PV Array Sizing

Horsefly
Horsefly Registered Users Posts: 470 ✭✭✭✭

Now that I’ve learned a bit, I’m confused about one aspect of designing an off-grid system: Sizing the solar PV array.

As some of you know, I first showed up here last September with lots of naive questions (http://forum.solar-electric.com/discussion/351167/mountain-cabin-off-grid/p1). In the course of lots of help here (you folks are awesome!), reading everything else I could on the web, and drawing on my now ancient electrical engineering degree, I managed to come up with a design (schematic is near the end of that same thread).  I remain greatly indebted to those of you who helped.

Along the way, we decided on a few changes, as one might expect. We added some room for a pellet heater sometime down the road, and we decided to move the ground mount PV array a bit further away from the cabin.

We decided split our purchase of components.  We bought some of the components before year end to take advantage of the great deal our host here had on free shipping and already great prices. So we already have the primary electronics: Charge Controller, Charger/Inverter, System Control Panel, E-Panel, combiner box, and all the requisite bypass and breakers.  

We can’t actually get up to our cabin to start doing much until sometime in May, so we decided to buy the other things this spring.  That meant the things that we either thought may still change, or were not covered under the free shipping from NAWS. Mostly, that’s the wire, conduit, PV panels, and batteries.  As a potential bonus, I figure there is at least some chance these components may continue to come down in price before we need them.

I come back here to seek the answer to what may turn out to be a philosophical question.

There seems to be two ways to calculate the PV power needed, and I can’t see that they would ever arrive at the same number, unless by chance.

The first way is what I would call the end-to-end approach. Lots of solar-related sites have web enabled calculators where you put in your load needs and the average peak sun hours, and it tells you how many watts you need from your PV array. I’m assuming this takes into account some guess at the losses / inefficiencies between the panels and the loads.  For me, I estimated our total daily energy consumption (conservatively) to be 2900wH. I came up with efficiency losses in wire, the charge controller, inverter, batteries, and panels, and came up with a compound efficiency of 50%. So I took 2900wH, divided by 0.5, and divided by my average peak sun hours (5.8), which gave me a nice rounded 1000W.   Easy breezy. Four 250W panels. I can do that.

The second method makes a little better engineering sense to me. Basically, it makes the battery bank the center of everything.  I think @BB (Bill) was the first I heard this from. You figure out the battery bank size based on DoD, days of autonomy, and inverter efficiency. Once you have a battery bank size, you size your PV array to provide charging sufficient for that battery bank. I got to a minimum 24v battery bank size of 284AH. Shopping around and rounding up, I arrived at a 310AH bank. From there I used Bill’s explanation of % charge per hour: “minimum acceptable” (5%), “good” (10%), and “optimal cost effective” (13%).

310AH * 28.8V (AGM charging voltage) / 0.75 (panel/PV wire efficiency) / 0.98 charger efficiency) * 5%/hour = 607W for “minimum acceptable” wattage from my PV array.  Similarly, the “good” (10%/hour) would be 1214W, and the “optimal cost effective” (13%/hour) is 1579W.

So the end-to-end method tells me I need a 1000W array. The “battery is everything” approach says I need something between 607W and 1579W of array, and 1000W is closer to the minimum. What to do?

More recently, I’ve seen here (and elsewhere) that solar panels are the best place to over-do. I.e., if you are going to spend more than you need to, spend more on solar panels. With the prices of panels coming down, maybe so?

Although I’m interested in how I should proceed, I’m also curious how you pros do it. I know some of you actually have built several systems, or even done this for a living.  I’m curious to hear how you folks approach this. Door 1, Door 2, or some other choice?

Steve




Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    The two different calculations for solar array sizing address two different issues. The first you did addresses loads+amount of sun for your area. I use 52% end to end efficiency (off grid AC system)--50% is certainly close enough.

    The second to to ensure you have enough charging current (5% to 13% typically) with a 0.77 derating for solar panels and charge controller.

    Since I really like to have a "balanced system" (typically, the battery bank supports 2 days of no-sun loads and 50% discharge--Or 4x daily loads)--Sometimes the results come out close to the same solar array, other times not. Depends on the amount of sun you get (location, weather, seasonal use).

    Then you have some choices to make... 5% charging (minimum recommended--good for weekend use). To 10%+ (good for year round use). More panels certainly gives you more energy to play with and you don't have to manage your energy usage as closely.

