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Calculating battery size?

GregM

GregM

New Member
Joined
Nov 14, 2023
Messages
110
Location
Datil, NM
The local grid supplier says I use about 32-34 kw hours per day.
I live at 7400' elevation in the Rockies.
The coldest I have seen here is -15° F but most of the time in the winter it is in the plus teens at night, if that cold. As of this writing it is 19.5° F

My array is 32 panels (so far) of 370 watts each, so 11,840 watts.

The question of the day is what capacity batteries should I use to make it through the night?
and
Is it better to have one big battery or two small ones?

From what I have read and seen, I like the idea of Trophy Batteries.

Thank you.
Greg
 
GregM said:
The question of the day is what capacity batteries should I use to make it through the night?
Click to expand...
That really depends on how much power you use at night.
Generally you size the battery for how many days you want to run without production. For just overnight, you can probably go with 2/3 of your daily use.
So, about 22kwh of storage capacity.
GregM said:
Is it better to have one big battery or two small ones?
Click to expand...
I prefer the redundancy of multiple batteries.
 
GregM said:
The local grid supplier says I use about 32-34 kw hours per day.
I live at 7400' elevation in the Rockies.
The coldest I have seen here is -15° F but most of the time in the winter it is in the plus teens at night, if that cold. As of this writing it is 19.5° F

My array is 32 panels (so far) of 370 watts each, so 11,840 watts.

The question of the day is what capacity batteries should I use to make it through the night?
and
Is it better to have one big battery or two small ones?

From what I have read and seen, I like the idea of Trophy Batteries.

Thank you.
Greg
Click to expand...

Your MIN-11400-TL-XH-US is a high voltage battery inverter ( 400V ), that leaves only a single choice which is the Growatt battery

You can have 1 to 4 in parallel according to the data sheet on your inverter

As for size, it depends on what your loads would be in kWr, a single battery is ~9kW leaving some room for not 100% discharge, I would suggest you buy what you can afford and error on more than less

Also, you may not be aware, you are required to use an auto transformer on the backup port of the growatt for 240/120 split phase

This is why the growatt inverter is so low cost, proprietary batteries and requirement of auto transformer
 
Last edited:
Solar Guppy said:
Your MIN-11400-TL-XH-US is a high voltage battery inverter ( 400V ), that leaves only a single choice which is the Growatt battery

You can have 1 to 4 in parallel according to the data sheet on your inverter

As for size, it depends on what your loads would be in kWr, a single battery is ~9kW leaving some room for not 100% discharge, I would suggest you buy what you can afford and error on more than less

Also, you may not be aware, you are required to use an auto transformer on the backup port of the growatt for 240/120 split phase

This is why the growatt inverter is so low cost, proprietary batteries and requirement of auto transformer
Click to expand...

Since the Growatt that I have is a Grid-Tie, I may be changing brands, even before I get started.
 
GregM said:
The local grid supplier says I use about 32-34 kw hours per day.
I live at 7400' elevation in the Rockies.
The coldest I have seen here is -15° F but most of the time in the winter it is in the plus teens at night, if that cold. As of this writing it is 19.5° F

My array is 32 panels (so far) of 370 watts each, so 11,840 watts.

The question of the day is what capacity batteries should I use to make it through the night?
and
Is it better to have one big battery or two small ones?

From what I have read and seen, I like the idea of Trophy Batteries.

Thank you.
Greg
Click to expand...
1 also live in the rockies, 9000 feet. I have 2.2 kw of pv panels and until last year 1100 ah of lead acid batteries. That's 550 ah of usable battery power. Lots of sunny days here so didn't use generator much and the batteries stayed in good shape. I built two 24 volt lifep04 280 ah batteries using cells and two jk BMS. Works much better than the fla batteries. They're usually fully recharged by 1 pm. My daily kw are about 14 kwh. Good luck.
 
GregM said:
Since the Growatt that I have is a Grid-Tie, I may be changing brands, even before I get started.
Click to expand...

You will find that anything else is 4X the cost for the inverter you have, the 48V inverters battery's are 50% or more lower cost but that's not the whole story.

I have a similar system, a Solis 5G HVES which is UL9540 listed and uses a 400V BYD battery. These are plug and play and while not the lowest cost per watt for backup, they use MUCH lower cost balance of system ( compare 4AUT to 8AWG for the battery cables as an example ).

For those not wanting to become experts in system design and learning dozens of codes, the plug and play HV systems are a good choice
 
Solar Guppy said:
You will find that anything else is 4X the cost for the inverter you have, the 48V inverters battery's are 50% or more lower cost but that's not the whole story.

