Battery Selection - System Sizing

nobadays

New to the forum...though have been trolling/gleaning info for sometime. Great information but alas I think we all have specific needs so searching the archives has not found a match for what I am looking for. 

Background - We are in the process of purchasing a cabin in CO for summer use: 5-6 months a year. It is off-grid and though completely wired for 120v power, it is well equipped for off-grid living - Every room has gas lights (and 120v lights w/LED bulbs), gas cook stove and gas refrigerator. Wood stove and radiant gas heating. Water is hauled to a 550 gallon cistern and pressurized using an RV pump with a stand alone 12v solar/battery system. The drawback is there is no solar for 120v power. The previous owners have relied on a 2000w generator to supply this need. We want to limit the use of a generator (we have 3500w and 4000w generators) for heavy, periodic uses such as power tools.

I have used the load calculator on this site, and put in everything I think we will likely use on a regular basis including - Satellite Internet, microwave, vacuum cleaner, laptop, etc.  Then I used another calculator to arrive at battery bank and panel sizing. First the numbers, then my questions! Thank you for reading all this!

Load Calculations: Average = 1867Ah -rounded up to 1900Ah
                               Max   = 2782Ah - rounded up to 3000Ah

First question: Do I design to the Average or the Maximum daily usage??

Here are the systems designs based around both:

Max daily battery bank @ 24v = 748Ah   Average daily @ 24v = 474Ah

Using Pueblo CO for Solar Exposure = 4.6Hrs/day (We will be south west of there in the mountains - west of Alamosa)

Solar array size Max daily = 847.8W    Average daily = 537W

Charge controller: Max daily = 40A    Average daily = 30A (I would likely use 40A anyway)

Next question: Do these numbers look right??

Now to the battery bank question of the subject line! 
           What would be the recommended batteries to get my 24v and the Ah as indicated??
          
As with most anyone... money is an object.  We have sat aside $5,000 dollars to build this system and hope we can stay under that!!

Thank you so much!!
Don 

Photowhit

Things look a bit 'hinky'. Consider that I have a 660 ah 24 volt battery bank and use upwards of 14 kWhs in the summer, though most of that load is an air conditioner and our heat comes with sun. And I charge that battery bank with a 4500 watt array...

If you use 4.6 hours of sun per day (assuming you used the summer months to calculate). And you need 2 kWhs of energy on average, assuming this is watt hours and not Ah?)

nobadays said:

Load Calculations: Average = 1867Ah -rounded up to 1900Ah
                               Max   = 2782Ah - rounded up to 3000Ah

You would need about 2-3 times the amount of energy from the array to support the loads, you will have about 50% losses using energy from the battery bank...

Sorry got to head out, I'll be back..

Estragon

Is it really 3000 AH of load (daily?), or should it be 3000 watt-hours?

3000wh/day is a reasonable sized off-grid system, and a ~750ah@24v bank is a reasonable bank to support that load.  40a is a bit light for charging in a system used daily (as opposed to one used only on weekend, for example).  3000wh@24v=125ah + charging inefficiencies, so you may find you need to run the generator sometimes to ensure you get fully charged every week or so, especially in the fall.

To get 40a into the batteries, you'd need ~ 40a x 28v charging ÷.95 (controller losses) ÷ .75 (panel temp adj. losses - may be a bit better if at altitude) = ~ 1600w.

Ballpark costs; 1600w panels might be $1200 or so (depending on racking etc), controller maybe $400,  750ah bank might be $2500, breakers, wire, etc maybe another $500.  

Not mentioned above is an inverter to supply 120vac from dc battery bank.  A good one run can $2000 or so, and a large one to run microwave type loads can take a lot of power just being on with light loads (eg 35w x 24hrs= 840wh/day).

Off grid, there are a lot of very personal choices. 

nobadays

Thank you so much for the help! I know these are rookie questions but...well I am one!

Sorry.... yes it is Watt hour not Ah.  Big difference!

