CadilacKid, Solar storage vs solar capacity

[Brewerbob]

First, empty your damn inbox (AGAIN).

Second, I've been searching thru solar threads lately and ran across a couple from 2018 that you were in. I've got a question for you...

What is the "best" storage vs capacity ratio? Yes in quotes because I know it's a loaded question. You can have 1.21 jijawatts and a AAA battery. Your controller is going to keep the cells turned off all the time. The opposite extreme would be you NEVER get anywhere near 100% charged. I've got roof space for 3500 watts. I don't know how many watts that would be at high noon, east coast but it ain't 3500 watts. I'm guessing in the AAA example that the voltage is just sitting on the input lines like the light switch that is turned off. Does it cause any harm to the solar panels or the controller?

Third, anyone else knowledgeable in solar set-ups, feel free to answer.

[john61ct]

Depends on many factors

Most importantly, whether other energy sources are also important contributors, or are you really shooting for solar-only?

how mission-critical is electricity?

how many weeks at a time off grid?

far from civilization?

insolation conditions, latitude, climate

vehicle - is heavy weight a problem?

How important is maximizing longevity?

LFP chemistry or lead?

I'm sure others I've missed as well.

Rule of thumb for 12V systems: between 50 and 100Ah of storage per 100W panelage

[JDOnTheGo]

Hi Bob,

I'll take a shot at those loaded questions! That way, several others can come along and tell you how wrong I am!!

The easy question first. You are correct, you can have a huge amount of solar electric "capability" and when not needed (battery bank full, no other demand), all that power just sits waiting for you to put it into action. (in very non-technical terms).

Assuming we are talking about cases like mine (full-timer, electrically "heavy" rig, traveling around the west, and you don't like running a generator) and depending on your electrical budget (you do need to have one of those), you ALMOST can't put enough solar panels on the roof of a bus. Yeah, yeah... c-rate mumbo-jumbo. The reality is that you have less usable square footage than you think and you will rarely get rated output from the panels. The best I typically see with my flat mounted panels is 80%. During the winter months, this falls to a high of about 60%. Thumbs up to those that want to install a tilting mechanism and have as few panels as possible. I'm lazy and like my panels just sitting up there working without me giving them a thought. Of course, those winter days with poor insolation mean your 3500 watt example is more like 1700 watts - and the days are short.

So, you get to decide how you want to "size" your system. Think every day will be perfect - sunshine & roses?? Plan accordingly. Think a good number of the days will less than ideal? Plan accordingly. Think you will be parked amongst the trees with lots of shading.... etc...

In terms of storage capacity, it again depends on your "plan" or "goal". If you plan to be in an area where no sun for 3-4 days is common then plan for 3-4 days of usable capacity. Regardless of that number, the backup plan is always running the generator. I know, I hate it too - that's why I have solar. Thankfully, this happens about three times a year for me. Some folks are happy to have enough battery to get them thru the night - that's ok - just not my goal.

Finally, all of this is dictated by roof space, battery storage space, and money. If you 'plan' fits into what I described above, put as much solar on the roof as possible and get as large a battery bank as you can afford/haul around. Yeah, turning it into an academic exercise is fun if that's what turns a person's crank.

Now, if you are talking about occasional weekend use and you would like to power one LED light from solar... well... forget everything I said above.

[rffffffff]

Batteries have a max charge rate that shouldn’t be exceeded, like c/8 or c/5 or something like that. If you ‘over solar panel’ the bulk charge stage, where the controllers put out max amperage may cook the batteries. I think that having panels to get right up to that point is probably ‘best’.

If your capacity is 860ah and the max charge rate is c/8 I you would want to limit to about 108 amps, which would translate to 1300 watts or so. (Using 12v) That being said, if you are charging at c/6 I don’t know if the controller would go over voltage and turn the amps down or not, which I would love to find out. If the bulk voltage setting on the controller will naturally limit to c/8 than more is better with panels, because it increases the low light charge and extends your charge day.

