Can someone offer this Newbie some advice please?

[hatchetbayfarms20]

I am trying to learn what I can about solar systems based on my need to implement an off-grid system for a new farm we are developing here in the Bahamas. I've been trying to understand the numbers and put together what our Loads List might look like. I've gone through calculations based on information found on the internet and think I've gotten that part covered. I got stuck at the very end in trying to figure out what our battery bank would look like based on the two example batteries that I found. As long as the numbers were simple, I understood about connecting the batteries in Series for Volt additive and Parallel for Amp additive. Once the numbers got a little more as in our numbers, that's where I got lost and couldn't get what my Series-Parallel configuration would look like. Can someone school me please?

Appliance	Quantity	Watts	Hours On per Day	Watt Hours per Day
AC 10k BTU	1	820	12	9840
Coolbot	1	5	12	60
LED Bulbs 60W	8	13	1	104
Drying Station	4	58	1	232
Cooling Fans	2	160	1	320
Small Fridge	1	110	24	2640
Microwave	1	900	.30	270

Days of autonomy: 3
Total Watt Hours per Day: 13,466

Temp. Battery Temp Multiplier
======================
80 °F 1.00
70 °F 1.04
60 °F 1.11
50 °F 1.19
40 °F 1.30
30 °F 1.40
20 °F 1.59

Stats
======================
13,466 Wh - 48 V - 3 Days - DoD 50% - Inverter 92% - Temp 80 F

Avg Daily Wh
======================
13,466 Wh / .92 Inverter Efficiency = 14,637 Wh

Battery Bank Capacity (Wh)
======================
14,637 Wh x 3 days x 1 Temp / .50 Battery = 87,822 Wh

Battery Bank Capacity (Ah)
======================
87,822 Wh / 12 V = 7,661 Ah
87,822 Wh / 24 V = 3,660 Ah
87,822 Wh / 48 V = 1,830 Ah

Battery
======================
12 V 1000 Ah Deep Cycle
12 V 200 Ah Deep Cycle

Battery Bank Needed
======================
?

 

[nosys70]

for such power, forget low voltage, even 48V is low, you should consider higher voltage. or split the load over several systems.
if you are off-grid, redundancy is a must.
you can find OPzV or OPzS module up to 3000ah.
Since you run an AC 12 hours a day, you should consider to keep cold air to cool batteries.
the calculation for the fridge is strange, a fridge is not running permanently , juste a few minutes per hours.
The biggest problem is motors (Ac, fridge) which can request a huge amount of power to start.
you will probably need a 5KW inverter.
there are now AC on inverter that have soft start, while if yours is already rated 800Wh for 10k btu , is it probably already a model with inverter.

it seems you decided on FLA batteries with your .5 coefficient.
you could probably go with LFP (cells up to 1000Ah exist) and get a better coefficient like .8

 

[DASH]

What's Your budget for this solar setup? The current battery configuration You have is going to be Your major cost by far.

 

[John Frum]

I am trying to learn what I can about solar systems based on my need to implement an off-grid system for a new farm we are developing here in the Bahamas. I've been trying to understand the numbers and put together what our Loads List might look like. I've gone through calculations based on information found on the internet and think I've gotten that part covered. I got stuck at the very end in trying to figure out what our battery bank would look like based on the two example batteries that I found. As long as the numbers were simple, I understood about connecting the batteries in Series for Volt additive and Parallel for Amp additive. Once the numbers got a little more as in our numbers, that's where I got lost and couldn't get what my Series-Parallel configuration would look like. Can someone school me please?

Appliance	Quantity	Watts	Hours On per Day	Watt Hours per Day
AC 10k BTU	1	820	12	9840
Coolbot	1	5	12	60
LED Bulbs 60W	8	13	1	104
Drying Station	4	58	1	232
Cooling Fans	2	160	1	320
Small Fridge	1	110	24	2640
Microwave	1	900	.30	270

Days of autonomy: 3
Total Watt Hours per Day: 13,466

Temp. Battery Temp Multiplier
======================
80 °F 1.00
70 °F 1.04
60 °F 1.11
50 °F 1.19
40 °F 1.30
30 °F 1.40
20 °F 1.59

Stats
======================
13,466 Wh - 48 V - 3 Days - DoD 50% - Inverter 92% - Temp 80 F

Avg Daily Wh
======================
13,466 Wh / .92 Inverter Efficiency = 14,637 Wh

Battery Bank Capacity (Wh)
======================
14,637 Wh x 3 days x 1 Temp / .50 Battery = 87,822 Wh

Battery Bank Capacity (Ah)
======================
87,822 Wh / 12 V = 7,661 Ah
87,822 Wh / 24 V = 3,660 Ah
87,822 Wh / 48 V = 1,830 Ah

Battery
======================
12 V 1000 Ah Deep Cycle
12 V 200 Ah Deep Cycle

Battery Bank Needed
======================
?

