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How many panels are needed ...

Postby Wheeler on Wed Jul 08, 2009 8:47 am

I am living in La Paz county Az. about 100 miles east of Phoenix, here is my question.


I am now using 16 T-125 batteries with a 240 ah each, they are is groups of 4 and 4 banks for a 24 v. system. I have 32 KC-120 panels with a rated power of 3,840 watts using an Outback MX-60 and a (I'll buy another MX-60 when I can afford it) trace C-40 controller, how do I determine the proper panel to battery sizing?

Also, my neighbor has 8 L-16 batteries in a 12 volt system, how many watts of panels would he need to be a balanced system?

Thanks for you help!
Wheeler
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Re: How many panels are needed ...

Postby crewzer on Wed Jul 08, 2009 1:41 pm

I am living in La Paz county Az. about 100 miles (west) of Phoenix, here is my question.

Also, my neighbor has 8 L-16 batteries in a 12 volt system, how many watts of panels would he need to be a balanced system?

Wheeler,

In reverse order:

To me, a “balanced” system means in part that the battery bank has been sized to provide three days of autonomy without dropping below a 50% state of charge (SOC). In other words, the battery bank’s design capacity is six times the (off grid?) home’s average daily gross energy need, including inverter efficiency. For a battery bank rated at 18 kWh (12 V x 1,500 Ah), that’s 3 kWh/day, or perhaps ~2.5 kWh/day net for a 12 V system with a 120 VAC inverter.

Assuming ~65% seasonal front end efficiency, the PV array will need to generate 4.6 kWh/day gross to meet that requirement. NREL data for Phoenix indicates that an average of 5.3 hours per day of insolation falls on a south-facing array with no shading and tilted up at latitude plus 15 degrees in December, so the array would have to be rated at 4.6 kWh/day / 5.3 hrs/day = 870W STC.

See: http://rredc.nrel.gov/solar/pubs/redbook/PDFs/AZ.PDF

A clean and well-aligned array rated at 870 W STC would deliver ~45 A into 12 V (nominal) battery bank at mid-day in the summer. That’s ~3% of battery bank Ah capacity (~1,500 Ah), which, in my view, is a tad low for a battery bank with operational daytime loads. 5% to 6% is typically a good working number, but that would require an array rated at ~1,500 W STC or higher.

I’ll (try to remember to) address the first part of your question later tomorrow.

HTH,
Jim / crewzer
090805 System Configuration: 966 W STC (849 W CEC PTC) 48V PV array, FM80, 24V x 400 Ah AGM battery bank, FX2524T w/ BTS, Hub-4 & Mate; Link-10 w/ BTS, & E-Panel.
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Re: How many panels are needed ...

Postby Wheeler on Wed Jul 08, 2009 2:32 pm

Thank you for the information! I will pass this along to my neighbor.

Ps. we are both living off grid.
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Re: How many panels are needed ...

Postby tallgirl on Wed Jul 08, 2009 4:22 pm

Crewzer,

If you go with "DC harvest = AC loads / derating factor" you're always going to get something in the range of 3%. If you use enough significant digits, it's usually in the range of 1/36th, plus or minus.

Also, in my opinion, you need to size the array for (n + 1) / (n) (or if you're not willing to go that high ((2n + 1) / 2n) days and the batteries for (n) days. If you size the array for (n) days, you never gain back SOC after a day of no production. What's the SOC after 3 days of no production? 50%, right? And after a day of production and a day of use? 50% again, and so on until the next day of no production when the generator has to be started.

There are post hoc ways of modeling that would require an experienced installer in the region, but you have to increase the DC-harvest to AC-load ratio or "Recharge Days" is infinity.
Julie in Texas

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Re: How many panels are needed ...

Postby crewzer on Thu Jul 09, 2009 4:19 am

Julie,

I see your mathematical point. However, the model I described generally seems to work when other factors are considered:

1) Some off-gridders tend to cut back on power use during / after long stretches of bad weather.
2) Some (many?) off-gridders have generators to help them through / recover from long stretches of bad weather. In fact, one popular model with generator back up calls for just two days of autonomy.
3) Sunny days tend to offer more hours of insolation than "average".
4) The December insolation data cited is "worst case"; all other months offer higher average daily insolation.

Your point is nonetheless well taken: There are many angles and variables to consider when spec'ing a system.

Regards,
Jim / crewzer
090805 System Configuration: 966 W STC (849 W CEC PTC) 48V PV array, FM80, 24V x 400 Ah AGM battery bank, FX2524T w/ BTS, Hub-4 & Mate; Link-10 w/ BTS, & E-Panel.
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Re: How many panels are needed ...

