Initial system setup - getting more confused

[medic149]

I've been getting a lot of great help from people on this forum and I really appreciate it. Seems like the deeper I go into developing my system, the more confused I get. This will be for a fully off-grid 320 sqft cabin with 2 honda 2200i generators to charge batteries when needed and run an air conditioner during really hot nights.

Any advice any of you would have on this so far would be appreciated again.

So far planning:
MPPT LV6548 6.5KW – 120v Solar inverter 8kW -2 x 4kW 250V MPPT BMS
More thank likely make my own 48V LiFePO4 battery setup - maybe purchasing through Michael here
No decision on which solar panels yet

Was going to use a 24V refrigerator, but don't know if I should just go with a 120V regular fridge. Is it worth it to run a separate 12 or 24V line to the house just for the refrigerator?

Here is my energy audit and system size information with the refrigerator estimated at 1000 watts a day.

Am I heading in the right direction or have a screwed something up?? Thanks again, everyone!

Minimum System size calculations
Full Load Wattage	3203	Watts
Full Load W-hrs.	7822	W-Hrs.
Full Surge wattage	1075
Inverter efficiency	96.00%	%
Full Load Inverter size (input watts)	3336	Watts
Full Surge load Inverter size	44	Watts
Daily Storage Requirement	4000	W-Hrs/Day
Max discharge from full CHARGE	90.0%	%
# days of storage capacity	1.0	Days
System Voltage	48.0	Volts
min Battery capacity W-Hrs	4444.4	W-Hrs.
min Battery capacity A-Hrs	185.2	Amp Hours
Max continuous current from bat.	69.5	Amps
Max Continuous discharge rate	0.38	C
# sunny days to recharge while under load	2.0	Days
Required Total daily Solar output (Sunny)	6000.0	W-Hrs.
Insolation # (Equivelent hours of full sun)	4.0	Hours
Min Solar Cell Wattage	1500.0	Watts
Max Charge Current	31.3	Amps
Max Charge rate	0.17​	C

 

[John Frum]

Looks like you want to do a 48 volt system.
Will your inverter be running all the time?
If yes the conversion efficiency for a 48->24 converter is going to be ~90% and the efficiency for the inverter is ~85% so you would be running an otherwise unnecessary 48->24 converter for just one application.

Considering your max wattage is ~3000 you could easily do a 24 volt system.
48 volts is also fine.
A 16s 272ah battery is ~14kwh which is more than your audit indicates.

These are my goto recommend https://www.samlexamerica.com/products/Category.aspx?cid=M11

You mention running a 24 volt line into the house, does that mean your system is in another building?

 

[medic149]

Looks like you want to do a 48 volt system.
Will your inverter be running all the time?
If yes the conversion efficiency for a 48->24 converter is going to be ~90% and the efficiency for the inverter is ~85% so you would be running an otherwise unnecessary 48->24 converter for just one application.

Considering your max wattage is ~3000 you could easily do a 24 volt system.
48 volts is also fine.
A 16s 272ah battery is ~14kwh which is more than your audit indicates.

These are my goto recommend https://www.samlexamerica.com/products/Category.aspx?cid=M11

You mention running a 24 volt line into the house, does that mean your system is in another building?

Thank you! I was wondering if I should just go with 24V instead.

 

[John Frum]

Thank you! I was wondering if I should just go with 24V instead.

Was there a reason your were leaning toward 48 volts?
You never answered about the dedicated power house.

 

[medic149]

Was there a reason your were leaning toward 48 volts?
You never answered about the dedicated power house.

Sorry about that. I get in the zone with reading things and looking up stuff trying to learn and learn. lol

My plan is to have a powerhouse/small shed to keep everything in and between the solar panels and the cabin. Then run electric lines from the powerhouse to the cabin. It would be pretty short run. Probably 30-40 feet tops if not less.

From the research I was doing, it seemed like people were saying go to 48 if you can, but I am very fine with 24.

 

[John Frum]

Sorry about that. I get in the zone with reading things and looking up stuff trying to learn and learn. lol

My plan is to have a powerhouse/small shed to keep everything in and between the solar panels and the cabin. Then run electric lines from the powerhouse to the cabin. It would be pretty short run. Probably 30-40 feet tops if not less.

From the research I was doing, it seemed like people were saying go to 48 if you can, but I am very fine with 24.

