Fusing Battery Strings? HELP PLEASE! -ANYONE???

ligwyd

Hello there,

New to the forum!

Hoping to find some help here with my current project.

I have 12 - 12 volt FLA batteries hooked up in 3 strings of 4 batteries each and then paralleled to get 48 volt battery bank.

Below are the batteries I am using.

https://discoverbattery.com/en/product-search/view/12VRE-3000TF-L

I am searching for the best fusing option and install for each of the 3 strings of batteries in my battery bank.

I have found a number of different products but am not sure as to the best way to go and so I am here.....

I have read that fuses should be no more than 7" from the battery string being fused. Is this best practice?

I found these buss bars which seem just fine for positive/ negative bus bars? Is this the best choice for bus bars or is this overkill?

https://www.bluesea.com/products/2104/PowerBar_600A_BusBar_-_Four_3_8in-16_Studs

Or would these be fine?

https://www.amazon.com/dp/B078VGH8DW/ref=twister_B078VX4Q5W?_encoding=UTF8&psc=1

I read in my charge controller manual page 2-22 in the event of a Ground Fault and the fuse in the Charge controller blows that the aux. output of the charge controller and battery cable connections can be subject to high voltage and as such should be rated for min. 300V (AC rating I am assuming)

https://41j5tc3akbrn3uezx5av0jj1bgm-wpengine.netdna-ssl.com/wp-content/uploads/2018/01/ML201711_Conext-MPPT-60-150-Installation-and-Owners-Guide-975-0400-01-01_Rev-H_ENG.pdf

It is for this reason that I am wondering if all my battery connections and fuses be rated for min 300V as well?

I found this Blue Sea product that seems to both act as a bus bar and fuses each of my 3 battery strings in one shot. Perfect I thought, until I see that it is only rated for 58VDC max. It is possible that I will equalize at 64 VDC and the fact that my charge controller manual is calling for min 300V rating, will this product not be the best for this application or is in fine?

https://www.bluesea.com/products/5196/MRBF_Surface_Mount_Fuse_Block_-_Common_Source

Then I find that it is best to keep the battery string fuses within 7 wire inches from the battery string so once again I am thinking this product will not be the best?

So I find these types of fuses:

https://www.go2marine.com/product/202660F/blue-sea-systems-terminal-fuse-block-1-terminal-stud.html

but the bolt for the cable lug is 5/16" and not 3/8" like I need?

Also, what is the best method to use for calculating fuse size for each battery string? I am told 200 amp fuse will be fine but want to be sure.

I am using a Schneider XW+ 6848 than can surge up to 12000watts for 30 secs. I am using 4/0 cable everywhere - for battery inter-connects and right back to the inverter.

Sorry to ask so much on my first post. I hope to be able to help others once I get my situation all sorted out!

Thanks in advance for help and support.

John

Estragon

I'd use something like the block in the goto marine link for string fusing. I think a 3/8 lug should fit, and with proper washers, should make a good electrical and mechanical connection.

For the +ive / -ive bussbars, I'd go with the higher amp rated bluesea one, not only for fault current carrying, but also to minimize potential voltage drop and/or mechanical stressing in normal operation, on the assumption the amazon one is made of flimsier material.

A separate overcurrent device shoul be installed between the inverter and +ive buss. My preference would be a DC rated breaker rather than a fuse, as a breaker makes a handier disconnect. It should be sized to the inverter makers specs, as long as wiring is as spec'd or larger (4/0 should exceed spec, I imagine) .

The string fuses protect against a battery failing shorted being fed current from multiple strings in parallel. The inverter breaker protects against an short in the inverter or its external DC wiring.

BB.

The hardware you are linking to is all good quality stuff. The fuses are very good for the application. And are available in pretty high current ranges (you did not say what your expected charging/discharging currents were for the battery bank/individual strings).

Here is a nice discussion about how to pick the proper fuse(s) and circuit breaker(s) for your DC power system:

http://forum.solar-electric.com/discussion/353232/oversized-wire-and-breaker

I do have an issue with using a fuse to detect ground faults in the solar array... We had lots of discussions about the way this is done (short answer is "I hate" the ~1 amp fuse/breaker between safety/earth ground and DC ground bus):

http://forum.solar-electric.com/discussion/9345/system-grounding/p1

Midnite changed their forum URL... Here is the updated URL used in the above discussions:

http://midniteftp.com/forum/index.php?topic=142.0

And the white paper that I cobbled together is here (ain't "link rot" wonderful):

https://us.v-cdn.net/6024911/uploads/attachments/512/1965.pdf

I am not sure how the Schneider high voltage MPPT controller does ground fault detection... But if it is the "fuse/breaker" between earth and DC ground. I don't like it and highly recommend that it be bypassed with a solid connection/bond between DC Battery Bus ground and Safety/green wire/Earth ground.