So I should design for Max Wh and not Average?  My feeling is that we will likely be using closer to the average than maximum in any given month.  We spend a lot of time backpacking so will be gone roughly two weeks a month during the months we are in CO.  The home is at 9100' if altitude has an effect.  But we may end up with partial shading in the mornings and evenings.

If the battery bank looks the right size I could always add panels as needed to ensure daily charging.

I am looking at the  EPEVER MPPT 40A charge controller.  Then an AIMS 2000w/6000w surge inverter.  My original thought was to use 6 x 315w panels so judging by your math, that would be better than the "calculator" indicates.

Thank you again... I look forward to more dialogue! 

Don

Estragon

Whether to use max or average is one of those personal trade-offs.  I try to do higher power stuff when I have extra pv (batteries are full or nearly so, and lots of sun left), or when I'm running the generator anyway, so my system works well being designed for average most of the year.  Being in Canada, a lot of generator use is inevitable in fall and winter though.

At 9100', your sun will be a bit stronger (less air to go through), and temps a bit cooler, so you may get closer to STC rated output at times.  About 75% of STC is what we expect in most climates.  

Any shade can cut output to near nil.  Some early/late shade may be okay, but any shade at all (including nearby wires, vent stacks, etc) from  ~10am - 3pm generally needs reconsideration of the location or dealing with the shade source.  It's also important to consider seasonal changes in sun angle.

IMO, a 6000w surge is too high for a 24v system.  That might be 250 - 300a with bank voltage sag.  If you really need surges of anything like that magnitude, a 48v bank may be better.

How far will panels be mounted from the batteries?

nobadays

Thanks again!

I see... maybe trying to size somewhere in between average and maximum would be a good idea. That said if I can swing it building for Max would give us some breathing room.  It's many the cost of batteries that are a shock.

Until we have more time at the house in different seasons I am kind of guessing we will have a bit of shade morning and evening... between 10 and 2 I think we will be in the clear possibly all year long. Maybe nothing a chainsaw can't remedy. 

I don't expect to ever have that kind of surge, that is the spec on that particular inverter/charger.  I may go with a 3000 or 4000w inverter just for longevity sake.  The AIMS inverters are highly rated for the price.

We're you saying in the earlier post that a 40A charge controller is too small? Would a 60A make more sense?

The array will be about 60' from where the power will hook up to the house, a place I can build an enclosure for batteries, charge controller and inverter. That brings up another question, should I build a small shed right at the array to put the equipment then run A.C to the house from there? Is this option better than running DC the 60' (higher voltage, less line loss?)

Thanks for your help.... I'm pretty handy and can build or wire up anything but I find all the math perplexing!

Don

Vic

Hi Don,

Would suggest,   that for an off-grid power system,   that you think long and hard before buying the power electronics.

AIMS inverters might (possibly) have fairly good reviews,   but many/most of those reviews are from folks that do not use them in off-grid systems.

IMO,  try to buy from companies that have been in the battery-based power system business for a number of decades.

Inverters that can produce much more power than you will need,   often consume a lot of idle power.   This seem particularly true of the inexpensive Import brands.

AIMS seems to be a marketing company,   seeming to source from a number of off-shore producers.   A number of us here do know some people that have not been thrilled with many of these lower-tier products.

Regarding the choice of solar Charge Controllers (CCs),   the same or similar things can apply.   You will probably want a CC that has as much flexibility,   perhaps one that can provide remote monitoring of your system,   etc.

The type of battery you choose can affect which inverter and CC will be the most compatible with it.   Cold temperatures at your site  can also affect the type of battery that you select,   and so on ...   FWIW.

Have fun,  and good luck,    VIc

nobadays

Vic....  Thank you for your input!  Wise advice I'm sure. I will price out some of the higher end components I just wish I had a bigger budget. I might be able to come up with a couple thousand more but that will be about all I'm afraid.... 

