solar and battery config with 12/2 wire bottleneck?

wattwatt

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I'm thinking of doing a quick and dirty installation ofsolar panels on mydetachedgarage roof (roughly 20' x 12' installation area)and storage based oneither 18650 lithium ionbatteries ordeep cycle 12V sealed lead acid batteries. There's a 12/2 wire (20 amp circuit) ran from the house, buried through thebackyard to that detached garage (for the garage door opener).If I wanted tore-purpose that wire to insteadbring power from this potential garagesolar installationto the house, any idea,roughly,what is thebestpower, panels,inverter and battery configuration I can achieveconsidering this 12/2 wire will likely be the bottleneck?
 

Cherry67

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crashintoty said:
I'm thinking of doing a quick and dirty installation ofsolar panels on mydetachedgarage roof (roughly 20' x 12' installation area)and storage based oneither 18650 lithium ionbatteries ordeep cycle 12V sealed lead acid batteries. There's a 12/2 wire (20 amp circuit) ran from the house, buried through thebackyard to that detached garage (for the garage door opener).If I wanted tore-purpose that wire to insteadbring power from this potential garagesolar installationto the house, any idea,roughly,what is thebestpower, panels,inverter and battery configuration I can achieveconsidering this 12/2 wire will likely be the bottleneck?

I guess the limit is 20 Amps, isnt it ?

I am sorry, there is not something like the best concept. There is only a advantageous compromise to your needs. So you have to share them first.
Isues are cost, quality, experience, equipment, lifetime expectation, ressources, Time available. Just as a starter.
 

Wolf

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crashintoty said:
................................... roughly,what is thebestpower, panels,inverter and battery configuration I can achieveconsidering this 12/2 wire will likely be the bottleneck?

Really easy the max is 20 amps at 120V thats 2400W.
Panels whatever you choose to compliment your storage system. You can never have too much solar only not enough.
12 Volt system will draw 200A from your batteries,24 Volt 100 Amps,and 48 Volts 50 Ampsat max load.
For your MPPT controller to work at the best efficiency your solar panel voltage should be at least 1.5 timeshigher than the nominal battery voltage.
I personally would go with 48 Volt nominal it is more efficient and cables cost less. That would mean at least 2 if not 3 panels in series.


Inverter well you are limited to 2400W

Wolf
 

OffGridInTheCity

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You could create AC 240v @ 20amps over 12/2 to send back to house. That would get you 4800watts. I just saw a pure sine wave 24v inverter that produces 240v single phase @ 3000w on Ali Express (https://de.aliexpress.com/item/32648808007.html). At the house side, you'd have to do something to step it down to 120v but I'm sure something exists for that.
 

wattwatt

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Wolf said:
crashintoty said:
................................... roughly,what is thebestpower, panels,inverter and battery configuration I can achieveconsidering this 12/2 wire will likely be the bottleneck?

Really easy the max is 20 amps at 120V thats 2400W.
Panels whatever you choose to compliment your storage system. You can never have too much solar only not enough.
12 Volt system will draw 200A from your batteries,24 Volt 100 Amps,and 48 Volts 50 Ampsat max load.
For your MPPT controller to work at the best efficiency your solar panel voltage should be at least 1.5 timeshigher than the nominal battery voltage.
I personally would go with 48 Volt nominal it is more efficient and cables cost less. That would mean at least 2 if not 3 panels in series.


Inverter well you are limited to 2400W

Wolf


  • does the 80% power safety rule thingie come into play here (i.e., 2400 * 0.8 = 1920 watts)?