I believe the use of a fuse/breaker between DC bus ground and safety ground is dangerous and not "legal" in NEC code (even though this method is in some versions of NEC).

What method you choose to implement is up to you... Grounding is a very complex set of issues.

Regarding cable fusing for parallel battery banks... Parallel battery circuits never balance perfectly. There is usually one string that has a bit lower resistance than the other(s) and it will carry more current.

And, if you assume that one fuse blows (or one cell/connection fails open), you will now have two parallel strings instead of three--And more current per string until the system gets repaired...

For fusing/breakers in a solar power system, I always like to be conservative... Whatever is the maximum continuous current you want your cabling to supply... I use a 1.25 (1/0.80) derating... If you expect 100 amps, then size the cable+fuse/breaker to 125 amps minimum.

Fuses/breakers, in general, are designed to eventually open at 100%+ of rated capacity and should not open at less than 80% of rated capacity.

With off grid DC power systems (including charging / discharging / AC inverter / etc.), most people are used to household wiring where you have a 15 amp circuit, and run a 15 amp hair dryer for 10 minutes.

With battery charging/water pumping/other continuous loads that can run for hours, you will get hot wiring and false tripped fuses/breakers if you run at 100% of rated current. Use the 1.25x derating for a more reliable/less trouble prone design.

So--Say you want 6,800 Watt inverter at 48 volts (surge current for 10's of seconds are not an issue--Other than checking voltage drop of wiring)... Worst case would be roughly:

• 6,800 Watts * 1/0.85 AC inverter eff * 1/42 volts minimum battery voltage = 190 Amps
• 190 amps * 1.25 NEC derate = 238 amp minimum rated fuse+wiring

Now, for three parallel battery strings, I would be suggesting a minimum of 1/2 rated current from each string:

• 238 Amps / 2 = 119 Amp minimum rated cable+fuse/breaker per string

But if you do 200 Amps rated wiring per string (and fuse)--You will be fine. Just make sure you have a separate ~238 amp+ rated fuse/wiring for cable from battery bus to XW inverter (otherwise you have 3x 200 amp rated current from battery bank into your XW DC bus).

As always, read the installation manual for the product.... The above is an approximation and may not agree with their specifications.

I have to go now--Please feel free to ask more questions.

-Bill

ligwyd

Thanks Estragon and Bill. I will read up on the links and then will get back to you if need be.

I do have a MIdnite Solar DC disconnect with 250 amp breaker in it attached to the end of the battery box and then feeds into the inverter so we are good there.

I will do some serious thinking on the GFP/ Bond issue. If there is a smarter way to do it, I'll be doing just that.

page 2-9 of my manual references this issue. Would really like to have it set up properly and safely.

https://41j5tc3akbrn3uezx5av0jj1bgm-wpengine.netdna-ssl.com/wp-content/uploads/2018/01/ML201711_Conext-MPPT-60-150-Installation-and-Owners-Guide-975-0400-01-01_Rev-H_ENG.pdf

Have good night. Appreciate your input very much!

John

Dave Angelini

Hi John,

If you use the Schneider -80-600v charge controller I will say that I disagree with Bill on this and we always have. A very rare case of this BTW.

The Schneider solar system Mppt's as well as Outback are designed for GFCI and if you use their XW Inverter, it can detect escalating faults and shut down. I have observed this along with clients testimonials of it saving their system in lightning and a grid fault from damaged power lines.

I also have never seen the negative bond problem Bill is describing in over 150 actual customer offgrid homes. It is not worth doing unless you have a very basic system. Good Luck

ligwyd

Noted! Thank you Dave.

More information the better.

