New system- 24v or 48v

[mschulz]

I have a small cabin with no power, wiring, etc. We are totally off grid. We are lucky enough to have running water all gravity fed. We use the cabin on the weekends but want to spend a little more time there.

I have calculated my needs to be 370 AMH (180 AMH for 5 days autonomy) battery bank. I plan to wire my panels on the roof with aprox. 40ft total wire run from panel to batteries. I was planning on wiring the panels to 48 volt and use a MPPT charge controller.

The question I have is should I run a 48 volt battery bank or 24 volt? I plan to use an inverter to run 120v through house for lights (LED) and 3 plugs for a computer, satellite internet receiver and wireless modem.

Thanks for any help.

[Sunking]

Higher voltage is always the better choice. But you had better re figure using Watt Hours per day and work from there. Using AH is not the right way to go about it.

You state you will use 370 AH per day which means you need a 1850 AH battery for 5 day antimony. But at what battery voltage did you figure AH

12 volts @ 370 AH = 4.4 Kwh
24 volts @ 370 AH = 8.8 Kwh
48 volts @ 370 AH = 17.6 Kwh

FWIW battery voltage is dictated by panel wattage and controller size A 80 amp MPPT controller has the following input limits:

1000 watt @ 12 volts
2000 watt @ 24 volt
4000 watt @ 48 volt

[mschulz]

Higher voltage is always the better choice. But you had better re figure using Watt Hours per day and work from there. Using AH is not the right way to go about it.

You state you will use 370 AH per day which means you need a 1850 AH battery for 5 day antimony. But at what battery voltage did you figure AH

Lets look at this way. I need to get 1941 watts per day at 120 volt to run the things I want. I would like 5 days autonomy 3 at min. I get 5 hrs of good direct sunlight on my roof through out december - more in the summer.

So I guess I need 5823 to 9705 watts at 120 volts for 3-5 days of autonomy.

[Sunking]

I am going to save you a lot of time and just tell you what you need. To generate 2 Kwh of usable energy per day in your area will require:

Panel Wattage = 600 watts
Battery voltage and Amp Hours = 12 volts @ 800 AH. 24 volts @ 400 AH, or 48 volts @ 200 AH
MPPT Controller = 60 amp @ 12 volt, 30 @ 24 volt, 15 @ 48 volt
Inverter = 500 watts

Your Move.

[russ]

I get 5 hrs of good direct sunlight on my roof through out december - more in the summer.

Say 4 hours in December & January

[Sunking]

Say 4 hours in December & January

Yeah I though 5 was a stretch, but I got lazy and did not look it up. Make that

Panel wattage = 800 watts

[mschulz]

Yes it is 4 hours on the chart but observed sun is 5. I will go with 4. I want to thank you all for the replies. I was planning on 4 250-300 watt panels so that would cover it. With 48 v you say 200 amphr in the battery, does that account for 50% dod? I am just checking my math. Thanks again.

[russ]

Yes it is 4 hours on the chart but observed sun is 5. I will go with 4. I want to thank you all for the replies. I was planning on 4 250-300 watt panels so that would cover it. With 48 v you say 200 amphr in the battery, does that account for 50% dod? I am just checking my math. Thanks again.

Observed sun is meaningless - you need to be getting strong sunlight

[Sunking]

Yes it is 4 hours on the chart but observed sun is 5. I will go with 4. I want to thank you all for the replies. I was planning on 4 250-300 watt panels so that would cover it. With 48 v you say 200 amphr in the battery, does that account for 50% dod? I am just checking my math. Thanks again.

You are welcome. Like I said Amp Hours are the wrong way to figure things.

You state you want 2 Kwh of usable energy per day right? You really need 5 days of autonomy (2.5 days to 50% DOD). That means a 10 Kwh battery. Once you have the Kwh capacity then you convert to Amp Hours and the formula is:

AH = Watt Hours / Battery Voltage.

So with

12 volts, AH = 10,000 wh / 12 volts = roughly 800 AH
24 volts AH = 400
48 volts = 200 AH

Note here for something to think about. Since your [anel wattage is l;ess than 1000 watts it is possible to use a 12 volt battery with a 80 amp MPPT controller but would be very foolish choice for multiple reasons.

1. 800 AH batteries are expensive and heavy. You would have to use 2 or 4 volt cells.
2. It only gives you about 200 watts more room (assuming 800 initial) to grow with as 1000 watts with a 80 amp MPPT is maximum.
3. Much larger more expensive wiring.
4. Fire risk of fire danger because of the higher currents involved.
5. It forces you to use a 80 amp MPPT controller which maybe an unessecary expense. Depends if you want room to grow.

24 or 48 volt is the logical choice. Give the option I would recommend 48 volts at this point for several reasons.

1. 6 volt 200 AH batteries are plentiful and less expensive. You would need 8 of them wired in series.
2. You van use a less expensive 40 amp MPPT controller and leaves you room to grow to 2000 watt input
3. Smaller less expensive wiring.
4. 400% more efficient than 12 volts, and 100% more efficient than 24 volts.

Only down side is 48 volt inverters are more expensive but more than offset by savings on the rest of the hardware. It is a Win-Win.

[mschulz]

24 or 48 volt is the logical choice. Give the option I would recommend 48 volts at this point for several reasons.

1. 6 volt 200 AH batteries are plentiful and less expensive. You would need 8 of them wired in series.
2. You van use a less expensive 40 amp MPPT controller and leaves you room to grow to 2000 watt input
3. Smaller less expensive wiring.
4. 400% more efficient than 12 volts, and 100% more efficient than 24 volts.

Only down side is 48 volt inverters are more expensive but more than offset by savings on the rest of the hardware. It is a Win-Win.

Perfect, that is exactly what I was looking for. Thank you.

[esteban]

Perfect, that is exactly what I was looking for. Thank you.

Weird... I'm in Reno too! If you get this going I'd love to take a look, I'm working to build a system myself.

[Sunking]

Perfect, that is exactly what I was looking for. Thank you.

You are welcome.

[Mike90250]

...
1. 6 volt 200 AH batteries are plentiful and less expensive. You would need 8 of them wired in series.....

Here Here!! Common Golf cart or marine 6v deep cycle batteries make a great starter kit . They are maybe 70% as good as the top of the line renewable energy batteries, but less than half the cost. And since it's common for the first set to get "toasted", you feel better about replacing them in 1 or 3 years.

[thastinger]

Just buy the stuff in my sig, works great together and has been trouble free/error free for almost a year now.
My 5 230W panels are wired in series dwon the the midnite 200
be sure to properly fuse, ground, protect the people and equipment

[paulcheung]

To the OP, If I were you, I would forget about the battery to last 3 to 5 cloudy days and get a small portable 800watts to 1500 watts gas generator and a battery charger along with the system. Amazon sell the generator for less than $200 and the battery charger for less than $150. way cheaper than use 8 batteries. Batteries replacement cost is very high.

I just setup a small system to run my pump on the fish pond in the yard, I use the 12volts system because I already have a 40 amps 12volt battery charger, and the 12 volts equipment are cheaper since my load is small, 2 kwh a day run 24 hours. I use 4 6volts 225amps deep cycle batteries 2x2. it work out for me. when I have rainy days I use the grid or the generator to charge the batteries.

Cheers.