Solar/battery ratio?

Just to clarify. Keep within the voltage limits. You oversize on the current.

I parallel East and West arrays at a fairly flat angle on the same MPPT.
I find I can all but double the theoretical number of panels before I max out the MPPT at noon.
(I use Outback MPPTs at this installation, they have cooling fans).

Although, perfect North at the perfect slope, Iā€™d still push it for 10% over-panelling to be running max on a good day. Good days are usually hot, and that downrates the output.
Incidentally, my North arrays are on are a 10m tower so they run cooler than they would flush to a roof.

The only real way to tell is to get some panels up there and see.

Edit: Incidentally, on my most recent trip to SA, I reconfigured all my panels so that the North arrays feed the PV inverters and the E-W arrays feed the MPPTs.
My thinking was that on dull days, the flatter panels do better and I lose most of my PV inverters if ESKOM is down. So I am better able to handle load-shedding on dull days.

On sunny days the PV inverters pump it out, but it must also be remembered that the hybrid Victrons also derate their exports if there is too much excess DC power.

OK, well that makes more sense. Its always going to hit the bottom for a while as its starting me off on 50% Min SoC.

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It is always difficult to respond these type of threads as there is a good chance that this was already addressed and I am just to lazy to read through all the posts ,so it this is the case please ignore :slight_smile: .

The panel/battery ratio could be calculated.
16x550 = 8800 w - Thumb suck efficiency - 7500 w peak
Around where I live (Mpumalanga) we get about 5kwh per 1kw of panels on a perfect day.
so for an array of 7.5kw you should generate about 37 kWh of energy per day
So 37 - 21 = 16 kWh that can be utilized to cover the rest of the loads. That should be about 2kw base load available.
Therefore , if your base load is larger than 1.6kw I suspect that you will have a problem. (I allowed for another 20% of losses).

Hope this makes a bit of sense.

Well, if you only use 16kWh then how are you going to discharge the batteries to recharge the next day? It seems that you are over complicating it. You only really have to look at panel production and average daily use - and make sure you produce more than you use.

All this is speculation in a way. What are you trying to do, the answers will be more direct if we understand that? What monthly consumption do you have?

Groetnis

Yeah, we should not forget that panels are the thing that produce the energy you need. Batteries do not produce anything but they can shift the energy you produce to align with your usage patterns. Size the panels for your daily usage plus a margin as your investment return/utility requirements/hardware/etc. allows.

Batteries only really have two uses:

  1. Stabilise the DC side
  2. Provide power when the panels arenā€™t producing enough (night/poor weather/early/late)

So thereā€™s really no such thing as battery to panel ratio.

To stabilise the DC side, you have a battery capacity to inverter capacity ratio. If you go lithium, the absolute minimum is 1:1 (unless everybody in your household as well as guests etc. are super discipled). Conversely, thereā€™s also perhaps a maximum battery to usage ratio: You do not have lithiums that cycle only between 90% and 100%, because it will shorten their lifetime.
To provide power when the panels arenā€™t producing enough, well, this depends on loadshedding needs/off-grid wants/inability or unwillingness to shift loads to times that the panels do produce enough/etc.

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That makes sense ā€¦ I donā€™t understand the replies?

Well, your chosen ratio can be inefficient.

I have an example. This doesnā€™t happen every day, but it does over weekends. I have 7kWh of battery storage. I prefer to cycle it no deeper than 60% DoD, so the battery is good for shifting 4kWh to a different time, typically early morning or in the evenings. All the rest of the energy has to be used in real time, during the day, or it will get lost.

So the amount of PV I need is whatever I use during the day, plus that 4kWh that goes back into the batteries.

What happens on weekends (for me), is that we use less energy. The batteries are full by 10AM, and I donā€™t have 3kW of constant loads to keep it busy with the rest of the day.

In my system, a 3:1 ratio (PV makes 3 times as much as my battery storage) is a little inefficient. I need more battery :slight_smile:

Haha that is true, but I donā€™t think you decide on your battery bank based on your amount of panels. Your amount of panels should be independent of your battery bank. It should be based on the loads you want to cover - which might include loads at night time.

Then your battery bank is chosen based on the loads you want to cover when the sun isnā€™t shining (enough or at all).

So if you sized your panels taking account of your night time loads (which it seems you did), then you need to make sure your battery bank accommodates that decision. If you sized your panels not taking account of your night time loads, then the battery bank should be as small as possible (if you want a good return on investment).

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Simply put, if you want to go off grid, you need a lot of battery storage. If you only want to ride out a blackout, you need ~2 hours worth of storage. Have a look at my planning and design posts to see the vast differences between a ride out type small system and the energy flows vs an off grid system.

