjesz akumulatorów Deep Cycle to nie zaleca się ich rozładowania poniżej 50% a najlepiej by było zostać w granicach 25%. Inaczej radykalnie zmniejszysz ich żywotność. W konsekwencji jeśli zmniejszysz wymagania czasowe do 3 dni, zakładana pojemność spadnie do 10kWh, ale jeśli aku ma pracować przy DOD 50% to potrzebujesz realnie 20kWh. Warto to mocno przemyśleć, bo moim zdaniem akumulatory LFP będą tańsze w dłuższej perspektywie.
Z tego co mi wiadomo, Victron nie ma dedykowanego MPPT do ładowania baterii z generatora. Z założenia ładowarka MPPT służy do ładowania akumulatora z paneli PV. Jeśli zależy Ci na zostanie w systemie Victrona, to możesz wymienić swoją przetwornicę na MultiPlus II czyli ładowarkę/inwerter od Victrona - za jego pomocą będziesz mógł ładować akumulatory.