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)