Opinions on battery sizing

BonesBones Registered Users Posts: 12
Hi all, i am new to this forum and have been slowly learning how to manage my own solar system over the last five years.

I currently have a 12v system with 280w of panel, and an unknown age 100ah AGM battery given to me by a friend.  The battery doesn't really have the capacity i'm after, and my charge controller (steca 3030 branded as netmetersolar) seems to let the voltage go up to 15.1v occasionally, even though it is set to GEL which I thought was supossed to limit charging at 14.4v.  I'm unsure why this is happening, but for this reason and the lack of capacity I am thinking of replacing my battery with either 2 x T105re (225ah at 12v) or 4 x t105re (450ah at 12v).

My power usage is quite low, i have a 300w inverter but only really draw around 100w maximum at any given time.

This leads me to my question:

In my situation, would you think about current power usage and get 2 x t105re and run them down to 50% dod, and use the saving to buy another 140w panel.


Think about my power usage going up in the future and buy 4 x t105re and run them to 20% dod while i save to get another panel by winter.

Thanks for any suggestions, i appreciate its a bit of an odd question but i'm going to get a quote on batteries in the next few days and want to make a good investment! 

Thanks again, Chris.


  • BB.BB. Super Moderators, Administrators Posts: 29,706 admin
    Having more solar panels can really be nice to help get long(er) life from your battery bank.

    If you have proper charging (5-13% or so rate of charge for solar--And really 10%-13% or so for full time off grid)--That is ideal.

    Roughly... everything done "right"--4 batteries at ~25% DOD (depth of discharge) will last a bit more than 2x a 2 battery bank with 50% DOD.

    However--The costs are about the same (4 batteries over 6 years vs 2 batteries over ~3 years of near daily cycling).

    On the flip side--It is not difficult to over discharge your battery bank (loads forgotten and left on, charger or wiring failure, etc.)--And now your 2x $$ battery bank is killed in two weeks.

    In the end--What is more important to you... If you have a couple of days of bad weather, will having more storage be helpful--Or can you just turn off your loads and/or fire up a genset and be happy.

    Design the system to meet your needs first--Then "fit" the batteries+solar panels+solar charger+etc. to support those needs.

    For a deep cycle lead acid battery--You probably will get optimum performance (best bang for your buck) if your DOD is around 20-25% per day, and avoid going below 50% DOD very often.

    Solar panels are a historic low point in costs... And batteries are getting more expensive... Running 10%+ rate of charge usually makes battery+load management much easier. 5% rate of charge is good for seasonal/weekend type use generally.

    -Bill "hand waving" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BonesBones Registered Users Posts: 12
    Hi Bill, thanks for the speedy reply =)

    Seeing as my panels give a max output of 16amps, that puts me at less than 4% rate of charge with a 450ah bank.......If I get another panel at 140w (8amp max), that will still only put me at 5% in ideal conditions which we get plenty of here in portugal in the summer.........the winter can be blue skies for weeks on end or grey skies for weeks on end! Firing up the generator is an option but only very occasionally, i dont want to be relying on it more than a couple of times a year.

    However, if I get a 225ah bank - With my 280w of panel i am sitting at 7% in ideal conditions, then adding one more panel would give me 10.6%.

    It looks to me like the 225ah bank is the better option as my charge controller is limited to 30amps at 12v which means one more panel max.  I could switch to 24v and get twice as many panels out of my cc, but that means getting a new (more expensive) inverter and losing my 12v loads which I use for the majority of my lighting.

    The last option I can see but dont really understand how to set up, is getting another 30amp cc and wiring it alongside my existing cc to get more panels into the system.  I can't see how this would work properly when boost or eq charges were triggered? what if one triggers and the other doesn't?

    Thanks for reading, Chris =)
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Bones said:
    My power usage is quite low, i have a 300w inverter but only really draw around 100w maximum at any given time.
    Welcome to the forum,

    The first step in designing a system is to define your loads.  In addition to specifying the PEAK load (which you have done), you must specify the daily energy load.  If you are drawing 100 watts all day long, that is 2.4 kilowatthours per day.  That would require (at a minimum) four of those t105 (and at least a couple more of those solar panels). 