I have a similar system, a Solis 5G HVES which is UL9540 listed and uses a 400V BYD battery. These are plug and play and while not the lowest cost per watt for backup, they use MUCH lower cost balance of system ( compare 4AUT to 8AWG for the battery cables as an example ).

For those not wanting to become experts in system design and learning dozens of codes, the plug and play HV systems are a good choice
Click to expand...
My original thought was to go Grid-Tie and then acquire the required components and change over. As long as the grid is alive, I can use the Grid-Tie system. My main concern today is grid failure as a result of the pushing towards EVs (etc) and the potential grid collapse as the infrastructure is not and has not been improved and maintain properly over the last decade.
As an example, California, has told people not to charge their EVs, all though they also have prohibited non EV sales in the near future.
 
Your inverter is already backup capable, so adding the battery and autotransformer to a critical loads panel is a straight forward path.

One word of caution is if you decide to stay with the current inverter, buy the parts NOW, most high voltage battery solutions have been orphaned due to low sales, not technical issues.

The Only manufacture I know of that is staying in the high voltage path for retail sales is Solis with the S6

High voltage is technically a better solution than 48V market, and why I choose it, but manufactures are in the business to make $$ and need heathy sales to support a product line.

As you know, you can just use your current one as gridtie, the prices are so low, they are basically disposable and add a different backup solution at any time.
 
Solar Guppy said:
Your inverter is already backup capable, so adding the battery and autotransformer to a critical loads panel is a straight forward path.

One word of caution is if you decide to stay with the current inverter, buy the parts NOW, most high voltage battery solutions have been orphaned due to low sales, not technical issues.

The Only manufacture I know of that is staying in the high voltage path for retail sales is Solis with the S6

High voltage is technically a better solution than 48V market, and why I choose it, but manufactures are in the business to make $$ and need heathy sales to support a product line.

As you know, you can just use your current one as gridtie, the prices are so low, they are basically disposable and add a different backup solution at any time.
Click to expand...

I'll investigate.
Thanks
 
Solar Guppy said:
Your inverter is already backup capable, so adding the battery and autotransformer to a critical loads panel is a straight forward path.

One word of caution is if you decide to stay with the current inverter, buy the parts NOW, most high voltage battery solutions have been orphaned due to low sales, not technical issues.

The Only manufacture I know of that is staying in the high voltage path for retail sales is Solis with the S6

High voltage is technically a better solution than 48V market, and why I choose it, but manufactures are in the business to make $$ and need heathy sales to support a product line.

As you know, you can just use your current one as gridtie, the prices are so low, they are basically disposable and add a different backup solution at any time.
Click to expand...

Are all Growatt units like that? Do the off grid models require proprietary batteries too?
 
GregM said:
Are all Growatt units like that? Do the off grid models require proprietary batteries too?
Click to expand...

I believe all other models from growatt are low voltage battery, typically 48V. You should always review the specifications for these details.
 
Solar Guppy said:
I believe all other models from growatt are low voltage battery, typically 48V. You should always review the specifications for these details.
Click to expand...

I was looking at the Growatt 12kw, but it appears the PV inputs will not handle the 32 370w panels I have?
 
GregM said:
I was looking at the Growatt 12kw, but it appears the PV inputs will not handle the 32 370w panels I have?
Click to expand...

As you are finding, it takes time and searching to find a solution that would work for your needs, there is no can handle all situations equipement.

Also you need to define all of the needs first and requirements and work from there

  • So if your need to to support X loads for Y time, that will define the battery capacity and inverter size to support the loads.
  • Does backup need to be split phase?
  • Do you require to meet building codes? if yes that eliminates almost all the low cost options ( being gridtie the answer to this is yes which requires UL1741-SA )

Then you may wish to have Solar for charging, and if your intent is to sell back to the grid, that may or may not have anything to do with the backup strategy.

It seems that AIO ( all in one ) inverters are the majority of the choices and the expensive ones will likely meet the battery and solar needs and code, but only you can make that determination by reading through all the specification documents.

While this site is to promote anyone can DYI and save money, I think that can be a bridge too far for many for building a complex home power system, that can legally meet codes, sell to the Grid and use the more expensive UL9540 listed components.

I've been professional involved in solar for 2 decades in equipment design side and I find it a challenge to do all the research, purchasing, permit packages and installation, but this is more of a hobby, building my own systems that I enjoy the challenges
 
My intent would be to go off grid.

My panels are.....
Open Circuit voltage is 41v
Current at Max Power 10.92 A
Maximum Power 370 W
I have 32 of these panels

With the grid-tie Growatt I plan to have 4 strings each in series to 4 pv inputs. 8 panels per string = 328v at 10.91amps.