    The reason for two sets of calculation is that some folks have "small" battery banks (2x daily load--weekend cabin, RV usage). Others have as much as 3 days (6x daily load) to carry through longer bad weather spells, etc. A larger battery bank needs more charging current to be "happy".

    For lead acid battery systems... The 4x daily load seems to be a sweet spot (cost of batteries, enough surge current for most people, charge relatively quickly, bank not too large that it rips a whole in your wallet if you "murder" your battery bank, etc.).

    You can do a lot more detailed calculations (and better estimates for your loads, inverter losses, and such)--For example, if you have a small load and large inverter than runs 24 hours per day (large inverter to manage surge loads, as an example)--Then your inverter tare losses can become significant and need to be explicitly added (instead of 6 watts tare 5 hours a night, 40 watts * 24 hours per day "on time" = 960 WH per day).

    In the end, if your system is within 10-20% of your predicted performance factors day to day--You are doing well. It is (usually) not worth getting down to 5% or better accuracy in design/predictions--There are just too many variables and "real life" will swamp your exact calculations.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Horsefly said; The first way is what I would call the end-to-end approach. Lots of solar-related sites have web enabled calculators where you put in your load needs and the average peak sun hours, and it tells you how many watts you need from your PV array. I’m assuming this takes into account some guess at the losses / inefficiencies between the panels and the loads.  For me, I estimated our total daily energy consumption (conservatively) to be 2900wH. I came up with efficiency losses in wire, the charge controller, inverter, batteries, and panels, and came up with a compound efficiency of 50%. So I took 2900wH, divided by 0.5, and divided by my average peak sun hours (5.8), which gave me a nice rounded 1000W.   Easy breezy. Four 250W panels. I can do that.
    Steve, First it's hard to imaging any place where a fixed array would received 5.8 hrs a day on average. Are you speaking for just the months you'll be there? You can't size for the average, you need to size for the minimum month. Other wise you will under size your system. Off grid system MUST be over sized as batteries must be fully charged often, once or twice a week to full charge.

    So where in general is this cabin? I guess with 2.9 Kwh's a day, your major use will be a fridge, lights, fans and a TV a little laptop time? Water heating, cooking by gas, (maybe a crock pot, rice cooker, foreman grill), no air conditioning...

    Array to battery does require a bit more thinking than other options, will you supplement the solar with a generator? How many days autonomy will you need? (days without sun running from battery only)
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    From personal experience and persistent monitoring I would double that 1600 W estimate to be able to use the cabin full time in the depths of winter...  This winter, 2016-17,  has been the worst I can recall for PV, have lived in the area since '73!.... and we are used to cold  (-30*C) days with more sun than clouds till now... so get a good handle on the number of days  of autonomy and add 50% to that....if you will be there in DEC or Jan.  Go for the 1600 for use  from ~ early Feb till Mid Nov. That will mean some close monitoring at the start and end so tha tyou leave with a reasonably full battery, no inverter left on and the CC connected.. It is the shoulder seasons not the summer you have to worry about. 
    I used method 2...

    Enjoy the ride...  have fun.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    So, just to run the math on your numbers... Starting with your load at 2.9 kWH per day:
    • 2,900 WH * 1/0.85 inverter eff * 2 days * 1/0.50 max discharge * 1/24 volt battery bank = 569 AH @ 24 volt batter bank
    A 569 AH @ 24 volt flooded cell lead acid battery bank will support a maximum (recommended) inverter of (569 AH / 200 AH per 1 kWatt) = 2.85 kWatt inverter (with ~5.69 kWatt surge).

    Now our two array calculations. First based on battery bank size (note that I am using 24 volts nominal for loads, and 29 volts for charging--Another fudge factor/loss estimate):
    • 569 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,071 Watt array minimum
    • 569 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,143 Watt array nominal
    • 569 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 2,786 Watt array "cost effective" maximum
    Notice, the larger battery bank needs a larger array for proper charging. Some battery mfg. recommend 10% minimum rate of charge (5% minimum seems to be a good number for equalization). Going less than 5% rate of charge, it is going to take a long time to recharge the battery bank (and you may not even recharge if you have some loads during the day "stealing" from the charging current--Why 10%+ solar array is nice--if you can justify it).