Don

littleharbor2

I might add, Be sure your controller has plenty of head room for high Voc. Tracer has many different models to choose from. Your high altitude Colorado location will give you higher than spec. voltages. If you go with the 300+ watt panels you mention I will assume they are 72 cell panels. You will have to series wire NO MORE than two panels with a 150 max Voc. controller.  Any lower Voc. limit and you may see magic smoke emanating from your controller.

nobadays

Littleharbor2..... Thank you for another piece to the puzzle!  I was wondering how I should wire the panels. This makes sense.   I will likely use the 315 watt panels as I can get a good price on them and 6 will do all that I  need.  I am looking now at an Outback CC, specifically the Outback Power FM60-150 VDC FLEXMax 60 charge controller.  I think that should stand up to the need.

I might get a system designed yet!

Don

petertearai

consider 48 volts , easyer than changing in a year or 2 . also price difference between the fm60 and fm80 is not alot . But will give you a bit more room for expansion  if you go fm80 .

mcgivor

@nobadays In the first post you said this,

I have used the load calculator on this site, and put in everything I think we will likely use on a regular basis including - Satellite Internet, microwave, vacuum cleaner, laptop, etc.  Then I used another calculator to arrive at battery bank and panel sizing. First the numbers, then my questions! Thank you for reading all this!

Load Calculations: Average = 1867Ah -rounded up to 1900Wh (corrected  from original Ah )
                               Max   = 2782Ah - rounded up to 3000Wh

Usually a refrigerator would use somewhere between 1 and 2 Kwh per day, you have gas so that is eliminated, the amount stated 1.9-3kwh seems high based on loads outlined above. The only high wattage are the vacuum and microwave, however these are typically intermittent short duration use, which makes me wonder if perhaps there has been an error in the calculation, if there was a refrigerator included then it would be about the figures for a modest off grid home.

nobadays

Petertearai....  48v would be sweet but if I'm not mistaken that would require a whole bunch more batteries..double?  I'm finding that batteries are the single most expensive element. So for now probably need to stay with 24v.  The fm80 might need a look at.

Thank you  Don

mcgivor

nobadays said:

Petertearai....  48v would be sweet but if I'm not mistaken that would require a whole bunch more batteries..double?  I'm finding that batteries are the single most expensive element. So for now probably need to stay with 24v.  The fm80 might need a look at.

Thank you  Don

The price for a given Kwh capacity would be the virtually same (same type & manufacturer ) regardless of voltage, the benefits are savings on conductors, smaller gauge at higher voltage , larger PV array possible with like charge controller with higher voltage, less need for parrallel battery strings with higher voltage to name a few. 

Photowhit

No more batteries for 48 volt, for the same capacity, just more in a string. If you have 8 - 6 volt batteries, each 6 volt battery stores about 220 ah at 6 volts or (6x220=) 1320 watt hours. Amps x volts = watts. Amps is a rate per hour, so the watt hours. So if you have 2 strings of them in parallel for a 24 volt system, you have 8 batteries at 1.32 kWhs or 10.56 kWhs of storage, if you have a single string of 8 - 6 volt batteries in a 48 volt configuration, you still have 8 batteries at 1.32 kWhs or 10.56 kWhs of storage!

I think you underestimated your solar insolation or calculated it with a fixed flat roof, I couldn't even get a single month to be 4.6 hours using PVWatts calculator.

It would be smart to use the lowest month if you were living year round and were not sure of your use profile.

There are a couple advantages to living in such a sunny area, both needing a smaller array, but also you can usually rely on having sun every other day at a minimum, so you don't need to worry as much about long term storage. Since you have generators and will not use much energy if you aren't there. I would only work toward having 2 days autonomy (amount of stored energy) So even at your max that would be 3 kWhs. 

I suspect you are likely to use a bit less than that, but being new to living off solar you might well push the limits. If you have teenage kids, double anything I say...lol. It's not as bad today, but it use to be anyone living off grid could tell you a few things that grid tied people can't. I can tell you how much energy pretty much every thing in my house uses! I can tell you what items are in use at any time, day, night home or not home. 

