Dipping my toe into solar energy.

KenthKenth Registered Users Posts: 4

Hi all.Been lurking for awhile.This site is a wealth of info.I have a small system that's been running about a year and a half.So far so good.I have 4 100watt panels wired in parallel feeding Tristar 45 charging 2 12 volt 50 AH agm batteries wired in parallel. A suresine 300 watt inverter gets some use.I now know these batteries should be 6 volt wired in series.I use max 500WH in the summer and almost nothing in winter--3 led lights and occasionally a trickle charger for lawn mower battery.Looks like I have enough charging capacity to upgrade to 6 volt 225 AH batteries.A couple days last summer my batteries got down to 12.2 volts--the reason for battery upgrade.Will my system work year round?.Do I have to exercise the batteries during winter or will they be happy just sitting with almost no DOD other than self discharge? I'm at 42 degrees latitude and the panels are angled at 22 degrees--roof pitch.I know the angle isn't good for winter but it is ok for summer when most of the energy will be used.I'm running a 12 volt cooler that uses 464 WH per day--manufacturer's number, not measured and 3 10 watt led lights,sometimes a small radio and a phone charger.If all goes well we would like to build a larger system.Thanks in advance for all opinions.

cheers kent


  • BB.BB. Super Moderators, Administrators Posts: 28,768 admin

    Welcome to the forum Kent,

    Down to the math. It does a good job of penciling in a system design, and you can see if it meets your needs (energy and costs). A 500 Watt*Hour system is relatively small and easy to maintain. Doing a rule of thumb design for off grid "cabin" use:

    First, size the battery bank. You need to decide if you want 500 WH every day, or on cloudy days, you can use less (or fire up a genset if needed). A 500 Watt sunny weather system may look like:

    • 500 WH * 1/0.85 AC inverter eff * 1/12 volts * 2 days storage * 1/0.50 max discharge = 196 AH @ 12 volt battery bank

    2x 6 volt @ 200 AH golf cart batteries in series would be a very nice fit. Flooded Cell Deep Cycle Lead Acid batteries are pretty rugged and easy to check with a hydrometer and 6 volts in series--A quick check with a DVM (digital volt meter) confirms if all is OK or not (battery voltages not the same, may need equalization).

    AGM batteries are very nice (more efficient, more freeze tolerant, store longer without charging, higher surge current, less electrolyte fumes/mess on the wiring). Downside is 2x cost of FLA batteries, and similar quality AGM batteries tend to have a 1-2 year shorter life. You cannot check specific gravity, so a bit more difficult to confirm state of charge.

    Now charging with solar. 2x calculations. The first based on the AH @ volts capacity of the battery bank. The second based on hours of sun for your location and daily loads. 5% to 13% rate of charge (based on battery 20 Hour discharge rate capacity) is typical. 10%+ is recommended for full time off grid (vs summer and weekend use).

    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 188 Watt array minimum
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 377 Watt array nominal
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 490 Watt array "cost effective" maximum

    And there is sizing the array based on your daily loads and hours of sun per day. Using a solar calculator like this can give you a good estimate of the long term solar harvest (based on your location and typical weather patterns).


    Say Albany NY, 62 degrees from vertical (28 degrees from horizontal) for summer harvest (optimal year round is closer to 47 degrees from vertical). Toss the bottom 3 winter months (less than 3 hours per day of sun), gives us February at 3.60 hours of sun per day:

    • 500 WH per day * 1/0.52 end to end off grid system eff * 1/3.60 hours of sun = 267 Watt array "break even" February

    Your 400 Watt array, is very good for the 2x golf cart batteries and will produce (February) an average of:

    • 400 watt array * 0.52 system eff * 3.60 hours of sun (Feb) = 749 WH per day (Feb) long term average

    The 200 AH @ 12 volt battery bank will support a ~500 Watt AC inverter maximum suggested--Your 300 Watt inverter is a very nice choice vs battery bank capacity.

    Your winter choices... AGM and simply charge to 100% State of Charge at end of summer/fall season and let the batteries sit until spring (no charging--up to 6 months at 75F. At lower temperatures, even longer between charging).

    With FLA batteries, you really need to charge them at least 1 day per month, or leave connected to your array (all other loads off/disconnected) and have the charge controller set to float the battery bank (something around 13.6 volts). And if the array gets covered with snow for a couple months, not a big deal. With batteries, for every 10C/18F below 25C/75F, they self discharge about 1/2 as much--So if your battery bank is below freezing--And near fully charged even FLA batteries will not freeze), your FLA batteries will be very happy if system is snowed in.

    A few folks have bolted a solar panel or two to a south facing wall (or tilt array to near vertical) to keep panels clear of snow and batteries charged.

    You do not want to "charge" the battery bank as FLA batteries will gas and "use water" -- You do not want to boil the batteries dry in winter (i.e., don't charge >14.x volts during periods of non-use and when you cannot check water levels).

