# Suggestions for off-grid island home in Maine

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Re: Suggestions for off-grid island home in Maine

From the PV Watts website--1,520 watts of solar panels fixed and 2-Axis in Caribou, Maine (derating factor = 0.52, all else using defaults):
```[FONT=Fixedsys]Results fixed 1,520W off-grid array Caribou Main

Month
AC Energy (kWh per month)
Energy Value (\$ @ 12.2 ¢/kWh)

1      3.36          87        10.61
2      4.34          101        12.32
3      5.23          131        15.98
4      5.75          135        16.47
5      4.99          112        13.66
6      5.09          108        13.18
7      5.15          110        13.42
8      4.99          109        13.30
9      4.26          93         11.35
10     3.45          80         9.76
11     2.38          54         6.59
12     2.73          70         8.54
=======================================
Year   4.31          1192      \$145.42  [/FONT]
```

And:
```[FONT=Fixedsys]Results 2-axis tracking 1,520W off-grid array Caribou Main

Month
AC Energy (kWh per month)
Energy Value (\$ @ 12.2 ¢/kWh)

1      4.08          107        13.05
2      5.30          125        15.25
3      6.48          165        20.13
4      7.64          183        22.33
5      7.13          166        20.25
6      7.66          170        20.74
7      7.10          158        19.28
8      6.74          152        18.54
9      5.43          122        14.88
10     4.11          97         11.83
11     2.73          63         7.69
12     3.29          85         10.37
=======================================
Year   5.64          1592      \$194.22 [/FONT]
```

Assume months May thru September:

Ave: 106 kWhrs per month (5 month average for fixed array)
Ave: 156 kWhrs per month (5 mn ave for 2-axis tracking)

(156 / 106) * 1,520 watt array = 2,234 Watt effective if 2-axis tracking

Assume \$4 per watt (shipping + taxes):

( 2,234 - 1,520 ) * \$4 = \$2,860 worth of "extra solar panels" vs "2-axis tracking"

So--the difference in power output between a 2-axis tracker or just purchasing and installing more solar panels for Caribou main is \$2,680 -- So what does a 2 axis tracker cost to install vs installing more panels on a fixed array?

Assuming \$1 per watt for fixed array hardware:

2,234 Watts * \$1 per watt = ~\$2,234 for fixed array mount

1,520 watts on a 2 axis tracker cost less than \$2,234+\$2,860=\$5,094 ?

Plus labor, materials, hole in ground, etc. for installation...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 29
Re: Suggestions for off-grid island home in Maine
crewzer wrote: »
Island Mon,

12 x 210 W = 2,520 W STC. This value pushes the FM80 beyond its spec limit. Reconfiguring the array from “48 V” ( 4 modules in series x three parallel strings) to “36 V” (3 modules in series x 4 parallel strings) will improve the FM80’s operating efficiency and therefore reduce waste heat. The “36 V” array’s summer voltage will still be high enough to achieve battery voltage targets for a 24 V system.

See: http://www.outbackpower.com/pdf/specs/flexmax.pdf

HTH,
Jim / crewzer

Hi Jim, I am now planning on going with a 48V battery bank, 16x trojan L16H's. I am not clear on how the FM80 will not suit my needs. I see on the attached document that you gave me, max system voltage 150VDC, max solar array STC nameplate 48V 5000 watts, can charge a 48V battery bank..etc.
What am I missing?
If you could explain I would appreciate it very much!

Island mon
• Solar Expert Posts: 5,558 ✭✭✭✭✭
Re: Suggestions for off-grid island home in Maine

"Assuming \$1 per watt for fixed array hardware:

2,234 Watts * \$1 per watt = ~\$2,234 for fixed array mount

1,520 watts on a 2 axis tracker cost less than \$2,234+\$2,860=\$5,094 ?

Plus labor, materials, hole in ground, etc. for installation...