John

ligwyd

Might be off topic, redirect me if necessary, but are there any negative attributes about using 600 volt charge controller besides being expensive?  As it is now I am sending  2340 watts MAX to each of my MPPT 60 150 charge controllers. If I had a 600 volt charge controller I could send all 4680 watts to it. Is it better to run two charge controller cool than one hot?

mcgivor

The advantages of a 600V controller are, it allows longer strings of modules which in turn reduces the wiring costs, lighter gauge conductors , less parallel runs which in turn need minimal combining with overcurrent protection and longer runs from array to controller. These considerations would generally be done in the planning stage of a system, if the infrastructure is already in place for 2×60-150 controllers then there is little advantage to be gained by changing to a 600V, in fact new lower current circuit breakers may be required. Having two controllers provides some redundancy, should one fail, your eggs are not all in one basket, which I would consider an advantage, being that both controllers communicate via Xanbus, and are integrated with one another as well as with the XW+6848 leaving things the way they are would be better. Just some thoughts, I understand that it was a hypothetical question.

Dave Angelini

Most of my systems will send around  6KW of nameplate solar into the 600V controller. It will not only current limit it's 80A output but also the input. It uses a 100 amp breaker on the output. As mcgivor said the lighter gage conductors, which I would add translating to some very long distances the array can be placed. I have one client with shade trees near the home and the array out 2000 feet.

So unless there is some reason to do this for future proofing, it does seem like you would not do this, hypothetically

Mcgivor, how are the cave dwellers? What is the scoop from your side of the pond?

mcgivor

 Mcgivor, how are the cave dwellers? What is the scoop from your side of the pond?

Cave dwellers need to learn SCUBA fast, time is running out, either that or its a long wait, months perhaps, nice to know you're up on current affairs.

BB.

Just to be clear, the amount of power (Watts) that you can send through the charge controller depends on the working voltage of the battery bank... For a ~6kWatt array on an 80 Amp charge controller:

• 80 amp * 58 volts battery charging * 1/0.77 panel+controller deratiing = 6,026Watt typical "cost effective" maximum array

At this size array, you will, very rarely, (few days a year, cool/clear days, few 10's of minutes a day) clip a little bit of output current on the array (i.e., 80 amps maximum output, may clip or limit available solar power by 0-10 amps).

If you life in an area that has sub freezing weather in the middle of the day (high altitude mountain etc.), you may get more clipping... Perhaps upwards of loss of 15% of available peak winter sun output (6,026 watts * 15% = ~904 watts "lost" due to clipping at 80 amps).

But for most people, "clipping" is a relatively rare event. "Over paneling" an MPPT charge controller is a very common configuration. MPPT controllers are designed to "throttle" their output based on rated output and operating temperatures (if controllers start to get hot due to weather/poor ventilation/etc., they will throttle back their output). "Good" MPPT controllers run just fine when clipping/throttling their output (keep the controller in good air flow/ventilation, do not install in a closet or under a shelf).

-Bill

ligwyd

Thank you very much guys. This is great info!

My mppt 60 150's can push 3500 watts max. 3500 dived by 260watt panels = roughly 13.5 panels. I did not understand the panel and controller de-rating calcs. Might have to dumb it for me

I like the idea of back up if one of my controllers goes down but I don't like having to spend 3 x the cost on wire/ conduit/ breakers/ install time just to have that back up. 6 of this half a dozen of that.................?

Is there a rule of thumb. for ex. 3500 watts max output plus another 25% (875watts - another 3 panels)

My manual says that each controller can be hooked up 6720 watts max (25 - 260 watt panels). Obviously this may not, in most situations be the best use of your solar panels.

I get that over paneling is good for max power in less than perfect conditions. Maybe making a one time investment on more PV beats spending $$$ on diesel?

Also, I am not sure where I seen it, but has anyone ever heard of a T-class style ignition protected 600V 200 amp fuse that has structural integrity so that is can be directly attached to a battery terminal and  have battery cable attached to the other end of the fuse?

Can the fuse be anywhere in line of batteries that are in series or does it have to be after the last positive battery terminal only?

I have not read the links your guys sent re: fusing but will ASAP. I'm sure many of my questions may be answered there......

Appreciate your time

John

BB.

More or less, solar panels are measured/rated for "flash" testing (STD--Standard Test Conditions). Basically turn on a bright light (aka sun in a box) for a few seconds at ~77F, and verify Pmp=Vmp*Imp for that panel model. This short burst of light keeps the solar cells at roughly 77F (25C).

California (and probably other states) do their rebate calculations based on PTC (PV USA Test Condition) which is much closer to a ~68F dayt with normal sun, air flow, near sea level conditions:

https://newenglandcleanenergy.com/energymiser/2015/12/01/stc-vs-ptc-why-solar-panel-testing-matters/

STC	PTC
Solar Cell Temperature	77°F (25°C)	113°F (45°C)
Ambient Temperature	77°F (25°C)	68°F (20°C)
“Cooling” wind speed	None	2.2 mph (1 m/s)

And the reality is that most panels mounted on a roof during the "warmer" 6 months of the year are going to be much hotter than 68F...