    Next, sizing the array based on hours of sun. Say Denver Colorado with array tilted to latitude (average best collection):
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Denver
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 50° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    4.27
     
    4.85
     
    5.58
     
    5.62
     
    5.81
     
    5.90
     
    Jul Aug Sep Oct Nov Dec
    5.72
     
    5.60
     
    5.87
     
    5.60
     
    4.52
     
    4.12
     
    You have quite a bit of sun, even in the winter (clear mountain air)--Assuming you want to supply 2.9 kWH per day even in dead of winter (little genset use):
    • 2,900 WH per day * 1/0.52 end to end system eff * 1/4.12 hours of sun (December) = 1,354 Watt array (December "break even" array)
    So--Some picks... Say you want full time off grid, recommend 10% rate of charge (solar panels are historically "cheap", batteries are expensive). That is a 2,143 Watt array. Such an array will produce:
    • 2,143 Watt array * 0.52 system eff * 4.12 hours of sun (nominal December daily average) = 4,591 WH per day (Dec)
    • 2,143 Watt array * 0.52 system eff * 4.85 hours of sun (nominal February daily average) = 5,405 WH per day (Feb)
    • 2,143 Watt array * 0.52 system eff * 5.50 hours of sun (nominal summer daily average) = 6,129 WH per day (summer)
    Now, if you decide you only want a 1 day+50% battery bank, then your battery bank will be 1/2 the size, and the solar array (for battery charging) will be 1/2 the size too.

    Or, you can decide on the larger battery bank and 5% rate of charge for a 1/2 size solar array (1,071 Watt array). And use the minimum 1,354 Watt array (December "break even" month)--That will give you closer to your 2.9 kWH per day that you asked of your system.
    • 1,354 Watt array * 0.52 system eff * 4.12 hours of sun (Dec) = 2,901 Watt*Hours per day (at 120 VAC).
    So we have some estimates of sizing (nominal battery bank, maximum AC inverter, range of solar array options)--And now you need to pick the actual components that meet your performance needs and cost targets.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    So the end-to-end method tells me I need a 1000W array. The “battery is everything” approach says I need something between 607W and 1579W of array, and 1000W is closer to the minimum. What to do?

    How much do you like to run your generator ?   I'd go for a 1600W array for the gloomy winter, and you will still have lots of time to bond with your generator

    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    Thanks all for the replies. As usual, @BB. gave me the most detail to noodle through, and with one minor exception, everything he said lined up with my own engineering. However, he brought up a key point I hadn't mentioned:

    BB. said:
    Now, if you decide you only want a 1 day+50% battery bank, then your battery bank will be 1/2 the size, and the solar array (for battery charging) will be 1/2 the size too.
    I forgot to mentioned (or maybe mentioned in the other thread) that we are only going for 1 day and 50% DoD. The cabin as it is now has been completely off-grid, wired for 120VAC and using a generator for the past 40 years. It is sunny there most of the time, but if we do have a day or two where there isn't enough sun, we can easily fall back to how we've done it for years: Run the genset when we need to.  That's how I got the 284AH battery bank (exactly half of your 569AH, with rounding), which I rolled up to 310AH based on the batteries I've chosen.  Between all the family members, it is mostly only used for weekends, and even then not every weekend. 

     However, this brings up another point regarding the size of the PV array: Although I'm starting out with this smaller battery bank, there is at least some probability that we will decide to add in a day or two of autonomy when we replace the current batteries. That would then require more panels, and if we over-panel now maybe we will be just sufficient.

    Looking at the rest of Bill's math, aside from using slightly different inefficiencies and battery charging voltages, I think I came up with the same numbers. I do need to go back and check some stuff though. I know at one point I had included 40W tare loss from the Conext SW4024, but now I'm not sure it went into my spreadsheet.  :*

    As for the solar insolation, discussed by both @BB. and @Photowhit:  The cabin is on the western slope of Colorado, between Montrose and Telluride.  I used the NREL Red Book section for Colorado, and focused on the Alamosa and Grand Junction tables. Where the cabin is has quite a bit more sunny days than the eastern slope towns, or the ski towns for that matter.  I ended up using the Grand Junction tables, which had an annual average of 5.8 peak sun hours for tilt equal to latitude. Alamosa was actually over 6 for an annual average. We're kinda lucky in Colorado, in terms of solar.  Also, the cabin is at nearly 9,000 ft elevation, so I'll be getting pretty good solar harvest - Probably better than the effective 5.8.

    @westbranch, you brought up using the cabin year round. Right now that isn't real practical, as there is no way to get to it for at least a few months of the year (except by cross country skis or snowmobile).  I initially used this reasoning to go for a "flatter" panel tilt (perhaps latitude minus 15 degrees), which would raise my peak hours during spring, summer, and fall to be above 6.0. But I decided to leave it at tilt = latitude.