[cadillackid]

JD pretty much sums it up perfectly..



and bob I emptied my mailbox.. lolol..



anyway i look at it just like at home.. you have 200 amp electrical panel providing power.. but rarely are you ever using 200 amps.. if ever..



your battery charge controller will keep you from over-charging your batteries..


from what I understand most of the inverter controllers allow you to "deficit finance".. meaning if you were say running your minisplit and. cooking a sandwich on an induction HOB.. pulling 4000 watts and your panels are putting out 3000, that your batteries can provide the extra 1000 until such time you turn off the HOB.. and you are using 2000 instead of 4000.. so 1000 is available for recharging your packs..

I realize there is all kinds of other math in there.. I just use it as a simplified example..



like JD says alot depends on your planned usage of KwH per day.. with some reserve or another method for provising electricity.. you travel to florida for the winter and all the sudden a whole day of clouds and rain shows up.. do you have enough battery to pick up the slack, or a generator to fall back on..



length of day varies a lot between winter and summer.. even though the weather id milder.. not only does lower sun angle cause your panels to push out much less power than in summer.. you have many less hours in a day to capture sunlight.. even down south where days are longer (after the fall equinox) than they are up north they are still much shorter than in summer


in summer months, sun angle and day length up north (after spring equinox) is preferrable over the south..



-Christopher

[rffffffff]

Originally Posted by cadillackid

JD pretty much sums it up perfectly..


your battery charge controller will keep you from over-charging your batteries..



-Christopher

Is this for sure? I am a little afraid to overdo it but I haven’t found anything that talks about what c/5 would do to voltage.

If I had 1000 panels and it charged my one battery to 14.2 (or whatever bulk is) would it exceed the c/8 amperage? Any idea?

[cadillackid]

a true charge CONTROLLER made for your type of batteries should not allow it to over-charge or over-current your batteries.. similar to when you plug your laptop in at home there is a charge controller that stops the battery charging even though the wall socket is still plugged in with plenty of power waiting..



just wiring the panel inverters to the battery bank offers really no protection against over-charging.. thats why I say use a Controller...

[Brewerbob]

Originally Posted by john61ct

Depends on many factors

Most importantly, whether other energy sources are also important contributors, or are you really shooting for solar-only?

I'll have shore power hook-ups but I don't like RV parks. And a genny.

how mission-critical is electricity?

100% for a fridge if nothing else. In warmer climates, AC.

how many weeks at a time off grid?

All of them.

far from civilization?

Not sure how this matters as even a mile of extension cord is pretty heavy and bulky. The further the better.

insulation conditions, latitude, climate

1" spray foam and 1 1/2" rigid except floor. 1" spray and 2" rigid for the floor with radiate heat.

vehicle - is heavy weight a problem?

33,000 GVWR, currently 20,600 empty (metal can empty).

[quote]How important is maximizing longevity?[quote]Forever or until I'm done with RVing or dead.

LFP chemistry or lead?

Capacity is capacity no? Thinking lithium.

Rule of thumb for 12V systems: between 50 and 100Ah of storage per 100W panelage

Thinking 48 v as that seems to be the sweet spot for stealing car lithium and where controllers like to be.

[Brewerbob]

Originally Posted by JDOnTheGo

all that power just sits waiting for you to put it into action. (in very non-technical terms).

I ain't no Chris but I know the difference between an electron and an Ohm.

Yeah, yeah... c-rate mumbo-jumbo. The reality is that you have less usable square footage than you think and you will rarely get rated output from the panels.

Don't know what a C-rate (charge rate???) is. I've got room for 12 panels.

The best I typically see with my flat mounted panels is 80%. During the winter months, this falls to a high of about 60%. Thumbs up to those that want to install a tilting mechanism and have as few panels as possible.[/quote]Lawn chair parts. I might go some kind of tilt but ignore for now.

I'm lazy and like my panels just sitting up there working without me giving them a thought. Of course, those winter days with poor insulation mean your 3500 watt example is more like 1700 watts - and the days are short.

So, you get to decide how you want to "size" your system. Think every day will be perfect - sunshine & roses?? Plan accordingly. Think a good number of the days will less than ideal? Plan accordingly. Think you will be parked amongst the trees with lots of shading.... etc...