    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BonesBones Registered Users Posts: 12
    Thanks for joining the discussion vtmaps,

    For me, this is where it gets a bit difficult.  I live completely off grid, and whereas most people can easily add up the wattage of their appliances and calculate how long they  run on an average day - I don't actually have any of these things, instead power is something of a luxury that I use when I have it for things such as Laptop, 12v lights and a fishtank heater to heat a small tank of water as a thermal mass and provide heat to the roots of tender plants through the winter. All of these things are essentially luxuries and as such are hard to calculate a typical daily draw........ Also my cc tends to show some sort of "phantom load" during the day so using my cc's  load ah log to estimate typical usage is not accurate!

    This is what's lead me to specifying my peak load, which I would definately not use all day.  I'll try and break down my individual loads and their probable hours of use:

    Laptop - 70w, around 2 hours per day (but sometimes i'll use it more if I have surplus power) = 140wh
    12v lights - 2 x 4 watt led's max 4 hours per night  = 16wh
    220v Lights - 2 x 20w cfl's max 4 hours = per night = 160wh
    Fishtank heater - tricky to estimate as it runs intermittently and depends a lot on ambient temp - 75w for maybe 6 hours per day = 450wh
    Circulating pump - 5w constant = 120wh

    That means that my daily energy load could be 886wh/day.

    As I said, most of these are luxuries so if the solar is low I'll usually sacrifice the laptop or the 220v lights to keep the aqaurium heater on - or cut the aquarium heater and pump and rely on a small gas heater until the sun returns enough to get the water up to temp.

    This makes my decisions around system size more about financial viability (I have very limited funds and dont want to make a situation where I have to keeping putting money into my system for it to survive) and making the best use of the investments I have already made in panels, charge controller and inverter.

    But, back to the point - My daily use could fluctuate between maybe 500 - 1000wh/day.  Obviously if I expanded my system i could find a use for extra power (which is obviously tempting!).

    Thanks again for reading, all opinions/suggestions welcome,  Chris
  • BB.BB. Super Moderators, Administrators Posts: 29,706 admin
    Probably your best best would be to insulate the planter and use a gas heater as backup to avoid generator time (use the "free solar" when the sun is available, run cooler in cloudy weather--slow plant growth but not damage the roots).

    1,000 WH per day is a very handy/smaller system and sounds like it will meet most of your needs. If you get weeks of dark/cloudy weather--Solar electric can drop to as low as 5% of typical sunny day output--Backup power sources and/or turning stuff off are your only options.

    A quick rule of thumb design (2 days backup power, 50% max battery discharge, 1,000 WH per day):
    • 1,000 WH per day * 1/0.85 inverter eff * 1/12 volt battery bank * 2 days storage * 1/0.50 max discharge = 392 AH @ 12 volt battery bank
    Then there are two charging equations--First is based on "size" of battery bank. Second is based on hours of sun and loads you want to run:
    • 392 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 369 Watt array minimum (weekend/seasonal)
    • 392 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 738 Watt array nominal (full time off grid)
    • 392 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 960 Watt array "typical cost effective" maximum
    And then based on amount of sun...

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 49° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    Jul Aug Sep Oct Nov Dec
    Toss the bottom three months (generator) would be February at 3.92 hours of sun "break even" month. Or you can try 2.76 hours for December:
    • 1,000 WH per day *  1/ 0.52 system eff * 1/3.92 hours = 491 Watt array February typical
    • 1,000 WH per day *  1/ 0.52 system eff * 1/2.76 hours = 697 Watt array December typical
    So--A balanced system for you would seem to work pretty well with around 392 AH @ 12 volt deep cycle lead battery bank and ~738 Watt array--It would probably reasonably meet your needs... And you could always add more panels to 960 Watts or so if you want more summer power, or run the genset/gas heating a bit less in the winter.

    Power usage and system design is a highly personal set of choices--The math is here to help you keep the over all system design blanced and (more or less) optimum use of your money.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BonesBones Registered Users Posts: 12
    Wow, thanks Bill - what a excellently comprehensive reply!!

    I can see straight away that I have been quite drastically underestimating the amount of cash needed to build a reasonable setup based on 1kw/day!

    I think you're right about trying to minimize using the aquarium heater, financially speaking i'm going to be unable to get a 392ah battery bank AND 4 more panels AND upgrade my charge controller (my current one is rated at 30a 12/24v).  I could theoretically switch to 24v and keep the same cc, but i've heard bad things about running the steca 3030 on 24v close to it's amp limit. I would then also need to buy a new 24v inverter and lose my 12v leds which are pretty good for their low power use.