    Bottom line: I think you guys have me convinced that I should go big on the panels. I just gotta keep my fingers crossed that the prices come down a little more before we buy!

    Thanks again!
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    Some points on bigger array relative to bank size (within reason):
    - you may be able to shift loads like pumping water to use extra solar available once batteries are mostly charged.
    - helpful on mostly cloudy days when you might still get an hour or two of decent sun or thin clouds.
    - you are way more likely to murder the bank than the panels.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    One issue with smaller battery banks--If you truly discharge the battery bank daily by 50% (weeks at a time)--There is the problem that there are not enough hours in the solar day to fully recharge the battery bank (it can take 8+ hours of charging i.e., full sun hitting the array).

    If many of your loads are during the day time--Then normally you are not discharging the battery bank to 50% daily as the solar array (more panels) are supporting your load directly.

    This brings up the idea of facing 1/2 the panels south east and the other 1/2 south west (virtual tracking) to get more hours a day (lead acid batteries do better charging at nominal current more more hours per day than "high current" with shorter number of hours due to a south facing array).

    It is possible that deep cycling your battery (i.e. 30% to 80% state of charge daily), that your battery bank will do OK. There is aging vs cycle "death".

    If you are there on weekends/a couple weeks at a time... You may have (for example) 60 days on solar. Even if you really deep cycle the batteries and get 500 cycles (vs 1,000-1,500 "less deep" cycle life):
    • 500 cycles / 60 days per year = 8.3 years of "cycle life"
    Vs something like 5-8 years aging life for a standard deep cycle battery (note: Cold climate/battery bank can help extend aging life by several years).

    You might be pretty happy with your smaller battery bank... And if you do "murder it" sooner (i.e., battery bank only last 3-4 years)... Cost wise, that is not a bad deal either (i.e., 1/2 size battery bank 3.5 years vs full size bank lasting 7 years is the the same overall costs).

    Otherwise, your inverter is a bit on the large size for the battery bank... 310 AH @ 24 volts (200 AH per 1kW) = 1.55 kWatt max continuous load (recommended). That number of 1.55 kWatts is also a typical maximum solar array for a standard charge controller (recommend remote battery temperature sensor to help keep battery from overheating with high charging current).

    You know your loads--(surge, continuous, etc.)--Those are the decisions you have to make and the "solar math" cannot answer directly.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    Good points @BB. and @Estragon.  

    I may have to look into that virtual tracking. I hadn't thought of that. As it is now, I will have two strings of either 2 or 3 panels, ground mounted. I could easily have them at slightly different angles. 

    I don't like the idea of "murdering" my batteries, but I realize as a rookie I am more likely to do so.  Even if I think of nearly everything, others will be using the cabin when I'm not there, and I'm not sure they all will want to keep as close of an eye on things.  The Schneider has some cool settings that would let me set a conservative Low Battery Cut Off (LBCO) voltage with a delay and a hysteresis value to prevent the inverter from just kicking back on when the batteries settle. I'm hoping to have a concept of operations that everyone can live with, where the inverter will just cut off and everyone knows to go start the generator, which will power the loads and recharge the batteries some.

    We have one big load: A well pump. That is the only thing pushing us to a large inverter. I know we have a bit of an upside-down load profile. Our daily consumption is only 2900WH, but our peak instantaneous draw can be 3900W (even higher during the inrush current for the pump). Hence my decision for a relatively large inverter.  The higher peak discharge is also one of the reasons I'm going with AGM batteries. 

    I do think @BB. is right that we should actually get pretty good life out of the batteries, since the place is really used mostly on weekends and never in the winter.  On the temperature thing: The batteries should be always pretty cool. The solar room will be in the northeast corner of the basement.  Even in the hottest part of the summer it will probably never get above 80 in that room.

    The inverter also has a "search" mode that it looks like I assumed. In this mode the inverter tare loss goes down to only 8W, but it doesn't provide any power until a big enough load is on the line. If I don't use the search mode, the tare loss is a steady 40W, which breaks my daily energy budget. The first time someone plugs in a phone charger (which is too low for search to wake the inverter up) it won't work, and I'll be in the dog house. Ugh!
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    I have a house rule that all the phone chargers etc. go on a single power bar with a lighted on/off switch. It gets switched off at night.