Postby crewzer on Thu Jul 09, 2009 5:09 am

I am living in La Paz county Az. about 100 miles (west) of Phoenix, here is my question.

I am now using 16 T-125 batteries with a 240 ah each, they are is groups of 4 and 4 banks for a 24 v. system. I have 32 KC-120 panels with a rated power of 3,840 watts using an Outback MX-60 and a (I'll buy another MX-60 when I can afford it) trace C-40 controller, how do I determine the proper panel to battery sizing?

Wheeler,

There are several perspectives to consider:

1) Trojan Battery recommends a charge current of 10%-13% of battery Ah capacity. Assuming 75% operational efficiency, your controllers should be able to deliver ~100 A to the batteries, although higher charge currents are certainly possible. 100 A is ~10% of the battery bank’s 960 Ah capacity, so, assuming other correct settings and good care, it should be happy.

See: http://www.trojanbattery.com/Tech-Suppo ... nance.aspx
and: http://www.trojanbattery.com/ProductLit ... sh_001.pdf

2) As I mentioned earlier, I like a minimum charge current of 5%-6% of battery capacity. Assuming a average daily discharge of ~17% of battery capacity, an average of ~4 hrs/day of full Sun, and various system inefficiencies, the batteries should be regularly recharged.

The MX60 controller is no longer in production. Down the road, I recommend you consider an FM80 controller to supplement the MX 60 and C40. I believe that an NEC-compliant configuration of your 32 modules and those three controllers in a 24 V system would like this:

Four KC-120 modules wired in series per string (48 V nominal)
Four series-strings in parallel (4 x 4, 16 modules, 1,920 W STC) connected to the 24 V battery bank via the FM80.
Three series strings in parallel (4x3, 12 modules, 1,440 W STC) connected to the 24 V battery bank via the MX60.

Four modules, wired 2 X 2 (24 V nominal) connected to the 24 V battery bank via the C40.

HTH,
Jim / crewzer
090805 System Configuration: 966 W STC (849 W CEC PTC) 48V PV array, FM80, 24V x 400 Ah AGM battery bank, FX2524T w/ BTS, Hub-4 & Mate; Link-10 w/ BTS, & E-Panel.
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Re: How many panels are needed ...

Postby Wheeler on Thu Jul 09, 2009 7:31 am

Crewzer,

Thank you so much for the information!

Here are the next questions,

a. what is the best book to purchase for me to understand what is said here and for general information on solar power?

b. when hooking up the controllers to the batteries, should I hook up the wires from all controllers to the same battery bank or should I separate them?

c. I have one more array of panels for a total of 4,320 rated watts of power, is this enough panels for 16 L-16's ? Over night I will use 50% of my existing battery storage running the swamp coolers and a TV, this is from dusk to dawn, the T-125's are almost 4 years old.

I have many questions but I find it difficult finding the proper words and supplying the proper information to get these questions answered, I imagine this will come in time, about the time I figure out what the hell I'm doing. :grin:

I am living off grid and have not used a back up generator yet, when I get the system moved and install a new Out Back inverter I will then get a generator.

Thanks again for the help!
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Re: How many panels are needed ...

Postby tallgirl on Thu Jul 09, 2009 11:41 am

Jim,

Thanks for the explanation. I've been spec'ing systems for use without a generator, as that's been the point of the systems -- not having to have a generator after storm outages. Finding gasoline or diesel after a hurricane can be quite difficult -- the last place I was able to reliably get fuel immediately after Katrina, driving from Texas to New Orleans, was Lafayette. That's what folks in hurricane prone areas are faced with -- no fossil fuels for running generators.

As an aside, the maximum summer insolation for Phoenix, AZ is 6.9 hours at latitude + 15. 6.9 / 5.3 = 1.3, so the "Recharge Days" is approximately 3 for the summer months
Julie in Texas

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Re: How many panels are needed ...

Postby Justme on Thu Jul 09, 2009 12:55 pm

tallgirl wrote:
1, I've been spec'ing systems for use without a generator, as that's been the point of the systems -- not having to have a generator after storm outages.
2, Finding gasoline or diesel after a hurricane can be quite difficult -- the last place I was able to reliably get fuel immediately after Katrina, driving from Texas to New Orleans, was Lafayette.
3, That's what folks in hurricane prone areas are faced with -- no fossil fuels for running generators.