Both 24 and 48 volts are doable as far as voltage drop goses for a fridge over an 80 foot round trip.
48 volts of course will be better.
So you could have the step down converter in the house if that is your only dc load.
If it was me though I would just run 120VAC and forget the DC fridge unless you have a good reason to do it.

 

[HRTKD]

If it was me though I would just run 120VAC and forget the DC fridge unless you have a good reason to do it.

Agreed! One set of wires from the shed to the house, instead of two.

medic149, check out the details specs on the DC refrigerators. Based on a few that I've looked at recently, they aren't all that efficient. I could run a propane absorption refrigerator for weeks with a 30 lb propane tank. (I'm not advocating that type of refrigerator for you, just throwing it out as an example of efficiency.) The larger selection of residential refrigerators seem to be more efficient than the DC refrigerators. This is my take based on just a small amount of research. Check it out for yourself.

I boondock almost exclusively in my RV trailer. In moderate weather I could easily stay out for two to three weeks with just two 30 lb propane tanks to run the refrigerator and the water heater and never really worry about the batteries. I might have to keep a closer eye on the battery state of charge if I used a DC refrigerator.

 

[John Frum]

If you want to minimize the startup demands get an inverter fridge.

 

[medic149]

Agreed! One set of wires from the shed to the house, instead of two.

medic149, check out the details specs on the DC refrigerators. Based on a few that I've looked at recently, they aren't all that efficient. I could run a propane absorption refrigerator for weeks with a 30 lb propane tank. (I'm not advocating that type of refrigerator for you, just throwing it out as an example of efficiency.) The larger selection of residential refrigerators seem to be more efficient than the DC refrigerators. This is my take based on just a small amount of research. Check it out for yourself.

I boondock almost exclusively in my RV trailer. In moderate weather I could easily stay out for two to three weeks with just two 30 lb propane tanks to run the refrigerator and the water heater and never really worry about the batteries. I might have to keep a closer eye on the battery state of charge if I used a DC refrigerator.

I know what you mean about the refrigerators. I was set on the Unique 13.0 cu/ft Solar Powered DC Fridge – AC convertor to run on a standard household AC plug, but for the price, I might be better off with just a regular 120 refrigerator. It would save running 24v wiring into the cabin since that was the only thing that would be on 24v.

 

[John Frum]

Sorry about that. I get in the zone with reading things and looking up stuff trying to learn and learn. lol

My plan is to have a powerhouse/small shed to keep everything in and between the solar panels and the cabin. Then run electric lines from the powerhouse to the cabin. It would be pretty short run. Probably 30-40 feet tops if not less.

From the research I was doing, it seemed like people were saying go to 48 if you can, but I am very fine with 24.

Lifepo4 batteries are safe enough to keep in the living space.
The powerhouse idea originated with flooded lead acid batteries that off gassed and got reinforced with the explody flavours of lithium.
But its not an issue with lifepo4.
Since lifepo4 batteries thrive at 20-25C they really probably should be in the living space.

 

[medic149]

Lifepo4 batteries are safe enough to keep in the living space.
The powerhouse idea originated with flooded lead acid batteries that off gassed and got reinforced with the explody flavours of lithium.
But its not an issue with lifepo4.
Since lifepo4 batteries thrive at 20-25C they really probably should be in the living space.

I did consider inside the cabinet, but wifey isn't happy with that lol and I'll keep the generators installed in the powerhouse and keep everything close to each other.

 

[medic149]

I'm trying to make sure that the Lishen batteries, 3.2v 270amp will work with my setup.

I think I need 16 of these batteries. Do you know the best way to build them? 4 cells in series to make 12v and then put all 4 12v packs in parallel?

 

[John Frum]

I'm trying to make sure that the Lishen batteries, 3.2v 270amp will work with my setup.

I think I need 16 of these batteries. Do you know the best way to build them? 4 cells in series to make 12v and then put all 4 12v packs in parallel?

With 16 cells you can make...
1x 16s 48 volt battery
2x 8s 24 volt batteries
4x 4s 12 volt batteries

Which system voltage did you decide on?

 

[medic149]

With 16 cells you can make...
1x 16s 48 volt battery
2x 8s 24 volt batteries
4x 4s 12 volt batteries

Which system voltage did you decide on?4

48v if I keep with the MPP LV6548. Would the 16 cells in series provide enough to cover my 8000 watt hour needs?

 

[John Frum]

48v if I keep with the MPP LV6548. Would the 16 cells in series provide enough to cover my 8000 watt hour needs?