Yeah, I know better than everyday being sunshine and 20 hrs of daylight tho I hope to have a few of those too. Trees and humidity are big in the east.

In terms of storage capacity, it again depends on your "plan" or "goal". If you plan to be in an area where no sun for 3-4 days is common then plan for 3-4 days of usable capacity. Regardless of that number, the backup plan is always running the generator. I know, I hate it too - that's why I have solar. Thankfully, this happens about three times a year for me. Some folks are happy to have enough battery to get them thru the night - that's ok - just not my goal.

Ah, now I got you. I need 1.21 jijawatts of storage and only one solar panel for cloudy day storage. 3x a year sounds great. Need to clean the cobwebs out of the carb.

Finally, all of this is dictated by roof space, battery storage space, and money. If you 'plan' fits into what I described above, put as much solar on the roof as possible and get as large a battery bank as you can afford/haul around.

I'll have a floating floor in the bedroom anyway so battery storage won't be a problem. Budget maybe.

Yeah, turning it into an academic exercise is fun if that's what turns a person's crank.


Now, if you are talking about occasional weekend use and you would like to power one LED light from solar... well... forget everything I said above.

Both. I want practical but it helps me understand with the extreme examples.

[john61ct]

Originally Posted by rffffffff

Batteries have a max charge rate that shouldn’t be exceeded, like c/8 or c/5 or something like that.

Depending on chemistry, that spec is just

"bank will not accept higher than"

Nothing to do with hurting anything.

Originally Posted by rffffffff

If I had 1000 panels and it charged my one battery to 14.2 (or whatever bulk is) would it exceed the c/8 amperage? Any idea?

Not an issue in the real world, just a waste of too much money spent on pnels not enough on storage.

[john61ct]

See, to the extent you have mains easily available, then you don't need to buy and carry "too much" storage.

A fridge is not mission-critical, so again, tilts balance toward panels over storage.

You are joking about aircon right? That you run the genset to feed, storage might give a little time-shifting is all.

> Insolation conditions, latitude, climate

Nothing to do with insulation. What's the weather like most days, how high is the sun in the sky? Look up the charts. . .

These also not answered

> vehicle - is heavy weight a problem?

Water, propane, other loads. . .

> How important is maximizing longevity?

The bank not you silly.

> LFP chemistry or lead?

Makes a huge difference!

Originally Posted by john61ct

Rule of thumb for 12V systems: between 50 and 100Ah of storage per 100W panelage

If your storage is at 48V then your Ah are "worth more" than the more normal 12V.

But please explain "stealing car lithium" and "where controllers like to be".

[john61ct]

Yes, the other approach is, "fill the roof" with panels, can't have too much.

Then, figure out your Ah per day budget and buy as many usable days' worth, depends on batt chemistry.

If using LFP, and assuming 12V, then 75Ah per day times 4 days is 400Ah, can always expand that if you find you need more.

If lead, 700Ah **much** more wasteful less easy / convenient, to add capacity later, so go to 1000Ah instead, still lots cheaper but lots heavier, doesn't matter.

[CMORGANSKOOL]

Originally Posted by john61ct

See, to the extent you have mains easily available, then you don't need to buy and carry "too much" storage.

A fridge is not mission-critical, so again, tilts balance toward panels over storage.

You are joking about aircon right? That you run the genset to feed, storage might give a little time-shifting is all.

> Insolation conditions, latitude, climate

Nothing to do with insulation. What's the weather like most days, how high is the sun in the sky? Look up the charts. . .

These also not answered

> vehicle - is heavy weight a problem?

Water, propane, other loads. . .

> How important is maximizing longevity?

The bank not you silly.

> LFP chemistry or lead?

Makes a huge difference!



If your storage is at 48V then your Ah are "worth more" than the more normal 12V.

But please explain "stealing car lithium" and "where controllers like to be".

My 5000 btu window unit uses about 50ah per hour or drains 100ah agm an hour. So you could technically run that AC on 1000 watts of solar and all night on about 10 batteries and cool a bedroom area. To cool a whole bus thoroughly I'm guestimating about 50,000btu. Which adds a 0 to everything and makes it impracticable for a bus 10,000 watts of solar, 100 batteries etc.