1, Not sure that I would follow that.
2, finding the broken array might be harder

I would put money on a genny being a better bet for power than a roof mounted solar array (or any array) after a hurricane.

3, In fact will elec power be that much of a need as the house is prob gone too? I would have thought food water & shelter were the big needs.
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Electricity in Storm Zones

Postby tallgirl on Thu Jul 09, 2009 1:28 pm

Justme wrote:1, Not sure that I would follow that.
2, finding the broken array might be harder

I would put money on a genny being a better bet for power than a roof mounted solar array (or any array) after a hurricane.

3, In fact will elec power be that much of a need as the house is prob gone too? I would have thought food water & shelter were the big needs.


1). Well ... it's worth what you paid for it ;) However, it's all very sound advice, based on living in a hurricane zone much of my life.

2). Finding a broken array, assuming the house is still standing, is pretty easy. We have horrible hail storms here and I just go look at the panels. My polycrystaline panels have done very well, the amorphous panels for charging lawn and garden tools haven't.

3). Most houses in a hurricane zone are not damaged in the least by a hurricane. Hurricanes are hundreds of miles wide, but don't destroy that entire path.

Because electric grids span geographic areas that are typically larger than the path of destruction, power in a hurricane ravaged area is often out much longer than it takes to repair minimal damage to the structure. I was in Southeast Louisiana four days after Katrina and from Madisonville to the west, all the way to Pearl River to the east (I didn't cross into Mississippi -- I was already worried about being arrested (or shot) by the National Guard), the totality of destruction was minimal. Damage to power lines was far greater, and it took weeks in many instances to restore electricity. During this time there were significant problems with fuel availability, while food was a lesser problem (I was spared eating MREs because Common Ground Collective and the Red Cross fed me so well) and housing, outside the area affected by the storm surge, wasn't a problem for most. Inside of Greater New Orleans proper, if the house wasn't affected by the flooding inside the city, or overtopping of the levees, it was immediately habitable, except for the lack of water, sewerage and electricity. Even with flood damage, many structures can be inhabited once the flood damaged materials are removed, along with flood damaged wiring.

Generators, on the other hand, are usually responsible for a number of deaths, both due to carbon monoxide poisoning and electrocution, as well as fire from poor fuel handling. They are also prone to theft as they tend to be portable, and people are killed either stealing, or defending, their generators. Many deaths from CO poisoning are caused by people bringing the generator inside to prevent it from being stolen. Noise from generators adds to the stress and makes things even more difficult.
Julie in Texas

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Re: How many panels are needed ...

Postby Justme on Fri Jul 10, 2009 2:05 am

I bow to your experiance. We are lucky in that we dont have any weather that bad (yet we seem to moan about it ALL the time lol).

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Re: How many panels are needed ...

Postby crewzer on Fri Jul 10, 2009 6:10 am

Julie,

Yeah... there's just no single solution to all scenarios, IMHO. I also suspect that many of the generator problems you described were related to people trying to "replace the grid" rather than help charge batteries. Off-gridders seem to do pretty well with managing their systems, including backup generators. Many RE products and systems include an AGS feature...

But, there's an exception to everything: http://www.wind-sun.com/ForumVB/showthread.php?t=4529

:(
Jim / crewzer
090805 System Configuration: 966 W STC (849 W CEC PTC) 48V PV array, FM80, 24V x 400 Ah AGM battery bank, FX2524T w/ BTS, Hub-4 & Mate; Link-10 w/ BTS, & E-Panel.
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Re: How many panels are needed ...

Postby crewzer on Fri Jul 10, 2009 6:55 am

Wheeler,

a. what is the best book to purchase for me to understand what is said here and for general information on solar power?

I don’t know if there’s a single best book. A couple of books and other sources that come to mind include:

Photovoltaics Design and Installation Manual , Solar Energy International
National Electrical Code (2008); Article 690 in particular
Photovoltaic Systems NJATC and Gosse
Home Power Magazine (http://www.homepower.com)
Articles by John Wiles: http://www.nmsu.edu/~tdi/Photovoltaics/ ... -Stds.html

Home Power magazine is oriented towards off-gridders, and their back-issues are available for purchase. (Full disclosure: I have done some work for Home Power magazine's sister publication, Solar Professional.)

b. when hooking up the controllers to the batteries, should I hook up the wires from all controllers to the same battery bank or should I separate them?