    - does the 10 to 15% watt loss come into play with the DC to ACconversion (i.e., 1920 = x * 0.85, x = 2259 watts max coming from DC side of theinverter)?
  • so, for example, if I have a panel array pulling 1000 watts from the sun, but have a 200 watt charge/MPPT controller, what happens to that remaining800 watts?
  • yup, I keep hearing on the interwebs that48V is the better voltage nowadays for homes, so I'm going with 48V
  • yikes! so the panels voltage should be producing 72V if I go with a 48V battery configuration??? Does the controller need to be rated for 72V or 48V?
  • when you say "2 if not 3 panels in series" you mean 24v panels and you're saying 3 panels because of the whole 1.5 times higher rule for the controller to be efficient?
 

wattwatt

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OffGridInTheCity said:
You could create AC 240v @ 20amps over 12/2 to send back to house. That would get you 4800watts. I just saw a pure sine wave 24v inverter that produces 240v single phase @ 3000w on Ali Express (https://de.aliexpress.com/item/32648808007.html). At the house side, you'd have to do something to step it down to 120v but I'm sure something exists for that.

Really!?! You can send 4800 watts over 12/2 wire? So watts don't matter, just the amps (i.e., 20 amps max is for120v or 240v)?
 

DCkiwi

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crashintoty said:
Really!?! You can send 4800 watts over 12/2 wire? So watts don't matter, just the amps (i.e., 20 amps max is for120v or 240v)?

CORRECT. only current creates heat ... which is the reason cables/wire has a current limit, according to conductor 'area' (cross-section).

On The Other Hand ... voltage requires insulation. So a low voltage is fine with hair thin insulation ... as the voltage goes up, so does the requirement for more (IE thickness) or better (think glass / ceramic) insulation.

EXAMPLE: I can run my drones tethered, via a very cheap and thin coax pumping 48V up the cable, at low[er] current, then at the top (on the drone) use a buck convertor to bring voltage back down to 8V (running 2s on the bird), at a MUCH bigger current than the power feed wire would ever take (~13A).

HOWEVER: each change in voltage/current is inefficient, and losses occur ... just sayin'. (eg 240 to 110VAC conversion is something which I would avoid, although ... it does fulfil the 'quick and dirty' spec.
 

Wolf

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does the 80% power safety rule thingie come into play here (i.e., 2400 * 0.8 = 1920 watts)?

Ah you do know a little bit about this stuff.
Sure I would always stay within the "safety zone"
What I gave you is a rough outline to get you thinking.


- does the 10 to 15% watt loss come into play with the DC to ACconversion (i.e., 1920 = x * 0.85, x = 2259 watts max coming from DC side of theinverter)?
Yup it does but that's not what you are worried about with a limited Inverter output thatyou will be restricted to. The most I would push through that wire is 18 amps (2000W)and that only for a short time. It also depends on the length of the cable. If its 100 feet you will have some voltage drop and all I would be running off of that feed is maybe a couple of lightbulbs and maybe a TV. Certainly nothing like a motor or a high wattage item.

so, for example, if I have a panel array pulling 1000 watts from the sun, but have a 200 watt charge/MPPT controller, what happens to that remaining800 watts?

They would fry your MPPT controller.
Never heard of a 200W MPPT controller but maybe they make one.
Most are rated for max PV voltage input and then max amps. If you are buying chinese than double it in other words if your PV array is putting out 16A don't buy a 20A MPPT controller but 30A.


yup, I keep hearing on the interwebs that48V is the better voltage nowadays for homes, so I'm going with 48V
Good choice


yikes! so the panels voltage should be producing 72V if I go with a 48V battery configuration??? Does the controller need to be rated for 72V or 48V?
Most reasonably good MPPT controllers have a min and max PV input voltage and "most" are autosensing for battery voltage and are also adjustable to the battery chemistry of your choice.Shop wisely and get the right unit.

Make a list of what you want to accomplish and research your points.

when you say "2 if not 3 panels in series" you mean 24v panels and you're saying 3 panels because of the whole 1.5 times higher rule for the controller to be efficient?

Yep I have 12250Wpanels 4 sets of 3 2 of the 3 series are parallel into 1 string as are the other 2 series of 3 panels. I have 2 seperate lines going to 2 seperate MPPT controllers feeding my 48V battery bank. My max PV array Visaround 100V at ~16A per string. That translates to about 20 amps of output per MPPT controller sometimes more. I have had asmuch as 50A get pumped into my batteries


Really!?! You can send 4800 watts over 12/2 wire? So watts don't matter, just the amps (i.e., 20 amps max is for120v or 240v)?