[Brewerbob]

Originally Posted by cadillackid

from what I understand most of the inverter controllers allow you to "deficit finance".. meaning if you were say running your minisplit and. cooking a sandwich on an induction HOB.. pulling 4000 watts and your panels are putting out 3000, that your batteries can provide the extra 1000 until such time you turn off the HOB.. and you are using 2000 instead of 4000.. so 1000 is available for recharging your packs..

I realize there is all kinds of other math in there.. I just use it as a simplified example..

I should absolutely hope so. A fricking diode will keep me from overcharging but it's a pretty stupid controller if I can't pull a 1,000 w out while putting 2,000 w in or vice versa.

length of day varies a lot between winter and summer.. even though the weather id milder.. not only does lower sun angle cause your panels to push out much less power than in summer.. you have many less hours in a day to capture sunlight.. even down south where days are longer (after the fall equinox) than they are up north they are still much shorter than in summer


in summer months, sun angle and day length up north (after spring equinox) is preferrable over the south..



-Christopher

Those happen to be my seasons too. Long northern summers and short southern winters. I'm sure, at least in the beginning, there will be a SW summer and a northern winter just because it's new. Northern lights, desert thunderstorms... the touristy stuffs. Then, it'll be "The Weather sucks, pack the flamingos, wheels up,... we're outta here.

[Brewerbob]

Originally Posted by cadillackid

a true charge CONTROLLER made for your type of batteries should not allow it to over-charge or over-current your batteries.. similar to when you plug your laptop in at home there is a charge controller that stops the battery charging even though the wall socket is still plugged in with plenty of power waiting..

A true controller??? Hell, the $19.95 charger at Advanced Auto will not cook your car battery. The dumbest one on the shelf cannot cook a lead acid battery. There's supposed to be a difference in the lithium chargers but I don't know the details. If it turns off a 14.2 v for a lead acid, it will turn off at 14.2 v for a cat. Maybe the lithiums don't like seeing 6 amps all the way up to shut-off? I dunno. It's getting hard (not that I've tried) to find a charger that is NOT auto sensing for lithiums.

[Brewerbob]

Originally Posted by john61ct

Not an issue in the real world, just a waste of too much money spent on panels not enough on storage.

And that is winner winner chicken dinner. It may be apples to oranges but there has to be a "right" number out there. The guys setting up solar farms are still storing that electric somehow are they not? It's acres of panels. True the load is almost unlimited as well but they aren't providing that electric until they have a buyer for it.

[john61ct]

Originally Posted by Brewerbob

there has to be a "right" number out there

Nope, it's all greyscale it depends.

You have all the info you need above.

A proper bank of true deep cycle batteries deserves proper regulated charge sources, ideally user-custom adjustable profiles.

Victron MPPT are excellent.

[Brewerbob]

Originally Posted by john61ct

See, to the extent you have mains easily available, then you don't need to buy and carry "too much" storage.

A fridge is not mission-critical, so again, tilts balance toward panels over storage.

What mission are you on?!? I like eating daily.

You are joking about aircon right? That you run the genset to feed, storage might give a little time-shifting is all.

Nope, not kidding. If (yes I know it's a big if) I'm making 3500 w/hr why can't I? I don't know what a mini-split draws but the more storage the better. If it's too damn hot, the bus has wheels and it's time for a 1,000 miles.

> Insolation conditions, latitude, climate

Nothing to do with insulation. What's the weather like most days, how high is the sun in the sky? Look up the charts. . .

Yeah, I get that but those I can't control without the use of the wheels. I throw all those variables into the same bucket as the panel is rated for X but you only get Y because of heat, humidity, blah blah blah. It may be over simplifying but like you said charts. Throw a dart at the map and they get an average of 157 days a year of sunny weather. Doesn't mean it'll be 157 this year or that I'll be there for those 157.

These also not answered

Pretty sure I did but I will again

> vehicle - is heavy weight a problem?

