Off Grid 1000w isolated circuit - need advice

If you intend to expand the system you may want to target a higher voltage on your PV system. Ultimately you will end up using much smaller wire and running less amps on the low voltage side. If I had it to do over I would target a 48 volt system.

Keith,

I'm wondering if it would be better to skip the tracker and use the money you save for more solar panels and batteries instead. 

John

Hi Keith,

I would agree that more panel or battery would be money better spent than a tracker. I had checked your inverter and saw it was good for 24 volts, thats all I was thinking you may want to do. I tried to check the volts at load of the Sharp NT 180V1 panel and I could not find what it was. I made the mistake of thinking that a 24 volt panel was a 24 volt panel. I bought Kyocera KC158G panels and they have a volt at load rating of 23.2 volts. To keep a 24 volt battery bank charged you need 27 volts. I ended up with a 20 volt battery bank. Not ideal but it works. I like your battery choice and I think you will be happier with that choice. I hope you enjoy putting this together it is a great learning experience and it is fun to do.

Dennis

Keith,
I was assuming that you were going to go up to 24 volts on the battery also. I would go to a 24 volt battery bank. Just series two 12 volt batteries. This accomplishes two things. It makes the draw on the batteries half as much which will make the batteries last substantially longer and it gives you more reserve power. The point I was trying to make on the charging was to have high enough volts at load out of the panel to maintain the batteries properly. To maintain the batteries properly you should be able to do a bulk charge at 2.37 to 2.45 volts per cell or 28.9 volts to recharge the battery. When the battery is fully charged you will want to fall back to a float charge of 2.2 to 2.23 volts per cell or 26.58 volts. Then on a regular basis several times a year you will need to do an equalization charge on the batteries at 2.58 to 2.67 volts per cell or about 31.5 volts. This prevents sulfation of the batteries and makes them last much longer.
Most charge controllers have these features built in and you just program the voltages, durations and how often the charge occurs. So for a 24 volt battery bank you will want a panel that is capable of doing an equalization charge to the batteries. Check the manufacturers charging recommendations for the specific batteries that you buy. The panel that you are looking at looks like a good choice to me. The Sharp panel will give you 35.86 volts at load so you should never have any trouble getting the voltage necessary to maintain a 24 volt battery bank and you will get efficient usage of the panel.
Regard,
Dennis

Keith,
Double check and make sure your inverter is good for 24 volts before you commit to everything else at 24 volts.
Regards,
Dennis

We are talking about electricity and electricity is all about the math.

Typically it all starts with a load assessment. You need numbers; Amps, volts, watts, hours, and for photovoltaics - longitude and latitude.

Working with the only numbers you gave us I will have to guess at the rest. 1000 watt inverter at 12 volts nominal.
I will guess at your region and say that the number of hours of equivalent full rated charge from a PV module is 4.
1000 / 4 = nothing less than a 250 watt PV array.
If this 1000 watt load is on for an average of 8 hours in 24 then - 8 x 1000 / 12 = nothing less than a 667 amp-hour battery bank.

When I say "nothing less than" I mean nothing less than! With any battery bank its best to stay within the top 20% of a 100% fully charged bank for a degree of autonomy and longevity of battery life. (Amp-hours times 5.) Since everyday is not a cloudless sunny day with absolute certainty, the best bet is to double the minimum of the PV array.

Keep in mind I guessed at a lot of these numbers. You can narrow it down I sure. Welcome to the world of photovoltaic battery charging!

Hi Keith,

I would not go with the nominal 18v panel into a 12v battery bank with a PWM controller (like the C35). If you look at the Vmp of the Evergreen 170 you'll see that it's 25.3 compared to the Vmp of a Sharp 170 that has a Vmp of 34.8v. Both of these voltages are much higher then what is needed to charge a 12v battery and you won't be able to make use of the full potential of the panels. 

If you use an MPPT controller, like the Outback MX60 or Xantrex XW or Solar Boost 6024H you'll be able to down convert the voltage to the make the best use of your input voltage.

You could also go with 12v panels wired in parallel in conjunction with a PWM controller.

I gather that your energy requirements are 1200 watt-hours per day and you have four equivalent sun-hours per day. If all things were perfect you would simply take the energy requirements and divide by sun hours to produce your array size (300w), but in the real world there are many things that force us to size the array larger. If we take into account deviation from standard testing conditions, voltage drop, inefficiencies in the charge controller and inverter and a variety of other factors that can cause power loss in the system we'll have to increase our array size anywhere from 25%-50% (depending on how conservative you want to be and how 'mission critical' your load is). This would give us an array size of 375w - 450w. Having a larger array also gives us the ability to recharge the battery bank from 'cloudy day' usage.     