Saw that after I posted my answer

Certainly because in a 120V system youare essentially using 1 wire for 20A Same with a single phase 240V systemHigher Voltage will do more work with the same amount of amps.
Problem is when you convert it back down to 120 there really is no gain. If anything there is loss.

20A X 120V = 2400W
20A X 240V = 4800W


Wolf
 

OffGridInTheCity

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Wolf said:
Saw that after I posted my answer

Certainly because in a 120V system youare essentially using 1 wire for 20A Same with a single phase 240V systemHigher Voltage will do more work with the same amount of amps.
Problem is when you convert it back down to 120 there really is no gain. If anything there is loss.

20A X 120V = 2400W
20A X 240V = 4800W


Wolf

There is a gain - you can step down 240v @ 20amps (single phase) to 120v @ 40amps (single phase) minus the transformer losses. Standard romex 12/2 is typically rated OK for 240v - I have this in my own home for the rainharvest shallow well pump. You could do even more at 480v if wire, inverter etc were availble (probably not).Here's an example of transformer rated at 5000watt (roughly) -https://store.maddoxtransformer.com...530616402003&gclid=CJLi7dLY2OICFY_CZAodHngALQ - for sake of discussion. Not recommending any of this - just for discussion purposes.

But of course - at some point the $ for transforming etc might be better spent on running new wire. You can buy 250feet of 6/3+ground for only $600 which would carry 240v@50amps :)
 

Wolf

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OffGridInTheCity said:
There is a gain - for sake of discussion. Not recommending any of this - just for discussion purposes.

But of course - at some point the $ for transforming etc might be better spent on running new wire. You can buy 250feet of 6/3+ground for only $600 which would carry 240v@50amps :)

You are correct I never thought of it that way 4800 Watts is 4800 Watts no matter how you send it.But as the OP said

I'm thinking of doing a quick and dirty installation ofsolar panels
I was answering in a quick and dirty way :p

Good point anyway for discussion purposes.

Wolf
 

Redpacket

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There's also the question of gear location:
Does all gear incl batteries, inverter, etc live in the garage so you're sending AC mains down the cable
OR
Just the panels in the garage & DC (safety considerations) down the cables > house where you've got the inverter, etc.

+1 Getting the correctly rated cable happening sounds a better way.
 

wattwatt

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Redpacket said:
There's also the question of gear location:
Does all gear incl batteries, inverter, etc live in the garage so you're sending AC mains down the cable
OR
Just the panels in the garage & DC (safety considerations) down the cables > house where you've got the inverter, etc.

+1 Getting the correctly rated cable happening sounds a better way.

I was under the impression that DC doesn't travel well long distances, so the 12/2 will carry AC. So essentially it'ssolarpanels, controller, battery, inverter all in the garage. Then12/2 wire with AC powerfrom garageto house... oh, do I need anew breaker panelwhere this feed comes in or do I keep running it all the way totie it into the main breaker panel?
 

DCkiwi

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crashintoty said:
I was under the impression that DC doesn't travel well long distances, so the 12/2 will carry AC. So essentially it'ssolarpanels, controller, battery, inverter all in the garage. Then12/2 wire with AC powerfrom garageto house... oh, do I need anew breaker panelwhere this feed comes in or do I keep running it all the way totie it into the main breaker panel?

Redpacket brings up an excellent point, I think.

'long distances' is ... i guess ... a relative term, but ... 20 metres aint that far for DC, as long as the current doesn't peak/spike too much.

by putting 'all in the garage' you are by definition would be putting the existing cable under peak/spike load/stresses, VS if you have just solar there, then your current is relatively smooooooth. in my experience not many houses need 2KW all the time, mine averages out at a tad under 500W (24/7/365, that is). so IFF I could get 10A @ 48V around the clock, and had big battery/inverter closer to loads, then .... that would power my whole house forever. but solar doesnt run at night, so ... i'd need a good 30-50A feed for the solar to do this ... see where Im going?