I don't see any indication of your peak ac load.
Need to calculate the momentary peak and the continuous ratings for the inverter.
If your pure dc load is not trivial you will need to look at that as well.

 

[zaps]

If you wanted your battery bank to use 48V, I think using 8 of the 1000 Ah batteries in a 4S2P configuration, or 40 of the 200 Ah batteries in a 4S10P configuration, might work. But, I'm new at this, and as nosys70 implied, such a battery bank would need hundreds of amps from a solar charger to charge in a day.

 

[zaps]

About that peak AC load, if everything listed in the table were on at once and only drew the watts shown, that totals only 2491 watts. At 120 VAC in the Bahamas, that's 20.76 amps.

 

[nosys70]

yes the system is relatively small if you read only these numbers.
the problem is the poster wants 3 days at full power on battery.
that is huge and over conservative.
if you get 3 days in a row with bad weather, you probably just need to turn off the AC.
if you get enough panels to run 13KW a day,without AC (80% of the bill) , and even with almost no sun you should still be able to run all the rest.
going off grid can be done two ways.
you adapt your behaviour to the situation (lower your lifestyle) , or you size the system like if you are still on grid.
but the bill will be probably pretty high (we speak here about 40.000 dollars system, most of it for the battery)

 

[DASH]

yes the system is relatively small if you read only these numbers.
the problem is the poster wants 3 days at full power on battery.
that is huge and over conservative.

Yes, if it was me, I would oversize the solar system to help make up for the days of poor sunlight, and that could substantiall reduce the battery size of the system. One has to be careful though, that the rate of charging the battery system doesn't exceed the recommendation of the battery manufacturer. With the number of batteries (even if reduced) this will probably not be a problem. Some controllers (outback, I think) will allow one to adjust the rate of charge.

 

[zaps]

I was thinking of a third way to go off grid... with a hurricane!

 

[nosys70]

yes the reasoning is how much solar panel power do i need to ensure that even at worse day i can still produce 13KW.
Because reasoning on the battery cause several issues.
1- price since you end up with huge battery.
2- charging it since your panel power will be to low to charge the battery even over several day.
3-So finally you would end up with a big battery that you would never discharge significantly, and if you do it (worst case) it would take forever to charge it again.

So the good path is usually to stay on the daily consumption, multiplying the solar panel power by 4 (panel are cheap) , and keeping enough battery to make sure you can run on battery overnight, but make sure you can do a full charge on the next day + powering you loads.

So to take back is calculation:
14KWday are needed, so if we take 5 hours of sun, it means 14/5= 2.8KWh of solar panels to charge the battery.
double that to power loads during these 5 hours (while charging the battery) = 5.6KWh
double again for safety (cloudy,rainy days) 11.2Kwh, round it to 12.
with that you charge the battery, power you loads during 5 hours, and power loads (except AC) for about 7 hours.

now you know that the battery is 48V, so 2800W/48=58.3A , so you need a charger able to deliver 60Amps and a battery able to gobble up 60A.
since usually battery charging is advised between 0.5C and 1C, you can say that a 150Ah battery is ok.
since you will hardly get 100% of capacity, you will better take a 280Ah battery and use only 200Ah.

200Ahx48V=9600wh.
That is insufficient to run the loads a full day on battery , but in theory, most of the time, the load should run from solar panel (at least 5 hours over 12)
The only problem here is when the AC is running.
if it is on daylight, the power should be covered mostly by panels. If it is on the night, we need a bigger battery.
but if it is proven that the battery is too small, no problem, we have 12KWh of panels, so we can increase battery up to 6KWh, that is 125Ah.
At 0.5C , this is still a 250Ah battery.
since we started on a 280Ah battery, this is not changing the battery , just pushing the discharge to 90%, that is still ok for a LFP battery.
250Ah x 48V = 12KWh
if we want to make the installation rock solid, i would choose 320Ah cells.