So, when you take into typical warm weather and mounting conditions, derating to ~81% of Pmp (really 81% * Vmp-std), is what you are going to get in a moderatly warm climate during non-winter days. Add in a bid of dirt and charge controller losses (MPPT losses are ~95% efficient in normal operation, and PWM controllers work "differently", but for warm/hot weather, the overall derating is about the same)--You can use ~77% or ~75% (your choice) to map "sales brochure numbers" into what you will typically see.

Yes, in cold climates/freezing weather/high altitude conditions you will probably see more power (especially in the winter)--I don't try to get too wrapped up in the numbers--The reality for solar power is if your measured output within ~10% of predicted output--You are doing fine.

Weather conditions can (day by day/minute by minute) vary much more than +/-10%. And long term solar "hours of sun" per day can easily vary by +/-10% from one month to the next year month (variable weather patterns).

Some solar charge controllers seem to be accurate within 1-2% of "lab grade" measurements.... Others seem to, on average, report numbers about 5% higher than "lab equipment" measurements.

We try to be conservative here--And not tell you that your solar system will generate 2x more energy on paper than you will find when you actually install a system (a typical off grid lead acid flooded cell battery to AC inverter system is actually about 52% efficient from marking numbers to actual output). But even though we give you our best estimate--They are just estimates--Not hard numbers.

For example, a paper design system would be evaluated as (fixed array facing south for Kelowna British Columbia, Canada):
http://www.solarelectricityhandbook.com/solar-irradiance.html

Kelowna
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 40° angle:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.28	3.46	4.43	5.01	5.05	5.04
Jul	Aug	Sep	Oct	Nov	Dec
5.67	5.54	4.95	3.47	2.34	1.99

Say you use the system (no genset needed) from February through October, or 3.46 Hours of sun per day with a 60 amp charge controller and 48 volt battery bank:

Optimum maximum array: 60 amps * 58 volts battery charging voltage * 1/0.77 panel+controller derating =4 ,519 Watt array

Harvest Estimate: 4,519 Watt array * 0.52 off grid battery system with AC inverter efficiency * 3.46 hours of sun Feb = 8,131 Watt*Hour per typical Feb/Oct day (assuming no shading by trees, mountains, chimney, overhead wiring, etc.)

That is the "break even" amount of average power per day for those two months. Sometimes, you may have more (streak of sunny/cool weather). Other Days you have less (marine layer/storm system moves in)--And you have the choice of using less power or starting the genset every couple of days.

There are more calculations (sizing battery bank, minimum array size for XXX AH @ YY Volt battery bank) and any difficult loads (large AC well pump, etc.)....

But this gives you an idea of what the math looks like (keep it very simple and accurate enough for our needs).

-Bill

BB.

Our host has T fuse holders and fuses:

https://www.solar-electric.com/cf300ampfuwi.html

I never have checked to see if a T fuse can be "direct mounted" or if they should always be in a fuse holder--Have to research that (perhaps somebody already knows here and can answer). T holders are generally used (and not cheap).

BlueSea makes a very nice/simple/small marine fuse holder:

https://www.bluesea.com/products/category/16/72/Fuse_Blocks/Terminal_Fuse_Blocks

Can be mounted on battery terminal or close by (idea is to ensure that any short circuit current (slice in insulation, wire fell out of crimp, etc.) has the over current go through the fuse/breaker to protect the wiring from setting fires (and why "close to source of high current" aka the battery, is recommended--Not 10 feet away at the AC inverter input).

Things to think about... If you need both a switch and a fuse for your "branch circuits"--A (properly rated) circuit breaker can do both functions (switch and fuse) -- And no expensive fuse to replace if one ever does pop.

Selecting the proper fuse/breaker is not trivial.... The links above do go into (way too much) detail about the issues.

Regarding fuse only in the + terminal or elsewhere/or two fuses, one + and one - terminal...

In general, "we" ground our negative battery terminals (and white AC neutral bonds) to "earth" ground (green wire ground, cold water pipes, etc.). When you have a "ground referenced" system... You only need to put a fuse/breaker in the + (hot) lead. The ground (return) lead is grounded and always near zero volts, so no protection is needed.