Fundamentally, you as the user of said solar system, need to make clear to yourself the purpose of the system, as that drives all design choices. I can explain if need be, but those other posts already do.

Groetnis

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For me its LOADS + CHARGE BANK to as close to 100% by the end of the day = PV CAPACITY.

Iā€™m struggling to find the link between comparing going off-grid with the original question the poster asked, ā€œIs there a rule of thumb ā€¦ panels and batteriesā€ and the answer from Paul.

There is a ā€œgeneral rule of thumbā€ being grid-tied, the ā€œspeculationā€ is on what the weather does, but Eskom is there to tie us over.

Off-grid, entirely different discussion, as you so eloquently posted in your threads on how you did it.

Like I said, just wondering, as Paulsā€™s post made a lot of sense to me, answering the original question.

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Let me put it this way, there is no ratio to speak of. With Grid tie there are no batteries, ergo no ratio. Going grid tie is wholly dependent on what for and how you use solar. If you only want to counter blackouts of two hours, the amount of battery is dependant on your loads for those 2 hours.

Then and only then do you calculate the panels required to charge the battery bank, and you do that according to your house load and rate to charge the batteries.

Now is this the only way to go about it, surely no, its just one way, not the right way nor the best way, and after all just another opinion. Rule of thumb, ja, its based on your use case. Maybe making sense nowā€¦.

Groetnis

I concur with the general line of thinking here: With Lithium batteries and/or grid-connected installations, there isnā€™t really so much of a ratio. With lead acid there was an actual ratio. You could squeeze it a bit, and with current limiting you could get around it a bit, but because of slow charging rate of those batteries, it constrained the size of the PV-array somewhat. Thatā€™s largely a thing of the past.

To perhaps drive that point home: It is perfectly acceptable to have grid-coupled PV only and zero battery capacity, which leads to a ratio that is mathematically undefined :slight_smile:

I think we are talking cross purposes here?

Hybrid grid-tied like say Victron with battery bank vs grid-tied like Solis with no batteries.
2 hours LS event to cater for vs using the batteries at night to further offset Eskom usage.

For me - 3 options:

  1. Grid Tie (theoretically no PV limits)
  2. Grid Tie with Storage (user defines amount of storage based on own requirements)
  3. Off Grid (Loads define PV capacity and storage requirements)

I was merely making my point more clearly, the grid tie has nothing to do with this, just illustrating the point about ratios. But let me not derail this.

Groetnis

My 2 cents worth

Im off grid with only 15kWh of batteries and currently only have 7kW of panels. hopefully during winter I can paint roof and install my extra 5kW.

What I have found is that I donā€™t need more batteries because I only average about 5kWh per night. Remember I donā€™t have electric stoves and big loads at night.

This has been the wettest summer in gauteng in a long time and only 2 or 3 occasions I had to run generator to top up batteries. I do believe that when my extra panels are up I probably wont need to top up.

So my final numbers will be like this: 15kWh battery, 12kW panels
My BMS limits the charging to 90A so what I do is run all big loads during the day which will allow me to use about 7kW throught the inverters and 5kW to batteries, if battery is flat and there is full sun.

Family is trained to check VRM before switching on things so they can use mostly sun and not battery during the day.

edit: forgot to add that it also depends if you going all north facing versus east/west. the east west can allow more panels with a better power delivery

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As per usual I am again late to the party and maybe I can attempt to clarify my train of though.
The original question was , from my understanding , how much panels do I need to be able to charge my battery bank after a nights use?
The answer in my mind is fairly simple. Big enough to be able to supply your standard daily consumption and to put back the energy that you take from you batteries.

I have been on Solar now for 6 years (I think) and of that there was a term of about 16 months that I were off grid. Keep in mind that I was forced to do all of this on a shoestring and therefore every watt had to be counted to make the system work. So I base most of my comments on a system that needs to be as efficient as possible.

I have seen a couple of banks only lasting two years because of people not looking after their batteries. Mindful that the tech has change since then but I do believe that the basics stays the same.
You need to design your system to meet your expectation. If you are going to take 10 kWh from your bank on a daily basis and you want put that back from Solar then you need to make sure that you have enough energy available and to a certain degree you can calculate that value.
If you have a Victron system with ESS it can be managed slightly better but the system can only use what you supply.

So after all of that :slight_smile: ā€¦

ā€œIs there a rule of thumb between the amount of installed Solar and battery size?ā€
My answer is , yes there is :
Make sure you know what your average load is during day time and then how much energy you will need in surplus to be able to put back energy that you consumed during night time from your batteries.
That is if you are using a Hybrid System and would like to be as efficient as possible.

As a side note :
I also agree that there can not be a Panel to Battery ratio as every household has a different energy consumption need.

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