YES, breakers are REQUIRED (AFAIK) at entry point to house ... rules and regs speaking ... but beyond that ... no such thing as too many fuses/breakers. ever hear of a plumber complain there were too many taps and choices as to how to turn water off while maintenance being done? nah ... me either. heard more than a few though complain there weren't enough taps. spoil yourself, if nothing else ... buy a spanking new breaker and sleep easy. Hope that helps.
 

completelycharged

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DC is more efficient at longer distances (hundreds of miles) than AC for power transmisison newtorks, hence the few 1MV and higher long distance links around the world.

There are also DC interconnects in several countries that are also 'monopole' and use an earth return loop (e.g. Victoria to Tasmania).... they 'effectively' only use 1 wire and the ground provides the other, like the principle of the early telegraph lines. I have never seen anyone try for a monopole DC power pack transmittion line though. The earth electrodes do corrode quickly an need replacing though.

The cable you have to the garage may be SWA rated for 600V, so another option is to put the solar feed down this cable and setup the solar to the Voc (max no load V) is say 550V and Vmp around 400V. If the cable is only rated 100% duty for 15A then you have 6kW to play with... Beware with HV DC and cables though because the failure mode can end up with the cable arcing and slowly burning like a fuse down the whole length if the arc is not extinguished. 500V can sustain a DC arc around an inch and the core separation in a typical multi core cabe is well under that.. This is the sort of experiment for youtube to show, take a typical 2.5mm 2 core SWA (600V rated) and then with an open end, short it to create an arc and then see how much of the end of the cable burns off before the arc goes out.

That said, I am using 2 core flex (yes, arctic flex) to run the supply back from my panels (4 in series for 100V, 1kWp) to the MPPT controllers and would never try flex with 200V DC as I think it would just burn through the whole length under a fault.

The garage in the end is the 'safe' option as to where to put hunderds/thousands of 18650 cells of various states of health and this is what i would base my decision on.

Inverter to back feed to the house, GTIL-2000.
 

DCkiwi

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completelycharged said:
DC is more efficient at longer distances (hundreds of miles) than AC for power transmisison newtorks, hence the few 1MV and higher long distance links around the world.

There are also DC interconnects in several countries that are also 'monopole' and use an earth return loop (e.g. Victoria to Tasmania).... they 'effectively' only use 1 wire and the ground provides the other, like the principle of the early telegraph lines. I have never seen anyone try for a monopole DC power pack transmittion line though. The earth electrodes do corrode quickly an need replacing though.

ahem. backfeeding through the ground is ILLEGAL in most civilised countries. for very REAL health reasons. (ever wondered why cows stand on two feet? or why the pigs are ALL sick right now, or why people die of heart attacks when they go swimming, or ... yeah.

do NOT use our EARTH as any part of your circuit ... anyways ... *whew* glad to have gotten that off my chest (PM's invited/welcome!).

Beware with HV DC and cables though because the failure mode can end up with the cable arcing and slowly burning like a fuse down the whole length if the arc is not extinguished.

very good advice. HV DC is ... nice, but ... 'hot' electricity (there's more than one type, afterall *tweaky*) is serious stuff ...

The garage in the end is the 'safe' option as to where to put hunderds/thousands of 18650 cells of various states of health and this is what i would base my decision on.

meh. I disagree (sorry CC!). EG my garage gets too cold (out of spec for my cells). I do have a shed option (planned, for future, soon, once I have a system:load/fans/waste-heat from 18650's+inverters to keep shed above 10degC) ... but for now, I'm sleeping solid with a few thousand cells online in my house ok. of course though ... I do have ----> MANY <---- fuses. did I mention that I had lots of fuses? point made.

anyways ... this (all/posts) is probably a lot of help to the OP ... and I'd like to hear more from him in terms of spec and plans and leanings ... we could all wax lyrical for much longer, but ... help us all out here ... lol.