-Bill "

Nice data Bill !
I would run your numbers with a single axis as the dual axis units are for grid-tie in my opinion. More to maintain and the key to offgrid is not the total watts but rather completing the charge as well as reducing the hours that you are using the battery each day! Trackers make the battery you have appear larger not to mention more days that will complete charge which increases battery life.

The intangibles are huge offgrid!
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

Re: Suggestions for off-grid island home in Maine

Here are the results for a 1-axis tracker:
```[FONT=Fixedsys]Results 1-axis 1,520W off-grid array Caribou Main

Month
AC Energy (kWh per month)
Energy Value (\$ @ 12.2 ¢/kWh)

1      3.84          101        12.32
2      5.12          121        14.76
3      6.36          162        19.76
4      7.34          175        21.35
5      6.49          150        18.30
6      6.74          149        18.18
7      6.47          143        17.45
8      6.36          142        17.32
9      5.30          118        14.40
10     4.07           96        11.71
11     2.68           62        7.56
12     3.06           79        9.64
=====================================
Year   5.32         1498       \$182.76 [/FONT]
```
May-Sept average: 140 kWhrs per month

vs 106 kWhrs per month for fixed; and 156 kWhrs per month for 2-axis

(140 kWh per mnth / 106 kWh per mnth) * 1,520 kW array = 2,010 fixed mount equivalent array...

Note that these advantages for tracking arrays over fixed seem to almost disappear for winter (poor weather and/or poor sun angles?). So--for winter you still may be left with the choice of more panels and/or more generator use...

-Bill

PS: Dave, I do agree with your observation that if you need power during "summer" daylight (such as A/C, shop equipment, etc.)--that trackers do off-load the batteries somewhat as the panels continue to support loads better at the beginning/ending of the day vs fixed mount arrays.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: Suggestions for off-grid island home in Maine
Battery Bank:
24Vdc 12x Trojan L-16H, 420Ah (20hr)
3 strings of 4 in series, total of 24V 1260Ah

I will have the positive(+) and negative(-) cables from each string attached to two bus bars.

I will be discharging no more than 25% of total capacity. Estimated storage of roughly 2 days.

Inverter:
24V 4000watt Magnum inverter 120/240Vac split phase.

-or-

two outback FX2524T's 24V stacked. Providing 120/240Vac
Island Mon,

Your earlier post indicated that you were planning a 24 V system. It's this nominal system voltage for the battery bank and inverters that determines what size array the charge controller can handle. The FM80's limit for 24 V systems is either 2,500 W STC or 2,000 W STC, depending on if the NEC 690.8 is applied

If you've changed your system configuration from 24 V to 48 V (nominal), then the FM80 will handle an ~2500 W STC array with ease. In fact, you'll find the FM80 crewzin' along at ~97.5% efficiency operating in bulk mode in the summer with the array supplying ~2,000 W at ~74 V operational Vmp. Please see the attached chart.

But, the array configuration will get a bit more complicated. Specifically, using flooded-cell batteries from Rolls/Surrette or Trojan, I'd recommend that you configure the array with five ES-A modules in series for each string. The winter Voc won't be too high (per NEC 690.7), and the summer operational Vmp (less than STC Vmp) will be high enough to achieve battery voltage charging targets while overcoming voltage drops in the wiring and inside the charge controller.

HTH, and please let me know if you have any further questions.

Regards,
Jim / crewzer
• Solar Expert Posts: 29
Re: Suggestions for off-grid island home in Maine
crewzer wrote: »
Island Mon,

But, the array configuration will get a bit more complicated. Specifically, using flooded-cell batteries from Rolls/Surrette or Trojan, I'd recommend that you configure the array with five ES-A modules in series for each string. The winter Voc won't be too high (per NEC 690.7), and the summer operational Vmp (less than STC Vmp) will be high enough to achieve battery voltage charging targets while overcoming voltage drops in the wiring and inside the charge controller.
Jim / crewzer

The structure that I am putting my panels on can only fit 12 of the evergreen panels. I was planning on combining the series strings of 4 via combiner box and running a pair of 1/0AWG cables down 20-30 ft to the charge controller. Using the 1/0 cable I would be keeping the voltage drop well below 3%.
My concern is, I do not have a large enough roof structure to have 15 panels (3 strings of 5 in series). My only other option is taking away two panels and having two strings of 5.