I really don't think you need to shoot for the huge battery bank. If you plan on using a fridge (which I would consider replacing! and run off solar!) I would think the 3 kWh per day would be likely correct. If you need an air conditioner, I would worry you had seriously underestimated your usage. I have a 660 ah 24 volt battery bank (@16kWhs), I would have preferred a 48 volt battery bank, but I have the battery from a cabin I built and needed to move (long story) It is plenty of storage and I get less sun than you do and use 3x the amount of energy you intend to during many of the months, I don't have a generator. I think you would be fine with a 8-6 volt 'golf cart' batteries. I would go ahead and go 48 volt and think this would support everything you have planned and likely the fridge too with very minimal generator run time.

To support a 10.5kWh array, you would want an array that could produce a minimum of 10% of the battery bank capacity (and up to 13%) or about 1,050 watts, Panels are rated at Standard Test Conditions. What they normally produce is around 75% of that,at elevation, you may find you get closer to STC numbers as @Estragon stated, but I'd shoot for 1400-1600 watt array.

Charge controllers are based on their output in amps, with a 24 volt system you will have 1600/24 volts= 66 amps(charging voltage for 24 volts if the bank is well below 24 volts) Again highly unlikely you will ever need to max it out unless you expand, I would go ahead and look at an FM80 for a 24 volt system, The amperage will be 1/2 that of a 24 volt system at 48 volts, so a FM60 would give you a lot of room to expand.

I know you are on a budget, but don't know if anyone has spoken about true sine inverters vs Modified sine inverters. Many cheap inverters are Modified sine inverters. Aims doesn't have a good reputation around here, you might search Aims inverters here and discover that. We have had 1 person who has spoken well about their industrial (or commercial?) line, but I haven't research further. To use your generator to charge the batteries you will want an inverter/charger that can converter the energy from your generator to charge the batteries.

I really think you would be fine with a smaller inverter, but Samlex has a 3000 watt unit that can be hard wired and runs around $1100, (I can't reach the website right now) If you have inspections or insurance, you will want a UL1741 (safe for home use, UL 458 is for recreational use) inverter. I think Samlex EVO line is about as cheap as those get, with Schneiders SW2500(3000 watt) running close at $1350

nobadays

Thank you so much for such detailed help!

Curious why you would switch from propane to electric for the fridge...

We will be new to living off-grid in a house but we spend months year round - two weeks to a month at a time - in our campervan so have a tiny bit of experience with solar from that (75w panel/cheap Chinese CC/2 Grp27 deep cycle batteries) and conservation of that power we produce...same as you but in our van, we know how much everything hooked to those batteries uses down to the Co2 sensor!

Mcgivor/Photowhit - I guess I'm still a bit fuzzy on the math when it comes to batteries... I was assuming that Amp Hours was the important number to look for so I figured with 8 - 6v batteries wired to produce 48v the a amps would stay the same @ 420A. When the same 8 batteries are wired to produce 24v they will double the amperage to 840A. The calculator I used recommended 748 Amps so that was my target. I do realize the 8, 6v batteries will store the same amount of energy regardless of 24v or 48v...so is the amperage out-put not important?  It would seem to me that the lower the amp out-put the safer the system would be - so 48v.

So... would an 8 - 6v battery bank wired to 48v be sufficient for my application?

I used the same calculator to arrive at the solar exposure/insolation. It looked low to me as well but thought designing to a lower number would be less detrimental than designing to more than I get.

Yes I do know about modified/pure sine wave and have been looking at pure sine wave inverters as if I'm not mistaken wall warts/laptop chargers don't like modified sine wave inverters and chargers for cordless power tools often will not work.  I will run power tools off a generator.

So to see if I have this right... I'm looking at now:
6 panels @315w = 1890w wired in three strings to a combiner box. (the panels could change but roughly stay the same Watts)
Outback Power FM-60-150vdc feeding an 8 x 6v battery bank wired to 48v (likely 8 Trojan L16 - 420a batteries)
Schneider Electric Conext SW 4048 3,800 Watts, 48VDC Inverter/Charger for Split-phase 120/240 VAC (I don't see the need for 240v)

Does this look right?