 

[svetz]

... got stuck at the very end in trying to figure out what our battery bank would look like...
...87,822 Wh / 48 V = 1,830 Ah...
...I understood about connecting the batteries in Series for Volt additive and Parallel for Amp additive.

Good job on the math, seems like you've a good handle on it. That you understand parallel strings to increase amps but are struggling with this makes me think what you're talking about is that you've discovered to get 1830 Ah @48V with 12V batteries you need more than 3-4 parallel strings of lead acid batteries and that's generally considered a bad idea for lead acid batteries as string interactions tend to shorten battery life beyond that.

The usual way around this is to use batteries of a lower voltage and higher Ah (see the battery FAQ). For example with this 2V 1215 Ah battery you could put 24 of them in series (48V & 1215 amp hours); so two parallel strings would provide 2430 Ah at 48V.

Here are a few alternate solutions for you:

• Abandon the thought of 3 days of battery reserve...instead use 1 day and get a generator to recharge the batteries for days beyond that if there's no sun. This is probably the most economical solution unless you're positive that most of the time you'll be without sun and power for more than a couple of days. If natural gas is available and not likely to run out in three days that's the way to go, otherwise perhaps a propane generator with a backup fuel tank on the farm? Unlike gasoline or diesel, propane can be stored indefinitely.
• Switch to LiFePO4 - Even if you don't plan to frequently cycle the batteries this may be a less expensive way to go and you can put more in parallel.
• Increase Voltage - drives up the cost of ancillary equipment and higher voltages are more dangerous
• Switched Banks

 

[hatchetbayfarms20]

My apologies to all of you that replied to my post. I wasn't ignoring it but rather like another post that I made, was checking my email since it would send one each time someone replied. This time that didn't seem to happen and only after thinking that it's been a while I decided to come to the forum directly and then found all of your replies. So thank you one and all.

Since I am new to solar period, someone your explanations I will admit went above my head and I apologize for that. Therefore my numbers may be a little off here and there, which I do know can also lead to getting inaccurate information, but, I am trying to learn what I can. When I started looking into this it was because I have seen youtube videos with others who said that they have been able to run small window ac units off of solar systems and what they described, didn't seem all that much. So, I thought that it would be as simple as they made it seem. From all that we will be needing energy for on the farm, the cold storage room which will use the window ac unit is the lion share of the power needed. The small fridge and microwave we can get by without for now.

What I was trying to determine at this point based on the numbers I attempted to write down was just how I would go about building out the battery bank to match the needs that I have listed. The budget, while it will be important is not really a concern right now. Once I can figure out the how, then the approach can be tailored to fit what we may actually go with. For example, I went with 3 Days of autonomy because of what I was following along in the examples that I was using. But as you know, I'm in the Bahamas, so we have a lot of sun hours here and don't really go too long without. They were always suggesting that I error on the side of worst case in my calculations.

Thanks again all, hopefully my newbie-ness is not too much and causing frustration for you. I will try and answer you as best I can. I really am trying to learn as much, at least the good basics so that my decisions can be informed.

Regards,
Marvin
Hatchet Bay Farms

 

[hatchetbayfarms20]

Good job on the math, seems like you've a good handle on it. That you understand parallel strings to increase amps but are struggling with this makes me think what you're talking about is that you've discovered to get 1830 Ah @48V with 12V batteries you need more than 3-4 parallel strings of lead acid batteries and that's generally considered a bad idea for lead acid batteries as string interactions tend to shorten battery life beyond that.

The usual way around this is to use batteries of a lower voltage and higher Ah (see the battery FAQ). For example with this 2V 1215 Ah battery you could put 24 of them in series (48V & 1215 amp hours); so two parallel strings would provide 2430 Ah at 48V.

