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solar and battery config with 12/2 wire bottleneck?
#1
I'm thinking of doing a quick and dirty installation of solar panels on my detached garage roof (roughly 20' x 12' installation area) and storage based on either 18650 lithium ion batteries or deep cycle 12V sealed lead acid batteries. There's a 12/2 wire (20 amp circuit) ran from the house, buried through the backyard to that detached garage (for the garage door opener). If I wanted to re-purpose that wire to instead bring power from this potential garage solar installation to the house, any idea, roughly, what is the best power, panels, inverter and battery configuration I can achieve considering this 12/2 wire will likely be the bottleneck?
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#2
(06-07-2019, 08:33 PM)crashintoty Wrote: I'm thinking of doing a quick and dirty installation of solar panels on my detached garage roof (roughly 20' x 12' installation area) and storage based on either 18650 lithium ion batteries or deep cycle 12V sealed lead acid batteries. There's a 12/2 wire (20 amp circuit) ran from the house, buried through the backyard to that detached garage (for the garage door opener). If I wanted to re-purpose that wire to instead bring power from this potential garage solar installation to the house, any idea, roughly, what is the best power, panels, inverter and battery configuration I can achieve considering 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.
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#3
(06-07-2019, 08:33 PM)crashintoty Wrote: ................................... roughly, what is the best power, panels, inverter and battery configuration I can achieve considering 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 Amps at max load.
For your MPPT controller to work at the best efficiency your solar panel voltage should be at least 1.5 times higher 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
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#4
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.
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#5
(06-07-2019, 08:59 PM)Wolf Wrote:
(06-07-2019, 08:33 PM)crashintoty Wrote: ................................... roughly, what is the best power, panels, inverter and battery configuration I can achieve considering 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 Amps at max load.
For your MPPT controller to work at the best efficiency your solar panel voltage should be at least 1.5 times higher 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 AC conversion (i.e., 1920 = x * 0.85, x = 2259 watts max coming from DC side of the inverter)?

  • 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 remaining 800 watts?
  • yup, I keep hearing on the interwebs that 48V 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?
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#6
(06-07-2019, 09:28 PM)OffGridInTheCity Wrote: 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 for 120v or 240v)?
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#7
(06-07-2019, 11:44 PM)crashintoty Wrote: Really!?! You can send 4800 watts over 12/2 wire? So watts don't matter, just the amps (i.e., 20 amps max is for 120v 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.
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all systems nominal... rock on.
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#8
Quote: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.


Quote:- does the 10 to 15% watt loss come into play with the DC to AC conversion (i.e., 1920 = x * 0.85, x = 2259 watts max coming from DC side of the inverter)?
Yup it does but that's not what you are worried about with a limited Inverter output that you 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.

Quote: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 remaining 800 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.


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


Quote: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.

Quote: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 12 250W panels  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 V is around 100V at ~16A per string. That translates to about 20 amps of output per MPPT controller sometimes more. I have had as much as 50A get pumped into my batteries


Quote:Really!?! You can send 4800 watts over 12/2 wire? So watts don't matter, just the amps (i.e., 20 amps max is for 120v or 240v)?

Saw that after I posted my answer 

Certainly because in a 120V system you are essentially using 1 wire for 20A  Same with a single phase 240V system Higher 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
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If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 8/7/2019
Total Number of Cells Recorded and processed                 6149
Total Cells required for PowIRwall                                   2856
Total Cells ≥2200mAh, ≥80%, ≥35mΩ, ≤75mΩ, ≥4.12V   2760
For Info Google Drive
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#9
(06-08-2019, 12:28 AM)Wolf Wrote: Saw that after I posted my answer 

Certainly because in a 120V system you are essentially using 1 wire for 20A  Same with a single phase 240V system Higher 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/prod...ZAodHngALQ  - 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 Smile
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#10
(06-08-2019, 01:14 AM)OffGridInTheCity Wrote: 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 Smile

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

Quote:I'm thinking of doing a quick and dirty installation of solar panels 
I was answering in a quick and dirty way Tongue

Good point anyway for discussion purposes.

Wolf
If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 8/7/2019
Total Number of Cells Recorded and processed                 6149
Total Cells required for PowIRwall                                   2856
Total Cells ≥2200mAh, ≥80%, ≥35mΩ, ≤75mΩ, ≥4.12V   2760
For Info Google Drive
Not your average Wolf       
            Cool
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