One more question - I may have asked it earlier... should I build a building by my panel array to house all this stuff, keep the DC run short or run the DC 60' to the house? I have space under an elevated deck to build the equipment building to keep it out of the house. (800sqft cabin - no room in there!)

You guys are great!  Thank you for helping out...so many other forums people just say "search the forums, this has been discussed." I have searched for three days (not counting non-specific trolling for some time) before I asked any questions... and learned a lot.

Thank you!
Don

mcgivor

Some important information, the SW 4048 will consume around 1Kwh per day wether loaded or not, be sure to factor this into the equation, additionally if thie SW4048 is the inverter charger of choice why not use the matching controller Schneider MPPT 60-150,  a System Control Panel "SCP"  or Combox would be needed to configure  the system, the Combox is probably the better choice if computerers are yor thing, this way everything is of the same family, able to communicate in the same language. Despite this statement there is nothing wrong with differing components as long as the operators understanding and experience is sound .

Battery question,  let's use 100 as a base
100Ah at 24V nominal is 2.4 Kwh (4 × 6 batteries )
100Ah  at 48V nominal is 4.8 Kwh (8 × 6V batteries )
To achieve the same Kwh capacity at 24V with the same batteries two strings in parallel  of 100Ah batteries would be required to equal the 48V single string.

Naturally with lead acid only 50% is available as a maximum discharge, to maintain a somewhat healthy bank, think very carefully, there are many factors to consider, don't rush the process, keep asking questions, eventually it will all come together and make sense.

Dave Angelini

Look at Schneider or Outback and stick with one brand for the whole system.

This is not ever going to be the place you live full time, right?

That question, to me, is one of the big ones on how much effort/$ you put into this.

nobadays

Mcgivor/Dave... I will look at these brands and see what they have to offer/ price points.  Makes sense to stay with one brand so they match up well.

Nope... don't expect to ever live there full time. Lived for many years in cold climates and lots of snow (-25 / -30) and truly don't miss it! Here in the mountains of AS where we live now it rarely gets into the teens and normally hovers in the 30s and 60s all winter with very little snow most years. In contrast the place in CO was 4* this morning and there is at least 8-10" of snow on the ground... and it is early in the winter.  Might be nice to visit for a week or two but we won't likely move there permanently. 

Thank you! 
Don

nobadays

Mcgivor...  thank you for the battery explanation. 

So if we use 8 - 6v @ 420Ah would the equation then be: 420 x 48 = 20,160 so 20.16Kw? or am I math challenged!

Just looked up the "System Control Panel "SCP"  or Combox" not knowing what these were.... yep gonna need one. Probably the SCP as the system will be operating in the winter while we are away and since we will have to rely on satellite internet that will not be on 24/7.

"keep asking questions, eventually it will all come together and make sense."  Well said and it is all starting to make sense I think!

Thank you for the answers!

Don

Estragon

To (hopefully) clarify the 24/48v thing...

I wouldn't say either is necessarily "safer".  High currents require larger wire, charge controllers, etc.  Higher voltage can create and sustain arcs more easily, and requires wire and insulation designed for the higher voltage.  There are safety risks with both, so it comes down to practical implications in a given application.

60' from array is relatively short.  At long distances, it's usually better to run higher string voltage with lower current to minimize voltage drop and required wire size, but this can limit controller choices to those capable of higher voltages.  In your case, strings of 3 might be (eg only) Voc of ~40v x 3 = 120v; too high for some controllers.  Strings of 2 for ~80Voc should be ok for most mppt BUT on a warm still day might produce <70Vmp, possibly too low to charge a cold 48v bank properly.  Using temperature data for you location and specs for the panels you're considering, you can see how different pv wiring combinations might work with a string size tool like:
http://www.midnitesolar.com/sizingTool/displaySizing.php

On the battery side, there are also trade-offs.  Lower voltage needs larger batteries and/or more strings in parallel for a given amount of power stored.  It gets hard to move very large batteries, and more strings means more wiring, cells to maintain, and potential balance issues.  A single string at 48v could be added to (while new) by adding another string (or replacing with 2 new strings if old) without replacing the inverter.  If 2 strings at 24v, adding equal capacity would mean 4 strings (getting problematic) or replacing the inverter to move up to 48v.