If you have a "floating" power system--Yea, you are supposed to have fuses/breakers in both Hot and Return leads. But many people only put in Hot/+ lead.

For example, in 120/240 VAC split phase residential power systems, we have single pole breakers for 120 volt branch circuits (hot+neutral). And 2 pole breakers in the 240 VAC branch circuits (Hot+Hot) and the two breakers are tied together so that if one breaker pops, the second breaker is also turned off and the failed branch circuit is "safe" to work on (as always, verify power with meter before servicing any electrical circuit).

We can discuss more--Just let us know what is of interest to you John.

-Bill

ligwyd

Hi guys and thanks as always for you time.

So it looks like I could have easily got away with one 60 amp controller. 18 panels x 260 watts = 4680 x .75 = 3510 actual test conditions. The controller can harvest 3500 watts each so a perfect match.

hmmmmmmm........I guess 2 looks kinda cool but probably better ways to spend the extra $500? Live an learn.

I'll add a note here that 2 controllers isn't all bad cause on STC days and cold winter sunny days,  I could be missing harvesting an extra 1000 watts or so with only one charge controller.. I am comfortable with the two but really will spend some time up front designing the next systems to the best of my ability.

Is there any way that the 250 amp dc disco that is mounted on my battery box protect each string from short circuit or does each string have to be fused? I think I know the answer that...

The blue sea stuff is great but only 58 v max. I know it would probably work but would be nice to have 600 v rating. I'm surprised someone is not manufacturing that exact product. Maybe not enough demand?

My 48v battery system is, I believe floating, but will look into the best way to set it up and do that way.

Will read up on all the links. Wow, so much to know

have a great night guys.

John

ligwyd

Regarding PV wire, which is the best choice? I have read that 7 strand copper(non-tinned because I don't think it is necessary) is the most common. I can get 19 strand where I live, is this better? I have read that the current flows on the outside of the wires so the more strands the easier it conducts? So far I am going with 10 awg single conductor black - red + uv resistant not direct burial to keep conduit size to a min and it is not being buried.
Any feed back. I am sure I am overcomplicated it as usual but really want to make the best choices I can. Is ROHS compliant important?
Thank you to all who have some feedback.
John

mcgivor

The tinning prevents copper oxide forming when exposed to the elements, the number of strands makes little difference as the skin effect is applicable to AC, especially at higher frequencies.

ligwyd

So tinned is the way to go?

Estragon

IMHO, tinning shouldn't be needed in most applications. Typically you'd use a factory made mc4 terminated cable cut in half to connect to factory mc4 terminated panels, with the unterminated end at a combiner box, or if field spliced, done with weatherproof connections.

I find finely stranded harder to connect, but it can be done. It has the advantage of being more flexible, which can be helpful in some situations (eg mobile/marine).

mcgivor

Tinned copper has its place, outdoors, hot or humid locations or harsh environments such as battery rooms with vented batteries, so is it the way to go? That depends on application, here is some information dealing with the subject https://www.belden.com/blog/broadcast/in-defense-of-tinned-copper

ligwyd

Fascinating Link! With mc4 connectors and the pv cable in conduit I feel comfortable with bare copper, but I will get prices on bare and tinned.  have seen some wholesalers stocking tinned. Not a bad way to go for wiring integrity and longevity. Wow. I love learning this stuff

With ever single connection in an off grid system being bare copper, or any electrical termination point for that matter, bates the questions if every wire or at least connection point should be silvered, tinned or soldiered?

mcgivor

In most applications bare copper is generally not a problem, I've found on battery terminals is where oxidation is most likely to occur, as well as electrolysis, my generator start battery cable turned green and almost consumed the nut. After replacement a spray of white lithium grease to keep out oxygen has kept things working with no issues since, there are aerosol products specifically for the purpose  https://eshop.wuerth.de/Battery-terminal-protector-BTRYTRMLGRSE-PROTECTOR-150ML/0890104.sku/en/US/EUR/   which are not commonly available in my location but the spray lithium grease seems to work. Just because it's called battery terminal protector doesn't mean it can't be used for other terminations. 

Dave Angelini

Probably the most important thing someone who does not have experience with solar can do is buy and use a torque wrench

ligwyd

Does Vaseline work just as good for battery terminal or is there a product that is the best? I wonder if you could put some kind of protectant on all wiring connections or would that not be a good idea due to a wire being more likely to slip out over time? re: a torque wrench I guess I'll call my local electrical wholesaler and see what they have and go around and check and re-check all of my connections.

littleharbor2

https://www.sanchem.com/docs/NO-OX-ID A-Special Electrical Grade.pdf

This is the ultimate stuff. Not overly costly.

ligwyd

I've came across that stuff before. I'll find some and get it on every connection I make and torque up properly and come back later and re-torque.