Here is the logic I use to calculate battery bank size- I'll let you do the math out for your system:

1. Identify the total number of watt hours per day you use

2. Identify the number of days back up you need.

3. Multiply watt-hours per day by back up days (step 1 by step 2)

4. Identify you planned depth of discharge. 50% is as low as I ever plan on. If you want to seriously live on the system (and not just use it for backup) I would go with a 25% dod or better. Convert this number into a decimal (.5 for 50% dod, .25 for 25% dod, .8 of 80% dod). Divide this number by the value in step 3.

5. Derate your battery bank for the lowest average temperature the batteries will be exposed too. Multiple a number bellow by the figure in step 4. This is the minimum watt hour capacity of your battery bank.

Temp in F     Factor
80+           1.00
70            1.04
60            1.11
50            1.19
40            1.30
30            1.40
20            1.59

6. Determine your system voltage. Divide the answer to 5 by your voltage. This is the minimum amp hour capacity of each parallel string of batteries (4500 watt hours / 12v = 375 AH)

7. Select a battery that looks close to your amp hour capacity you calculated in 6. If you can’t find a battery that is close to the capacity, look for one that is as close to half the capacity you need, or close to a quarter the capacity you need.

8. Once you find a battery divide your system voltage by the battery voltage. This will give you the number of batteries you need in series.

9. Divide the number of from question 6 by the amp hour of the battery.  Round up the nearest whole number. This is the number of series strings in parallel.

10. Multiply the answer from question 8 to the answer from question 9. This is the number of batteries you need total.

For Example:

1)   I use 6000 watt-hours per day
2)   I want three days back up
3)   (6000 Wh/day) x (3 days) = 18,000 Wh
4)   I plan to discharge the batteries no more the 40%. (18,000 Wh) /( .4) = 45,000 Wh
5)   My batteries will never get bellow 60 degrees. (45,000Wh) x  (1.11) = 49,950 Wh
6)   I am planning a 48v system. 49,950 Wh / 48v = 1040 AH.
7)   The price per watt-hour in the MKL16 looks good, I want to see if it will work for me. It’s 370 AH at 6v
8 )  48v / 6v = 8 batteries in series
9)   1040 AH / 370AH = 2.8… I’ll round up to 3
10)    8 batteries in series x 3 strings in parallel = 24 batteries.

At this point I would look at another battery option to see if it worked better. I’ll start again at step 7:

7) I want to reduce the number of battery interconnections (especially parallel connections) so I find a Surrette battery that is 4v and 1104 AH.
8 ) 48v / 4v = 12 batteries in series
9) 1040 AH / 1104 AH = .94… I’ll round up to 1 (this actually means I have 1 string in series, so no parallel connections at all).
10) 12 batteries in series x 1 string in parallel = 12 batteries.

At this point I would email my AES sales rep and ask for a quote on 24 MK L16’s and one for 12 Surrette 4-KS-21PS (with shipping) to compare prices. I would break down each battery bank into total watt-hour (battery voltage x battery amp-hour capacity x number of batteries) capacity and then find the price per watt-hour of energy storied (total price of batteries delivered / total watt-hour capacity). I would take to my friends (and fellow forum members) about what they think of the brands, compare warranties, and then buy the batteries I felt best about.



I would recommend going with:

3x Kyocera KC125G 125W 12V Solar Panel
1x 15' MC1 Connector #10 AWG Male/Female
4x GROUNDING LUGS WITH SET SCREWS QTY-1
Mounting of some sort- Ground/roof/pole
1x XW Solar Charge Controller MPPT 865-1030
1x 63 Amp Din Rail Mount Breaker
1x 12 Amp 150VDC Din Rail Mount Breaker
1x MNBabyBox 4 Slot AC or DC Breaker Panel
An appropriately sized battery bank. I have not done the math out, but I'm guessing it will come out to be close to:
2x 8A4D AGM 200 Ah (20 Hr) AGM Battery
1x BATTERY INTERCONNECT CABLE 4/0 13" BLK
1x BATTERY INTERCONNECT CABLE 4/0 13" RED

I would wire the three panels together in series and then into the MPPT XW controller that will charge your two 12v batteries (wired in parallel) at the best voltage and current. This unit also have a built in ground fault protection.

This list comes out to $2,849 and if installed this year you could use it to take advantage of the federal tax credit for 2008. (www.dsireusa.org for more info on the tax credit)

http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/

For a start, try; Average, Annual, and Flat plate tilted South at latitude. This web site reflects a "fair" assessment of the the number of hours of equivalent full rated charge from a PV module. Expect a little less if anything.
Keep in mind that this South is solar South not geomagnetic South and each region has a different degree of deviation from magnetic South.

Does anybody know of a web site with charts showing degrees of deviation from geomagnetic South?