All in all, spending a bit more on a higher voltage controller like the Outback so you can reliably charge a 48v L16 bank makes sense to me.

I don't see much difference in where to locate batteries if using 120-ish volt pv strings.  You could play around with a voltage drop calulator to see.  One factor in the choice might be which would make it easier to avoid batteries getting hot in summer.  A shaded, possibly insulated enclosure will help life expectancy of both batteries and electronics.

Photowhit

nobadays said:

Curious why you would switch from propane to electric for the fridge...

I have said, and run numbers, for new installs, propane only makes sense for very minimal seasonal use. The best most efficient fridges, used 20 lbs a month. If you will have solar anyway a slightly larger system will pay for it's self quickly.

For seasonal use living there 6 months out of the years and an average fridge burning 20lbs every 3 weeks, that's 160-180 lbs of propane for 6 months, around. To me I wouldn't want that expense and tether to another energy system, though I heat with wood. Using gas already, I guess I can see it as not a bad trade off. I wouldn't want to be taking everything out of the fridge even once a year. I got stuck in Florida for 2 months last winter, came home late at night and a after getting the wood stove going, I popped pot pie in the microwave, something I couldn't do with gas fridge.

They are expensive, but you already have yours. Maybe stick with it until it dies, then replace with an electric. I certainly wouldn't buy another!

nobadays said:

The calculator I used recommended 748 Amps so that was my target. 

I'm guessing that was an RV calculation, or at least for a 12 volt system, more accurate to use kWhs of storage.

nobadays said:

So... would an 8 - 6v battery bank wired to 48v be sufficient for my application?

Batteries come in different sizes, they make 6v 330-450 ah batteries that are commonly used for solar (use to call them 'floor scrubber' batteries, but as common to use them for solar these days.) I think 8 - 6v golf cart batteries 215-230 ah, would be a bit minimal, but you have such good weather and it's close, I would go with them. 

Oddly we suggest these as inexpensive 'starter batteries' for people new to off grid solar, and I might be setting you up for failure from chronic under charging. but I think it's close enough to work for you. If they are stored outside they will have somewhat diminished capacity in the winter cold. I don't know what your energy profile looks like. If your use is seasonal, more energy use in winter for lights and indoor entertainment, then you might want to move up to the next 350-450 ah batteries.

nobadays said:

Yes I do know about modified/pure sine wave and have been looking at pure sine wave inverters as if I'm not mistaken wall warts/laptop chargers don't like modified sine wave inverters and chargers for cordless power tools often will not work.  I will run power tools off a generator.

Other electronics don't like modified sine wave inverters. Laptops are commonly run off them, most of those car inverters are modified sine wave, the battery charging creates a buffer for the electronics. I might have had a internal inverter for a laptop screen die from running on a modified sine wave inverter, 15 years ago, but the laptop had a bad battery so I took it out rather than waste the energy charging the battery.

Yes, many battery chargers for tools will die on modified sine wave inverters. I checked with DeWalt back in @2001. they said their older chargers would but the newer one that had come out around 2000 would be fine on a modified sine wave, I framed up an 'A' frame using one without issues.

nobadays said:

6 panels @315w = 1890w wired in three strings to a combiner box. (the panels could change but roughly stay the same Watts)

One of the issues with 72 cell 300+ watt panels is their voltage is a bit too low to run in pairs for charging 48 volt bbattery bank, you want about a minimum of 30% higher voltage coming into the charge controller than going out a 48 volt battery bank charging at around 58 volts. They are often just a bit too high a VOC (volts open circuit) to run 3 in series, particularly in cold climates.

Schneider makes a design tool to help calculate array/system;

https://www.schneider-electric.us/en/faqs/index?page=content&id=FA225778

nobadays said:

One more question - I may have asked it earlier... should I build a building by my panel array to house all this stuff, keep the DC run short or run the DC 60' to the house? I have space under an elevated deck to build the equipment building to keep it out of the house. (800sqft cabin - no room in there!)