Here are a few alternate solutions for you:

• Abandon the thought of 3 days of battery reserve...instead use 1 day and get a generator to recharge the batteries for days beyond that if there's no sun. This is probably the most economical solution unless you're positive that most of the time you'll be without sun and power for more than a couple of days. If natural gas is available and not likely to run out in three days that's the way to go, otherwise perhaps a propane generator with a backup fuel tank on the farm? Unlike gasoline or diesel, propane can be stored indefinitely.
• Switch to LiFePO4 - Even if you don't plan to frequently cycle the batteries this may be a less expensive way to go and you can put more in parallel.
• Increase Voltage - drives up the cost of ancillary equipment and higher voltages are more dangerous
• Switched Banks

Howdy Svetz,

Think you hit it on the head. Once the numbers got like mine, I was having a hard time trying figure out just how to build the battery bank. What everyone seemed to be saying was that 2 or 3 string max was recommended. My numbers were causing me to go beyond that amount for strings and thus my confusion. You've even brought something to my attention that I thought was the way it all worked. And that is when strings are actually more important and a concern. I only went with what you see based on my following along information that I was finding. But didn't know that strings were used based on the type of battery you chose, in particular lead acid. Can you give a good example of an LiFePO4 that I might look at using?

I do have a Portable Dual Fuel Generator (uses propane) as backup and part of our plan. In fact, that was going to be what we were thinking about using in the first place, but figured solar might be the better way to go.

Thanks as always. For your response on my first post and now this one.

 

[hatchetbayfarms20]

I don't see any indication of your peak ac load.
Need to calculate the momentary peak and the continuous ratings for the inverter.
If your pure dc load is not trivial you will need to look at that as well.

Hi SmoothJoey.. I will have to look into that. I didn't know. Thanks

 

[hatchetbayfarms20]

yes the reasoning is how much solar panel power do i need to ensure that even at worse day i can still produce 13KW.
Because reasoning on the battery cause several issues.
1- price since you end up with huge battery.
2- charging it since your panel power will be to low to charge the battery even over several day.
3-So finally you would end up with a big battery that you would never discharge significantly, and if you do it (worst case) it would take forever to charge it again.

So the good path is usually to stay on the daily consumption, multiplying the solar panel power by 4 (panel are cheap) , and keeping enough battery to make sure you can run on battery overnight, but make sure you can do a full charge on the next day + powering you loads.

So to take back is calculation:
14KWday are needed, so if we take 5 hours of sun, it means 14/5= 2.8KWh of solar panels to charge the battery.
double that to power loads during these 5 hours (while charging the battery) = 5.6KWh
double again for safety (cloudy,rainy days) 11.2Kwh, round it to 12.
with that you charge the battery, power you loads during 5 hours, and power loads (except AC) for about 7 hours.

now you know that the battery is 48V, so 2800W/48=58.3A , so you need a charger able to deliver 60Amps and a battery able to gobble up 60A.
since usually battery charging is advised between 0.5C and 1C, you can say that a 150Ah battery is ok.
since you will hardly get 100% of capacity, you will better take a 280Ah battery and use only 200Ah.

200Ahx48V=9600wh.
That is insufficient to run the loads a full day on battery , but in theory, most of the time, the load should run from solar panel (at least 5 hours over 12)
The only problem here is when the AC is running.
if it is on daylight, the power should be covered mostly by panels. If it is on the night, we need a bigger battery.
but if it is proven that the battery is too small, no problem, we have 12KWh of panels, so we can increase battery up to 6KWh, that is 125Ah.
At 0.5C , this is still a 250Ah battery.
since we started on a 280Ah battery, this is not changing the battery , just pushing the discharge to 90%, that is still ok for a LFP battery.
250Ah x 48V = 12KWh
if we want to make the installation rock solid, i would choose 320Ah cells.

Nosys70,

More information to digest for me. Now to get my brain to wrap itself around it. Thank you very much

 

[hatchetbayfarms20]

If you wanted your battery bank to use 48V, I think using 8 of the 1000 Ah batteries in a 4S2P configuration, or 40 of the 200 Ah batteries in a 4S10P configuration, might work. But, I'm new at this, and as nosys70 implied, such a battery bank would need hundreds of amps from a solar charger to charge in a day.

Thank you Zaps

 

[svetz]

... What everyone seemed to be saying was that 2 or 3 string max was recommended. My numbers were causing me to go beyond that amount for strings and thus my confusion....

The basic theory is that lead acid batteries in parallel run the risk of uneven charging and discharging, potentially shortening the batteries' life.