    So, with these things in mind, and looking at the figures you kindly worked out - I think my best plan of action is to try and half my projected 1kwh/day. I think this would be do-able as the figures I gave for usage were based on the max I would use, and I would rarely do all of these things in one day, and definately not every day (especially if i'd invested in quality batteries and had a v. strong incentive to never discharge below 50%). 

    Cutting that 1kwh/day in half would allow me to get quality batteries, afford one more panel AND keep my current cc and inverter. 

    This would make my system 225ah T105-re's, 420 watts array - looks like it sits comfortably between full time off grid usage and typical cost effective maximum (assuming I can just halve the ah and array figure you provided as i dont fully understand the equations for this - if you feel like breaking it down for me i'd love to learn :smile: )

    It also looks like it would put me comfortably over the typical December figures minimizing winter genny use (hoping for near average weather). 

    Then, by the time the batteries have given all they've got in maybe 5-10 years, I could have piece by piece upgraded cc and array ready to give a bigger battery bank the kind of amps they'd be after..................

    While i'm musing about compromising on my *ideal* system, I suppose I could always forget the quality batteries for now - get cheaper 100ah 12v SLA's which i would feel less precious about (and run them lower than ideal) and use the saving and the 2 years while i kill them to get more panels and a bigger charge controller.  This would double my current capacity without locking me into a 5-10 year period where I can't expand the capacity of my bank if i find the means to.................

    Hmmmmmmm! It seems there are quite a few options and many different angles to approach from!

  • BB.BB. Super Moderators, Administrators Posts: 29,706 admin
    If you can get "golf cart" 6 volt ~200 AH batteries--Those are usually the best bang for your buck (at least in the USA) for smaller systems like yours. They are also fairly rugged and forgiving (you can always add water if you over charge them a bit).

    Sealed batteries tend to be more sensitive to over charging (vent, and you cannot add water). Plus, you can not use a hydrometer to monitor state of charge of the electrolyte (makes understanding what is going on with your battery bank a lot easier).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike74820mike74820 Registered Users Posts: 44 ✭✭
    my system is a lot like bones setup. but i learned early on these little setups need to be dc only. i run a computer monitor . 12volt computer 3 set of 12 volt lights and all the usb junk off the computer, and i can tell you if i used my 400 watt pure sine wave inverter, my battery was dead pretty much after 15 hours of use. now that i switched every thing to dc only, my midnight battery monitor never goes below 70%  another thing ill say for these little systems at least get the xantrex c35 pwn controller, it is adjustable and make all the difference in charging your battery, just so if some one asks i have a 105ah battery and my load is around 2.5 amp a hour and i use my system on average 15 hours a day
  • BonesBones Registered Users Posts: 12
    These are exactly the reasons I was looking at getting quality flooded cells in the first place........ I like the idea of a quality product, not only for my own piece of mind but also for environmental reasons. Plus, being able to monitor the individual cells will, I think, teach me a lot about how to keep batteries in optimum condition. Plus being able to occasionally go as deep as maybe 60% dod *very* occasionally could help me through a period of crappy weather.  So far i've personally found the kind of mystery of what's going on inside sla's and the agm battery on an individual cell level a bit frustrating for my naturally inquisitive personality :smile:

    Having very limited funds and no steady source of income can sometimes make investment decisions very difficult..........

    Thanks for the reminder that quality batteries will give me more for my money, and teach me a lot more :smile:

    I must remember also that my cc being of quite low quality and little prone to overvoltage (as well as seemingly reading 0.2 under compared to my multimeter, so potentially never getting to 100 soc) could cause sla's to vent a lot and be trash in no time, whereas this situation could be corrected with flooded cells.