I'm fine with leaving my battery bank outside, there is little chance the batteries will freeze, but there will be a noticeable reduction in capacity in very cold weather. I like ground mounted panels, for many reasons, like when I finish typing, I'm headed out to clear the snow off my array with the expectation of sun this afternoon. Something to consider in very snowy areas is to mount a panel or string of panels vertically, so even after snowy periods you will have quickly cleared panels.


Sounds like a great project!

Estragon

Another thought, just to throw it out there, would be for you to get a small PSW inverter to run off the existing 12v rv pump system for a while to run the lights and maybe the sat-modem.  You'd still use the generator for the larger loads, and to charge the small 12v bank to offset the added load.

This would allow for better measurement of actual likely loads, and for you to get a better sense of personal trade-offs you can't or won't make.  From concept to final installation, my solar project took ~3-4yrs.  Part of that was to build structures for panels etc., and logistics getting gear to cabin, but also helped with getting a good sense of actual needs.  For example, my campstove toaster worked, but was a PITA so electric toaster went into the must-have column.  For the 3-4yrs I used a pair of GC batteries and small inverter for lights, inet, phone charging, etc. for most of the day, and ran genny for a couple hours/day.

In doing this way, I think it made a big difference in getting a rig that pretty much does what I want the first time around.  I read lots of "ready, fire, aim" stories that end up in prematurely failed batteries and/or expensive equipment changes because of bad planning/expectations

nobadays

Photowhit/Estragon... Thank you for more info!

As far as batteries go I was looking at getting 8 x 6v Trojan L16 420Ah ... If I have an 1800+ watt solar array would this nt be plenty to keep these batteries charged at my calculated usage?  (avreage-1900kWh  Max- 3000kWh)

The panels I'm looking at buying (though I'm open as long as they stay around $200 ea) are as follows:
         Rated voltage: 36.81vdc - wired in series = 73.62vdc
         VOC: 45.75vdc - wired in series potentially = 91.5vdc         
So at 30% higher than the 58vdc going to charge the batteries this would be low at the rated but fine at the VOC... If I fall in between great if not.... I might need different panels with a higher rated voltage... or wire 6 panels into 2 strings which would put the VOC at 137.25vdc correct?  Is the 137.25vdc too high for a 150vdc rated CC?

We are talking about going up in January and I have been thinking about pulling a battery out of our RV and taking the 75W panel and a small inverter just to see how it works to run the lights.  I do think though with the help of folks on here we can put together a system that will be...kinda sorta... one and done.

Thank you!
Don

Estragon

Note that panel voltages need to be adjusted for temp.  Voltage goes down with higher temp, and vice versa.

Voc needs to be adjusted up for record low temp, and that voltage needs to be well under the controller max.  The high voltage scenario can arise as weak light hits panels on a winter cold morning.  Voltage rises with very little light.  There will be little/no current, but potentially damaging high voltage.  I'd want to have at least 10-15v headroom over the record low temp / high Voc to allow for local variation from record data, or more if the record was recorded at lower altitude etc.

Once sun hits the panels and they start producing useful current, they will drop in voltage.

Vmp needs to be adjusted down to account for high panel surface temp from direct noon sun heating and ambient conditions, and for design voltage drop from wire losses between pv and controller input.  Temp compensated target charging voltage could be >60v (especially with Trojans, which IIRC want high-ish charging voltage) with a cool/cold bank but hot panels, the array will put out a lower than STC Vmp voltage.  This could happen, for example, on a warm spring day with no wind on the panels.  Not a disaster (like too high Voc could be), but you may not get high enough voltage to charge properly without a decent cooling breeze.

You really need to plug the numbers into a string tool to get a sense of whether the proposed string wiring for these particular panels would work well in your location.

nobadays

Good info...pieces coming together!

I could not access the Schneider string calculator so used one for the Midnight Sun Classic.  I think what I plan is good to go...I'll try to attach the pdf of the results.