The 12kw off grid unit only has 2 PV inputs and it does not appear that the 12kw can handle this level of input.
 
GregM said:
My intent would be to go off grid.

My panels are.....
Open Circuit voltage is 41v
Current at Max Power 10.92 A
Maximum Power 370 W
I have 32 of these panels

With the grid-tie Growatt I plan to have 4 strings each in series to 4 pv inputs. 8 panels per string = 328v at 10.91amps.

The 12kw off grid unit only has 2 PV inputs and it does not appear that the 12kw can handle this level of input.
Click to expand...

If you want to go off grid ( dependent 24/7 ) you should be looking at only tier 1 providers and have multi year proven performance and reliability.

Growatt in my view would not be an option, gridtie is a completely different calculus, it's all about payback
 
Schneider XW line, MidNite Solar

These are components ( charger controllers , Inverters ) that you build systems, with redundancy.

Offgrid with AIO can be used, but you need to have hot spares available ( online or parallel ). In the AIO market Sol-Ark seems to be popular, but you need two if them if this is your only power source. Solar-Ark is a China designed product resold by exclusive Sol-Ark ( its a DEYE product ).

I was small part of the large team that created the XW line when it was Xantrex and it will be coming up on 20 years of proven service. I view off grid as you are your own power company and need to think in that mentality, you are the support when something goes wrong and you must have things separated out and redundant to keep power running when something fails ( always will )

My own system is a pair of Solis 5G HVES which has been replaced by the S6, but this is not off grid, but partial backup and peak shaving to keep my export to the grid under the 10 kWh limit for Dukes tier 1 net meter agreement
 
GregM said:
The local grid supplier says I use about 32-34 kw hours per day.
I live at 7400' elevation in the Rockies.
The coldest I have seen here is -15° F but most of the time in the winter it is in the plus teens at night, if that cold. As of this writing it is 19.5° F

My array is 32 panels (so far) of 370 watts each, so 11,840 watts.

The question of the day is what capacity batteries should I use to make it through the night?
and
Is it better to have one big battery or two small ones?

From what I have read and seen, I like the idea of Trophy Batteries.

Thank you.
Greg
Click to expand...
Rule #1 - You never have enough batteries.
Rule #2 - After you get the batteries you think you need, refer back to rule #1

Conservatively, you're harvesting at least 50,000kwh per day. You should have bettery for at least half that. 25kwh. Your grid supplier says 34kwh. IMHO you should always have enough battery to get through 1 very cloudy day.
You should really have a 48v system. a 48v/300ah is your absolute minimum 48v/600ah would be much better.
 
GregM said:
Care to name one or two?
Click to expand...
I really like Victron. I will be buying some Schneider solar charters but not wild about them. I need 60v and Victron does not do 60v. I'd take a Schneider inverter any day. Anything Schneider on the AC side of the equation should be pretty good. I worked for them briefly as a consultant, They were nice guys. Victron is what I could call an A- company. Good stuff, but expensive and lacks some protections. Midnight and MorningStar for solar chargers are very much geared toward Lead/Acid batteries. Their manuals don't talk much about lifepo4. One of them, I think Midnight didn't discuss Lifepo4 at all from what I could see in the manual.

For inverters I liked the idea of Magnum but they seem to be hard to get and costly to put together a full network of inverters.
 
GregM said:
Care to name one or two?
Click to expand...

If you don't intend to disconnect entirely from the grid, then you can use the grid for your backup. Solar and battery are prime and the grid is your fallback. In that case you can use lower cost AIO inverters without a lot of worries.
 
Dave911 said:
If you don't intend to disconnect entirely from the grid, then you can use the grid for your backup. Solar and battery are prime and the grid is your fallback. In that case you can use lower cost AIO inverters without a lot of worries.
Click to expand...
The grid is always a "backup" if available in the way of chargers.

My PV charging and grid charging are somewhat seamless. I use Victron SmartSolar chargers for everything. From the grid, I have multiple 60v power supplies that feed the SmartSolar chargers 60v. The SmartSolar charger then converts it to the proper 48v with a proper charging algorithm. Where the float value for the normal chargers is 53.8v, the float for the grid chargers are 51.60 so grid is only involved when required. They all have an absorption value of 54v so once the grid chargers kick in, they charge to the same max voltage as the solar chargers. There is some overhead in having the grid supplies on but oh well. That's just how it works. Someday I might rig up an AC solenoid based upon DC voltage but not right now. Too many other higher priorities.
 
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