Thanks Alot.

John

Vic

ligwyd said:

Does Vaseline work just as good for battery terminal or is there a product that is the best? I wonder if you could put some kind of protectant on all wiring connections or would that not be a good idea due to a wire being more likely to slip out over time? re: a torque wrench I guess I'll call my local electrical wholesaler and see what they have and go around and check and re-check all of my connections.

IMO,   Vaseline works just fine on battery connections  --  coat everything after a light burnish of terminals and lugs.     Would NOT use  any compound on battery terminals that has any conductive material in it.

Would AVOID  any fine strand cable.  It is not rated for use on most busbars,   and screw type terminals.   For battery and inverter cables,  it IS the best,   but,  elsewhere,   many lugs/terminals/bussbars are designed for coarser stranded cable  --  usually 19 strands,  for the cable sizes that you will be dealing with on the DC side of things.

Agree,   that you should not need Tinned cable.

One additional note about your PV cables in conduit;   Most PV cables,   rated for exposure  have large diameter insulation.  It is best to use conduit fill tables for the exact type of cable that will fill the conduit.   You will probably not need to increase the PV cable size for the 12 current-carrying conductors in the conduit,   but be sure to look at that.   Also,  if your system will need an inspection,   recent code revisions require that the conduit on rooftops have an additional calculation for roof temperature verses the size of cable  in conduit that is exposed to the sun.

Opinions differ,   Vic

ligwyd

Thanks Vic, I thought of the 6 strings there would be only 6 current carrying strings but I guess I thought wrong. I guess was thinking of the negative being neutral. So, 12 current carrying conductors then. My conduit will be pretty much in open air attached to the aluminum solar array and then down the side of it and then through a weather head and then attached to the end of the sea can - lots of air flow. Will crunch the numbers.......

Vic

Yes John,   each conductor in the pair from each PV IS a current-carrying conductor.  A ground in a conduit is not counted as such.

Regarding conduits on rooftops,  it is the horizontal runs that are on,  or close to the roof,   that should have this temperature calc done.

If your system will be inspected,  perhaps asking the entity what will do the inspection about which Code that they are using,   and for any special local things that they will look for.   Since you are in Canada (and I am not),   some of the details will be different than hear.  And,   always it seems that the actual inspector will have  a certain way of looking at things.

Have fun with that system.   Looks like it will be Top Notch.   FWIW,   Vic

ligwyd

Thanks Vic/ Gotta decide, source PV wire and then I'll figure out the conduit.

Quick rabbit trail here.

I chimed in on an older post titled "matching Battery Bank Size to Panels" here

http://forum.solar-electric.com/discussion/6105/matching-battery-bank-size-to-panels

and had a question: is there a ball park figure or something for a battery bank Ah to PV watts ratio?

my couple comments are a the the end of the post. Estragon replied and gave some valuable feedback and I also wanted to see if you might add anything to the equation?

Sorry for posting this here. I kinda like all my questions on one post for easy review.

I guess I was thinking that in any well designed system on your average fairly sunny day you should have your battery bank full by noon and have the rest of the day for float, opportunity loads etc. It would make sense that there would be some basic rule of thumb in this area. maybe I am trying to simplify something that is not so simple and actually is very design specific.... I get that, but there has got to be a little more info/ understanding that I am lacking here.  ex 5000 watt STC pv for 500 Ah battery bank........??? You know what I am getting at here? 

I'll also add that when I began this project my start was not to calculate my loads to see how much power I needed (I know seems stupid) but my thought was build the best system you can afford and live with what it produces. We are so wasteful with our resources, I thought living on a power budget would just smarten you up in this area of your life. Conservation is KING. Many will calculate how they live on grid and not want to change their habits and want to live the same way off gird. Well you can do that but it is very costly! I did an experiment for 2 years before I started this project to see how little power we could live with. We are a young family of 5 and were able to get our daily usage down to 3kwh/day and so I set out to build us a system that could make at least twice that each day. Sorry to babel. Looking forward to your feedback.

ligwyd

does this answer my question?

https://www.wholesalesolar.com/solar-information/battery-bank-sizing