Thank you
Don

nobadays

Oops!  I found the error of my ways!

So, as we discussed this system at one point it was advised to have 3 strings of 2 panels. Then it was pointed out that there likely would not be sufficient voltage for charging, and that was correct.  So re-configured to 2 strings of 3 panels. The problem there, with the 315W panels I was designing around is that the VOC could spike in my location burning out the charge controler (Schneider 150/60). It was advised by Estragon I plug the numbers into a string calculator. Since I could not locate the Schneider string calculator I used one from Midnight Sun.

The error came when I plugged the numbers in I used - 3 strings of 2 panels. The calculator said that would work... unfortunately that still leaves me with insufficient voltage to efficiently charge the batteries. When the same numbers were plugged in using 2 strings of 3 panels... Estragon was proved right.  The calculator said that the max VOC was "Marginal." Not something I want to design around.

The solution seems to be, different panels. After a little shopping I located some 305W panels with a slightly lower VMP and VOC which will be well within the parameters of the CC and still give me all the charging capacity we should need, and less than $4.00 more than the other panels.  Thank you again for helping out!!

Now if I can just size the battery bank proportionately I will have a completed design. I have been looking at 8 - Trojan L16, 420Ah 6v batteries in series to deliver 48v.  However when I have run my numbers through a battery bank calculator - I have found 2 and they both spit out different numbers - it seems this may be over-kill for the system design, almost double by one of the calculators. If this calculator is correct, that amounts to almost $1,000 of extra storage capacity I don't need.

Any thoughts on sizing the battery bank? Trustworthy calculator/s you can point me to? 

Thank you!
Don

Estragon

Battery bank sizing comes down to personal factors which are tough to capture in a calculator.  A couple rules of thumb we use as a starting point:
- max drawdown to 50% of capacity
- capacity for 2 days no sun
This results in a bank size of 4x estimated daily loads, so a 3kwh daily load = 12kwh bank.  12kwh/48v nominal = 250ah bank.

Batteries have less capacity when cool/cold (mine can be ~1/3 less in winter), and lose some capacity as they age, so going a bit bigger and cycling a bit less deeply isn't a bad idea as long as you have capacity to charge properly.

Some of the squishier factors include willingness and ability to; use the generator when needed, run optional loads when extra solar is available, restrict loads in bad weather, etc. 

With usage in the same 3kwh/day area as your estimate, I find the 350ah bank does the job.  With a fairly dry spring/summer, I didn't run the genny at all from March to mid-Sept this year.  When the switch flipped to wet, gloomy fall weather in mid-Sept, a bigger bank would have let me go a bit longer between genny runs (but the runs would be longer for a bigger bank, so no real fuel savings).

A string of GC batteries, ~225ah@48v = 10,800wh might work if you're okay running the genny more, and might cost well under 1/2 the cost of Trojans.  

nobadays

Estragon...thank you for your reply!

Using the GC batteries is what I was looking at. Not only would it save money but I think they would be sufficient for our expected loads. We have been designing this system for the max load at 3000k but our average load is less than 2000k.  That coupled with the fact that we tend to be gone backpacking...at least 2 weeks a month, and that this system will primarily be used from June through September, I think we will be ok with a smaller battery bank. 

One of the reasons I had first thought about wiring to 24v was to keep the cost down if we decided we would need more storage... add 4 6v batteries instead of 8.  

I think we will go with the GC batteries.... any preference as to brands? Costco...Trojan...Centennial?

Thank you 
Don

Estragon

Freight/distribution can be a big factor in battery costs.  I'd probably go with whatever's economical locally.  Trojan is well regarded, but I personally wouldn't pay a big premium over USBattery or whatever is sold in volume locally.

If you do find you want to expand capacity, unless you really need it urgently, my approach would be to live with the one string of GCs if possible.  It's not all that unusual to "murder" your first bank for one reason or another while learning and adapting to being off-grid.  If you do need to expand urgently by adding a string, you'll want to do so early.  New batteries added to old tend to age with the old.

nobadays

Thank you for all your advice!
Don