It’s OK for multiple charge controllers to charge a common battery bank. You just need to “normalize” the charge controllers so that they ideally all read the voltage and use the same target voltages.

c. I have one more array of panels for a total of 4,320 rated watts of power, is this enough panels for 16 L-16's ? Over night I will use 50% of my existing battery storage running the swamp coolers and a TV, this is from dusk to dawn, the T-125's are almost 4 years old.

Depending on which L-16’s you spec, 16 of ‘em configured 4 x 4 would be rated at 24 V x ~1,500 Ah. A 4,320 W STC array should be able to deliver ~110 A on a hot day, and more in the winter. 110 A is 7.3% of 1,500 Ah, so that’s a good start. However, the array also has to supply enough energy to recharge the batteries on a frequent basis.

Assuming 80% energy efficiency (Wh out / Wh in), taking 50% (11.5 kWh) out of your existing 960 Ah (23 kWh) battery bank means that you’ll need to put 14.4 kWh back in. Assuming 75% operational efficiency and ~6.5 hours/day of “full” Sun, this will require a PV array rated at 3,000 W STC.

Assuming 80% energy efficiency (Wh out / Wh in), taking 50% (18 kWh) out of your proposed 1,500 Ah (36 kWh) battery bank means that you’ll need to put 22.5 kWh back in. Assuming 75% operational efficiency and ~6.5 hours/day of “full” Sun, this will require a PV array rated at 4,600 W STC.

Changing the array tilt for summer operation (tilt = lat or Lat – 15) and harvesting ~7 hrs/day of full Sun would require an array rated at 4,300 W STC.

YMMV. You may also find that your loads vary seasonally, For example, fewer lights but more swamp cooler in the summer, but more lights and no cooler in the winter. If you go with the 4.3 kW array, you'll need to consider something two FM80 charge controllers plus the C40.

Bring your questions any time!

Regards,
Jim / crewzer
090805 System Configuration: 966 W STC (849 W CEC PTC) 48V PV array, FM80, 24V x 400 Ah AGM battery bank, FX2524T w/ BTS, Hub-4 & Mate; Link-10 w/ BTS, & E-Panel.
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Re: How many panels are needed ...

Postby tallgirl on Fri Jul 10, 2009 3:16 pm

crewzer wrote:Julie,

Yeah... there's just no single solution to all scenarios, IMHO. I also suspect that many of the generator problems you described were related to people trying to "replace the grid" rather than help charge batteries. Off-gridders seem to do pretty well with managing their systems, including backup generators. Many RE products and systems include an AGS feature...

But, there's an exception to everything: http://www.wind-sun.com/ForumVB/showthread.php?t=4529

:(
Jim / crewzer


Ouch! Wow, that's a tragedy.

Most of the problems weren't with trying to replace the grid, per se, but with the misleading size figures quoted by the low-end generator manufacturers. Other problems were the fact it was a major disaster area (I wish I had photos of me in my cowboy boats holding a chainsaw after I'd cut 18 to 24 inch trees -- you'd have a clue) and there were desperate people, some of whom were criminals. I understood why a 3500 watt generator wouldn't start a washing machine, but the generator manufacturers sort of omit "Will not start heavy motor loads no matter how many watts they are when running" from the advertising materials.
Julie in Texas

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Re: How many panels are needed ...

Postby Wheeler on Fri Jul 10, 2009 5:48 pm

crewzer wrote:Wheeler,



YMMV. You may also find that your loads vary seasonally, For example, fewer lights but more swamp cooler in the summer, but more lights and no cooler in the winter. If you go with the 4.3 kW array, you'll need to consider something two FM80 charge controllers plus the C40.

Bring your questions any time!

Regards,
Jim / crewzer



Thank you so much, with the information you have provided it gives me a better understanding of what I am doing and what questions to ask in the future. :grin:
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Re: How many panels are needed ...

Postby crewzer on Fri Jul 10, 2009 6:31 pm

Julie,

but with the misleading size figures quoted by the low-end generator manufacturers.

Yep... that is indeed another problem. However, at least some reputable gen. mfrs. are finally getting better... check out the application chart below from a Yamaha 3K inverter generator.

(I wish I had photos of me in my cowboy boats holding a chainsaw after I'd cut 18 to 24 inch trees -- you'd have a clue)

That would indeed have been worth a picture or twelve! :wink:

I'm done... Have a great weekend!

Jim / crewzer
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090805 System Configuration: 966 W STC (849 W CEC PTC) 48V PV array, FM80, 24V x 400 Ah AGM battery bank, FX2524T w/ BTS, Hub-4 & Mate; Link-10 w/ BTS, & E-Panel.
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