    For water I use a cistern to gravity feed toilets and lawn service in summer, so pressure tanks will generally last until I can use solar to fill. That lets me keep the big inverters off at night.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Ah Ha!.... Even if I think of nearly everything, others will be using the cabin when I'm not there, and I'm not sure they all will want to keep as close of an eye on things.   the Unknown Factor! and probably UNLIMITED  USE...
    Hey Dad (?) the light went out on us, any ideas?   There are a bunch of blinking Red lights and a squeell every time we restart the system...
    This definitely tells me you need to think about the panel orientation for the times OTHERS will be there, alone...Will they be there in the winter?  Have you considered the impact of someone getting one or more of these?
    https://youtu.be/QAy1d4VItLc

    hope the link works...

     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    I hear you @westbranch.  About once per winter someone in the family goes in for at least a weekend.  So I am thinking some about what that could mean for the solar. I'm mostly worried about how to keep keep the panels clear enough for some float on the batteries.  If someone does go in, I'll make sure they have orders to clear the panels off!

    Those ATV snow machines are pretty cool. We may need to have a family meeting to invest in those next (after we pay all the bills for solar!).
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    I will never, well almost, go back to a snowmobile now that I have the tracks.  Some years we get > 7 feet of snow and it drifts too in parts...  Been stuck in some of the drifts, badly....  the tacks are amazing... well worth the price and capable of towing a small trailer  like https://www.costco.ca/Highlander-II-ATV-Trailer-.6-m³-(20-ft³).product.100250277.html

     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    @westbranch - do you know how it would handle slush under snow?
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Have not encountered that yet, but nothing seems to stop the rig now..  it almost walks up walls would be a good phrase to describe it... I thought it was going to flip... on me... 
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    So after reading and rereading this thread (and reading @BB.'s awesome posts a couple more times after that :) ), I think I've come to the conclusion that calculating the required PV array size based on the batteries is the only correct way to do it.  In an off-grid system, it really is all about the batteries. It's easy to think about power from the PV array going to the loads, but the fact is the loads are drawing power from the battery, and the PV panels are only recharging those batteries (well, mostly). 

    I think the end-to-end calculation may be a little simpler, but it falsely de-emphasizes the batteries, and probably undersizes the PV array in most cases.

    The only reason I'm bringing it up is because there are so many web sites that provide the handy calculators that just have people put in their load requirements (W and WH) and gives them an estimate of how big their PV array should be.  I don't think they should do that, and it probably is a disservice to the poor folks that are seeking help.  It's not like I'm going to go out and protest to get it changed, but it seems to be a bad idea.

    Aside from the mathematics of the two approaches, many of you guys with lots of experience made it clear that and over-sized PV array is quite a bit better than and under-sized array. I think these on-line "tools" will more likely to point people to an array that is smaller than they should get.  I also think these tools should take the opportunity to teach people the importance of the battery bank, but I guess that is a different subject.

    Steve

    P.S.  If it isn't already obvious, the engineer in me just loves to read posts by @BB.! 


    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    It is why I like to do the naked calculations in the open... When you do spread sheets and web calculators--The details of "why" you do things get lost.

    You can program the calculations into a spread sheet pretty easily--But then you know why and what they are doing (and what the base assumptions are).

    You can "shorten" the calculations if you want--But it assumes that you use some sort of standard assumption (like 4x daily loads in a sunny region). Once you go outside the box (which most people end up doing)--Then you really have to go back to doing the two sets of calcuations and looking at the hours of sun, how much you want to run the genset, etc. to tweak the system to make you happy.

    In the "olden days"--Many installers/solar retailers told folks that if they wanted to get more power from their systems, to add more batteries. And they ended up with not enough solar panels to keep either their loads or their batteries happy.

    The "right answer" was more panels--But back then, "more panels cost $10-$20+ per Watt (and batteries were "cheap"). Today panels are closer to $1 per Watt, and batteries are expensive. Now adding panels (and over paneling), is the cheaper and better way to get "more capacity" from the systems.

    However, for folks with poor sun (or poor winter weather)--Solar will always be difficult. You simply need good direct sun (and little to no shading) to get good production from solar electric panels.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    I think back on the rules of thumb we use to use, and since batteries have changed very little, I suspect that is why we use to use 3-5 years for a set of golf cart batteries, and now I expect 5 years out of cheap GC and as much as 7 from Higher end...

    Though I still use 5-7 years for L-16 and don't see much improvement in other tall cell batteries, perhaps because they were in more expensive/better thought out systems...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.