Inverter to back feed to the house, GTIL-2000.

above my pay grade, so no comment. completelycharged is on the ball though, so ... give us some feedback crashintoty!

PS if you ARE in the USA, then you can bet your utility is backfeeding through the ground. naughty them ...
 

Redpacket

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So to summarize the OP has a roof area of approx 20 feet x 12 feet for panels & a 20 A circuit back to the house.
For the safety reasons mentions already, the 20A circuit should carry AC mains not DC.
It seems with the available area, the OP won't be generating massive solar input & therefore having an inverter >20A output would drain down any stored power very quickly.
Assuming you're located in the USA, 120VAC @ 20A is 2.4kW continuous.
This should be adequate for running at least part of the house for now & you can always upgrade later.

PS suggest going lithium for the batteries, you'll get so much more storage & battery life.
 

DCkiwi

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Redpacket said:
So to summarize the OP has a [...] 20 A circuit back to the house.
For the safety reasons mentions already, the 20A circuit should carry AC mains not DC.
It seems with the available area, the OP won't be generating massive solar input & therefore having an inverter >20A output would drain down any stored power very quickly.

Then why not pipe the solar as DC to the house???!?!??? (notice my DC bias?)

PS suggest going lithium for the batteries, you'll get so much more storage & battery life.

agreed, except for the PS part.

performance. lifespan. power density ... all plus's in my books. and then there's the aspect of giving old cells a Second Life as Storage (get it?!). save the planet. use what others can't/discard.
 

completelycharged

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Basslink (Victoria to Tasmania) : https://en.wikipedia.org/wiki/Basslink
"Basslink is a monopolar with metallic return"
http://www.basslink.com.au/basslink-interconnector/operations/

https://www.abc.net.au/news/2019-06-02/hydro-tasmania-confirms-basslink-outage/11172028

It does create some interesting issues though as you can hook up to the power flow with a couple of earth spikes.... bit different from getting power via a large coil under a HV overhead line though :) Ahem, both illegal, as your stealing the power or absorbing the EMF pollution depending on how you view it...

The smaller GTIL-1000 could work ok, I like these units as they are relatively self contained, cut-in / cut-out and with a CT to allow your home power demand to be netted off, at a lot cheaper price than some other inverters....

Agree, temperature can cause issues for 18650's, although I just don't like the failure mode of 18650 cells because if they are all packed together they tend to cascade once cell to the next in a firework show. Fuses can prevent one cause for failure.

One option I have not seen people do is to dig a hole and put the pack in, 2m down. Temperature would be stable year round and excess heat from the pack through the year will raise the temperature up high enough to avoid low temperatures above ground, even well below zero...

Agree with Redpacket, go lithium, lead-acid is a slow decline legacy technology that worked well for over 100 years. Same reason we dont have an abacus in our pocket..
 

DCkiwi

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completelycharged said:
The smaller GTIL-1000 could work ok, I like these units as they are relatively self contained, cut-in / cut-out and with a CT to allow your home power demand to be netted off, at a lot cheaper price than some other inverters....

question: what is 'CT'?

genuine question, I dont know that acronym, so maybe others dont too?? i'm very ignorant about most things to do with inverters, so ... spiel away! : )
 

Wolf

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Boy its been busy here overnight.

DCKIWI


do NOT use our EARTH as any part of your circuit ... anyways ... *whew* glad to have gotten that off my chest (PM's invited/welcome!).

Ya better tell mother nature to stop with those lightning strikes I understand its her circuit not ours but don't you think we can use just a wee bit of it.
CT is current transformer that tells the GTIL how much current you are drawing from the grid.


I agree a GTIL 1000 would be Ideal to supply 1 leg of his grid supply.
Throttle it to 900W and the max you will be drawing from a 48V nominal bank is ~19 Amps
Put your constant usage devices such as phone chargers, PC, maybe even the fridge on that leg of your split phase.
With lots of solar and a good battery bank you would essentially not draw any power from the grid on that leg.

Wolf
 
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