AFAIK, LiFePO4 doesn't have this limitation; probably because it's so common to use a BMS to keep all the cells at the correct voltage and so they stay balanced. On their car packs Tesla fuses each cell, if you go DIY with many in parallel you might want to fuse each...or at least each string in case of a bad cell.

...Can you give a good example of an LiFePO4 that I might look at using?....

Sorry, I'm just a theory guy. But... Will's keeps his DIY solar blueprints updated with what he believes is the best gear at the best cost. In fact, a DIY battery will save you a ton of $.
As it's a business you might be looking for a turnkey system, in which case something like the Tesla powerwall (NMC) with 3200 cycles or the Enphase Encharge (LiFePO4) with 4,000 cycles might be worth looking into.

A big system like yours...keep in mind the round-trip efficiency of lead acid is only 80%, Lithium is generally higher...for example the Encharge is 96% for the battery and with inverter/charger (AC round-trip) 89% efficient. The less efficient it is, the more panels, wiring, racking you'd need for solar. In a big setup like yours those additional costs add up.

...I do have a Portable Dual Fuel Generator (uses propane) as backup and part of our plan...

A combination backup plan for something as big as you envision seems like the way to go. After a year of operating costs you can project your propane fuel costs to determine what the expansion trigger $ points are for solar and battery (at least until you get your first big generator maintenance bill ;-). Best to know the $ trigger points because you can be sure propane prices will go up and battery prices will fall.

 

[hatchetbayfarms20]

The basic theory is that lead acid batteries in parallel run the risk of uneven charging and discharging, potentially shortening the batteries' life.

AFAIK, LiFePO4 doesn't have this limitation; probably because it's so common to use a BMS to keep all the cells at the correct voltage and so they stay balanced. On their car packs Tesla fuses each cell, if you go DIY with many in parallel you might want to fuse each...or at least each string in case of a bad cell.


Sorry, I'm just a theory guy. But... Will's keeps his DIY solar blueprints updated with what he believes is the best gear at the best cost. In fact, a DIY battery will save you a ton of $.
As it's a business you might be looking for a turnkey system, in which case something like the Tesla powerwall (NMC) with 3200 cycles or the Enphase Encharge (LiFePO4) with 4,000 cycles might be worth looking into.

A big system like yours...keep in mind the round-trip efficiency of lead acid is only 80%, Lithium is generally higher...for example the Encharge is 96% for the battery and with inverter/charger (AC round-trip) 89% efficient. The less efficient it is, the more panels, wiring, racking you'd need for solar. In a big setup like yours those additional costs add up.


A combination backup plan for something as big as you envision seems like the way to go. After a year of operating costs you can project your propane fuel costs to determine what the expansion trigger $ points are for solar and battery (at least until you get your first big generator maintenance bill ;-). Best to know the $ trigger points because you can be sure propane prices will go up and battery prices will fall.

Thanks Svetz. Guess I will keep digging. I will check out Will's DIY solar blueprints for more information as well.

 

[Hedges]

Suggestion to make the system much smaller:

If you left off the A/C load, system could be sized for 3600 Wh/day. Then put in enough PV to run A/C as well, and control it with a "battery full" signal.

Something like I have, but smaller:
one SI 6048U
One Sunny Boy, anything from 3000W or larger should do, but just get the 7.0 or 7.7 kW model for future expansion.
1.5 kW to 4 kW (depending on PV capacity) 120/240V auto transformer to match Sunny Boy voltage to single Sunny Island (assuming your appliances fit the 120V US Sunny Island, otherwise get 240V model.)
3kW to 5kW of PV panels. Make two strings, one oriented toward 10:00 AM sun, the other toward 4:00 PM
four, 100 Ah 12V SunXtender batteries.

I expect you to cycle the batteries to less than 50% depth of discharge each night. Should last 3 to 10 years, depending on how much less.
Program a relay from Sunny Island to only enable the A/C circuit when battery above 90% full.
The second relay can auto-start your generator at a suitable depth of discharge.
PV array can be oversized to as much as 12kW. With two strings at 90 degrees to each other, they would produce about 8.4 kW peak, with production flattened over the day. Consume all you want while the sun shines.

 

[DASH]

Solar Panels are cheap. Batteries are not. You may find that what works best for You is Solar during the day, and maybe a generator for part of the night. It would be really cool, if You could somehow create a chiller that would work during the day (consuming solar electricity), and then release the cold it stored during the night.