Empty metal can currently weighs 20,600 lbs. GWVR is 33,000. No idea what the build out minus solar will weigh but I doubt it'll be an issue.

Water, propane, other loads. . .

Water heater I assume? Planning on a diesel heater for both water and air. Propane, don't want any. Stove is diesel too. I want a single fuel. May throw in a wood stove just because they are cool. They are a shitton of work to clean and feed tho so maybe not. That's the plan anyway. And everyone knows how plans go.

> How important is maximizing longevity?

The bank not you silly.

Yeah, I know. I was thinking in terms of how long the system should live (reliability type thing). If you mean how long should the bank bank hold out on a full charge, that's kinda the point of my question in the first place. I'm making "3500 w"; how much bank is too much bank? It can be rainy for a week. If it is rainy for a week AND I need AC, I'm at the wrong latitude!! I am going to need that fridge tho. The beer isn't going to keep itself cold.

> LFP chemistry or lead?

Makes a huge difference!

LFP = lithium???Don't speak at me with acronyms!! From a single day of reading and zero budgeting, I think Leaf batteries and 48 v is the best set-up. If 96 v s better because controller or wire size fine. If 36 v is better because of availability when something goes poof fine.

If your storage is at 48V then your Ah are "worth more" than the more normal 12V.

But please explain "stealing car lithium" and "where controllers like to be".

That's where I get fuzzy. I understand Ohm's Law. More voltage gets you more watts at lower currents. Lower currents are smaller wires and less loss. So what I really want is 1.21 jiga VOLTS and a tenth of an amp. There's a catch tho. There's only one controller in the world that can do that, it's int eh basement of the Pentagon, it's bigger than a 1000 skoolies, and it costs 1.21 jiga dollars.


Reading up on that 48 V lithium Leaf set-up the guy needed something like 96 or 128 (some multiple of 4) to sense all of the individual batteries. Widget A (don't know if that was the controller but it read like an add-on in addition to the controller) was only good for 12 inputs so he needed a boatload of them. The 12 v system at 1.21 jiga amps needs wires the size of a small tree but not as many sensing lines. Had to do with parallel versus series. It may have been for the battery configuration, or the panel configuration, or both.



But anywho, the 48 v set-up sounded like a nice middle ground between the 12 v/1kA versus the 256 v/1A system.

[Brewerbob]

Originally Posted by CMORGANSKOOL

My 5000 btu window unit uses about 50ah per hour or drains 100ah agm an hour. So you could technically run that AC on 1000 watts of solar and all night on about 10 batteries and cool a bedroom area. To cool a whole bus thoroughly I'm guestimating about 50,000btu. Which adds a 0 to everything and makes it impracticable for a bus 10,000 watts of solar, 100 batteries etc.

Thx, I like this answer. Smaller words and I know these acronyms!!

So, another Chris question because this is definitely up his alley, can you have 2 mini-splits and one "outside" unit? I don't know what the two halves are called. Like C Morgan, I want to cool the living area and NOT the bedroom. Or the bedroom and NOT the living area. There's going to be an hour in between that's going to suck so if the mini split can be set-up (rated BTU wise???) to do both awesome. If it's the difference between 5,000 BTUs and 150,000 BTUs (heat is 150k OEM I think) then forget it.

[john61ct]

Originally Posted by Brewerbob

What mission are you on?

Some people have gear their life depends on, requires redundancy.

> Nope, not kidding. If (yes I know it's a big if) I'm making 3500 w/hr why can't I?

Can but silly, just run your gennie.


All those variables just nudge toward higher or lower ratio, never exact numbers.

> I doubt it'll be an issue.

Tilts toward more storage.

Water is heavy, is all.

Longevity of the bank. Careless / heavy use of a smaller bank, replace every few years.

Bigger bank well coddled can go over a decade, up to you.

> I'm making "3500 w"; how much bank is too much bank?

7000Ah

> LFP = lithium??? Don't speak at me with acronyms

Your job to google & learn.

Forget repurpose EV packs or other LI chemistries, way too unsafe for mobile living.

Stick to 12V, simple is best, pay more for copper less for gadgets. Maybe 24. . .