272 ah * 3.2 volts * 16 cells * .8 depth of discharge = ~11,141wh

 

[medic149]

Thanks! I actually came up with that 11,141 kwh in one of my many calculations I did lol

 

[John91]

I've been getting a lot of great help from people on this forum and I really appreciate it. Seems like the deeper I go into developing my system, the more confused I get. This will be for a fully off-grid 320 sqft cabin with 2 honda 2200i generators to charge batteries when needed and run an air conditioner during really hot nights.

Any advice any of you would have on this so far would be appreciated again.

So far planning:
MPPT LV6548 6.5KW – 120v Solar inverter 8kW -2 x 4kW 250V MPPT BMS
More thank likely make my own 48V LiFePO4 battery setup - maybe purchasing through Michael here
No decision on which solar panels yet

Was going to use a 24V refrigerator, but don't know if I should just go with a 120V regular fridge. Is it worth it to run a separate 12 or 24V line to the house just for the refrigerator?

Here is my energy audit and system size information with the refrigerator estimated at 1000 watts a day.

Am I heading in the right direction or have a screwed something up?? Thanks again, everyone!

Minimum System size calculations
Full Load Wattage	3203	Watts
Full Load W-hrs.	7822	W-Hrs.
Full Surge wattage	1075
Inverter efficiency	96.00%	%
Full Load Inverter size (input watts)	3336	Watts
Full Surge load Inverter size	44	Watts
Daily Storage Requirement	4000	W-Hrs/Day
Max discharge from full CHARGE	90.0%	%
# days of storage capacity	1.0	Days
System Voltage	48.0	Volts
min Battery capacity W-Hrs	4444.4	W-Hrs.
min Battery capacity A-Hrs	185.2	Amp Hours
Max continuous current from bat.	69.5	Amps
Max Continuous discharge rate	0.38	C
# sunny days to recharge while under load	2.0	Days
Required Total daily Solar output (Sunny)	6000.0	W-Hrs.
Insolation # (Equivelent hours of full sun)	4.0	Hours
Min Solar Cell Wattage	1500.0	Watts
Max Charge Current	31.3	Amps
Max Charge rate	0.17​	C

I know how you feel I just posted same comment lol. I think your ahead of me though so congrats!

 

[GXMnow]

Where are you located? How cold does it get? What are the expected "Sun Hours" for the worst case in winter? If it does stay below freezing, you may need to have heat in the power house for the batteries. You can't charge LFP batteries below freezing. There are now some packaged LFP batteries with built in heaters, and others have used heating pads to keep the cells warm. If you are in a very cold climate, it just makes more sense to keep the batteries in the heated living space rather than throwing a bunch of your energy heating a shed. But if you are in a warm climate, and going below freezing is rare, then it is not a big deal.

For fully off grid, you do want the solar panels to provide all the power you need for a day, plus a bit more to make up for a cloudy day and get the batteries charged back up. The battery bank should also be able to fully run you for over a full day, again, to allow for that day without sun. Fire up the generator on the second day without sun. Looking at your energy audit in the first post, I think you are going to want a bit more than 1,500 watts of solar. Something in the chart seems a bit off. Only 44 watts of surge load? 3,203 watts maximum power, but then only needing 7,822 watt hours. Seems a bit light, unless a lot of load is very short term. But then also only claiming you need 4,444.4 watt hours of battery to run that 7,822 watts looks a bit off as well. While it is true, you can be using some of your power while the sun is up, you may run into issues if you have clouds when you need power, and your battery is not there to back you up. If your energy audit says you need 7,822 watt hours, I would have at least that much battery. Even though LFP cells can run down to below 10%, they will still last much longer if you keep them above 40%. If you use just a single 16S string of the common 270 amp hour LFP cells, that gets you over 12 KWH of useable storage. Then put up eight 300 watt solar panels and you get 9,600 watt hours on an average 4 sun hour day. That easily covers your 7,822 watt hour usage estimate, and allow for a bit of fudge factor when you leave a TV on over night. And if you have poor sun for 2 days, you might still make it without needing to start the generator. The extra solar panel or two does not cost much when you plan out the whole system. The batteries do get a bit expensive, but right now, the 270 AH cells are about the lowest cost per KWH. If you want to use pre built packaged cells, it will certainly cost more. You need to weigh that out for yourself. IT would take something like 12 Battle Born 12 volt 100 AH batteries to match that string of 16 270's.