    Thanks a lot for your time and input, and helping me not just go round in circles in my own head.  Very good to have people with experience able to add some much needed perspective to my situation :smile:

    Thaks again, Chris
  • wellbuiltwellbuilt Solar Expert Posts: 416 ✭✭✭
     Vt ,mike ,and bb. helped with my system last year. 
     I wanted to go with 1000w of solar and 4 6 v gc battery's  but after some one ventilated my air stream trailer with a 12 g  and buck shot I scaled down a bit , and have been using 2 6 volt  gc battery and 230 watts of solar. 
     I have a 300 watt morning star inverter and a cheep tracer 40 amp mttp cc .
     For 8 months last year we never dropped below full charge 12.6/12.7 .
     I'm at float buy 10 am and charge phones ,iPad ,fish finder battery's after lunch.
     Last fall I started a propane heater with fan 12v and needed a  c pac unit to sleep with at night .
     Some lites are 12v but most run on ac  power.
     I have a Samsung led tv it takes 18w it streams movies from a thumb drive with 100s of movies. 
     My radio is 12volt but works better on ac .
     My battery's cost 80 bucks each the solar pan nails I had , the cc was 190 
     in the winter I run a 12 volt car charger for 11/2 hr a day @ 10 hrs per gallon of fuel. With a 180 dollar generated. 
            Works great so far.
    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • BonesBones Registered Users Posts: 12
    Hi wellbuilt, Thanks for getting involved :)

    What kind of panels are you using with your mppt? I am currently struggling to find quality 12v panels for a good price and my cheap 12v inverters fan bearing sounds like its on its way out so I'm considering switching to 24v.  This would mean 4 6v batteries and a 24v inverter which bumps up the price a lot.

    Alternatively I could get 2 24v panels and two batteries and get an mppt cc to use 24v panels with 12v of batteries. Is this what you are doing?

    Thaks, Chris
  • BonesBones Registered Users Posts: 12
    Or would it be better to wire the panels in series to 48v for the mppt charger?

  • mike95490mike95490 Solar Expert Posts: 8,469 ✭✭✭✭✭
    With systems over 500w, MPPT seems to be more cost effective, running thinner wires and a more expensive MPPT controller.
    the rule of thumb for battery voltage, is every 1,000 watts required of the inverter, add 12V.
    1,000w = 12V
    2,000w = 24V
    3,000w+ = 48V  
     The other factor is battery bank, if you have lots of small loads, 800W 24/7  a 12V bank would need to be massively parallel, and a higher voltage is benificial for easier watt hour storage, and less copper loss in the wires
    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 ,

  • wellbuiltwellbuilt Solar Expert Posts: 416 ✭✭✭
    Well my panels are Kyocera 1 70 watt 1 140 watt  for 12 v charging . 
     They spent 10 years on a sail boat from 2000 to 2010 but where old when I bought them . 
     The panels where take offs from a road crew . 
    They are run in series and give about 10 amps +-  but are mounted almost flat  and to the SE ( out put is kinda low) 
     I have 215 ah 6 v battery's but don't use much power from them ,because once the battery's are full  at 10 am the rest of the days power is free from the sun.
     My phones, I pads  and battery operated tool get charge in full sun  showers2/4 are at 600 pm after beers .
     Cooking is on propan / or fire .
     I have 3 small 12 v fans run on a 32 watt panel 1 out side on the porch 2 inside the speed is controlled buy throwing a old  t Shirt over the panels to cut down on the power .
     Even on rainy days I get 3 amps or so .
     I mite only use 3 or 400 watts a day from the battery's to 
     Nov ,dec ,Jan  I run the charger 2 hrs in the am every day and get a full charge every week .
     It snows here every  day enough to to cover the panels.
      I think I would try to heat your plants with water from a black hose on the ground  out side and run your small pump off a solar panel when the sun is out (no battery)
     If you  Dump your heater I think a 12 volt system will work for you since you don't really have any equipment to run.
     I would get 2 6v g c batteries 215/230 ah  , stay with 12 volts ,get a 40 amp mppt charge controller ,you can use it for 12/24 volt  charging gand it is easy .
     500 watt  12v 1000 watts24 v  
     You will need to match your old panels with a new one .
     Maybe run one old panel 140 watts and one new panel 100 watts x 2 for 480 watts total or just buy 2 new 250 watt panels for 24 v hook them in series .
     A good lnverter will burn less power  and every thing will run better.
     The tracer charge controllers come in different sizes here 10 20 30 40 60 amp 
     I like the 40amp cc because it will charge 2 gc battery's at max power
      4 gc batteries use 2 40 amp controllers ect  
      It depends on size of your panels , batteries 
    I hope next year I will have a new system in the house .

    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
Sign In or Register to comment.