Will 3 strings of 4 in series not supply enough voltage during the summer months? Keep in mind that avg high temps are in the high 70's.

Thanks,

Island Mon
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: Suggestions for off-grid island home in Maine
Will 3 strings of 4 in series not supply enough voltage during the summer months? Keep in mind that avg high temps are in the high 70's.
Island Mon,

I consider both the “bottom end” and the “top end” when addressing this issue.

At the “bottom end”, Trojan says 59.2 V is required for the end-of-bulk absorb target, and 62.0 V is required for the EQ target voltage. These targets are for batteries at 80 F; colder batteries need higher voltages. At 70 F, figure on increasing each target voltage by ~0.6 V, so 59.8 V and 62.6 V respectively.

See: http://www.trojanbattery.com/BatteryMaintenance/Charging.aspx

Adding in ~2 V for losses in the wiring, breakers, and terminals (~3%), and another 2 V for the voltage drop inside the controller’s DC-DC converter, the gross targets measured at the array may be ~63.8 V and 66.6 V.

At the “top end”, the ES-A 210 module’s STC Vmp spec is 18.7 V. This spec is based on a module cell temperature of 25 C, or 77 F. However, summer-mid-day module temps are typically 25 C (pole mount) to 35 C (parallel roof mount) or so higher than ambient. Assuming 25 C (~77 F) ambient and a 30 C cell temp rise, the module cells will operate at ~55 C, or ~30 C above STC conditions.

Evergreen’s NOCT specs provide a glimpse into this behavior. The NOCT specs (80% insolation and 20 C ambient) indicate cell temperature of ~45 C (25 C above ambient) and an operational Vmp of 17.0 V.

Evergreen’s temperature coefficient for module Vmp is -0.43%/C. At 30 C above STC spec, the module Vmp should be 18.7 V x ~87%, or ~16.27 V. Four modules in series will supply 65.1 V. This value will likely be further reduced if the array is roof mounted and/or mid-day ambient temperatures exceed “the high 70’s”.

My view is that the calculated 65.1 V operational Vmp is too close to the 63.8 V and 66.6 V targets identified above. The result may well be a system that works satisfactorily part of the time, but will likely struggle if the array is roof-mounted, when ambient temperatures are high, and/or when it’s time to EQ the batteries.

This is an example of why I’m such a fan of “60 V” array applications for systems with 48 V flooded-cell batteries.

HTH,
Jim / crewzer
• Solar Expert Posts: 29
Re: Suggestions for off-grid island home in Maine

Jim, thank you for a very thorough explanation.

I know that I mentioned using Trojan L-16's, however, what if i chose to use a similar 6v AGM battery. I see that the daily charge rate is between 55.2-56.4V and the float charge is 52.8V.
I assume the AGM's will be compatible with the FM-80

I understand what you are saying, but I am trying to keep strings of 4 in order to keep my setup of 12 panels. I cannot fit any more than 12 panels on the roof, hence why I am being so difficult.

Island Mon
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: Suggestions for off-grid island home in Maine

Island Mon,

AGM's change the picture considerably! Assuming an end-of-bulk/absorb target voltage of ~57 V at 77 F, you should be fine, especially since EQ is typically not required, although Concorde's tech manual discusses otherwise.

I've been using East Penn Deka (also known as MK) AGM batteries for several years, initially with the MX60 controller, and now with an FM80.

You can read more about my experiences with AGM batteries from these discussions on the OutBack Forum:

http://www.outbackpower.com/forum/viewtopic.php?f=1&t=848
http://www.outbackpower.com/forum/viewtopic.php?f=1&t=1750

BTW, have you already purchased the Evergreen modules? There may be other products that, configured as 4 x 3 or 3 x 4, will work "more comfortably" with flooded-cell batteries.

HTH,
Jim / crewzer