    The last "issue" is the cooler. As long as it keeps your food cool and does not use more than 500 WH per day--Everything is probably fine.

    If, however, it does use more, then a propane refer or efficient AC or DC compressor based fridge is probably more efficient (AC chest freezer can use 250 WH per day when using a refrigerator temperature controller).

    The one big issue with typical North American refrigerators is that they pull 600-1,000 Watts everytime they start (induction motor compressor)--They are cheap and efficient, but need a larger power system (1,200-1,500 Watt AC inverter minimum and larger battery bank) to manage the starting surge.

    There are DC refrigerators that run from 12-24 volts and do not have high starting surge (typically manual defrost). And there are a few new inverter/linear compressor refrigerators that will run from a 600 Watt AC inverter or so--But these smaller inverter/linear compressors refrigerators tend to be very rare in the USA at this time.

    Hmm, looks like Home Depot is starting to list a bunch more DC refrigerator/freezers... They tend to be much more expensive than AC versions (hopefully, that will change over time):


    Your thoughts?


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • KenthKenth Registered Users Posts: 4

    Thanks very much for the reply Bill.I'm going to pull the trigger on the golf cart batteries.I really want to get a handle on maintaining FLA batteries before attempting a larger system.No matter have much I read about it,hands on always works better for me.As far as my little cooler goes,It's a Unique Off Grid 80L and really suits our needs well.I don't know how to measure accurately it's power usage.It's 12v DC and wired directly to the battery---can't use a Kill-A-Watt type meter.As you can see I'm not an electrician.I disconnect it in the winter so no draw from it at all so the only draw would be from led lights used occasionally and maybe a trickle charger for batteries.My controller is provides 13.4 volt float.

    It looks like panels should be adjustable for angle--something in incorporate into a larger system.

    I still have a cheap Chinese controller that I started with--worked fine for a year before I upgraded to the Tristar.It's rated at 12/24v 20A. I have found a source for Unicor 24 volt 195w 7.21A imp panels.Is this something I could connect a single panel to my 12 volt batteries?I hate to get rid of the batteries if they are still good--going to do a load test on them before I do anything.Thanks again for your help.

    cheers, kent

  • BB.BB. Super Moderators, Administrators Posts: 28,768 admin


    Yep, that is a nice compressor based cooler and is a good choice for your off grid system.

    To measure DC current, a DC Current Clamp DMM (digital mutli-meter) is a great diagnostic tool, and can help estimate energy usage:

    https://www.amazon.com/gp/product/B07546L9RT (low cost meter)

    https://www.amazon.com/gp/product/B019CY4FB4 (mid-cost meter)

    Note, there are a lot of AC current clamp meters that also measure DC Voltage, but not DC current--You have to make sure the current clamp meter you get does support DC Current measurements (not just AC/DC voltages). Also, DC Current Clamp meters need "zeroing" when used, and possibly every few minutes or so.

    And, there are now lots of pretty cheap DC Amp*Hour/Watt*Hour meters out there these days--You can wire them up to your load output and monitor energy usage (Watts is a rate, like miles-per-hour; Watt*Hours is an amount like total miles driven).


    For measuring specific gravity, a hydrometer (always rinse with distilled water before putting away--otherwise floats will tend to stick over time):


    Regarding connecting your "24 volt panel" to your batteries... In solar power, the details really do matter. Need to know the Vmp&Imp ratings of the panels, and the specifications for your charge controller. The "24 volt" panels can have Vmp range from 24 to 36 volts...

    For a 12 volt battery and PWM charge controller you want Vmp-array ~ 17.5-18 volts, and for a 24 volt battery, Vmp-array ~ 35-36 volts

    To float your AGM batteries, you want around 1-2% of their AH rating (100 AH ~ 1-2 amps of float current) with a charge controller (AGM batteries do not like being overcharged at all).


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DaangeroussDanDaangeroussDan Registered Users Posts: 23 ✭✭
    edited April 5 #5

    I recommend AGM's https://www.youtube.com/watch?v=ycjrC-3A79E

    Using solar trackers will greatly improve your input as wells https://www.youtube.com/watch?v=-6vCpbBpuVY

    As for solar charge controller, there is no reason to us pwm any longer, MPPT is the only way to fly and are very inexpensive now days https://www.youtube.com/watch?v=1oAi44IAQwQ

    As for your batteries, purchase a small "battery desulfator" off ebay, and hook it up to your battery bank during low input months (winter), here is a link to the type I use from Ebay; https://www.ebay.com/b/Battery-Desulfator/48618/bn_7023229918

    Visit my youtube page for over a dozen tutorial, click on "uploads" or "videos" button to see my list of topics and vids.https://www.youtube.com/channel/UCl-G3EJMjjdCKJkJdPTvkgg/videos?view_as=subscriber

  • KenthKenth Registered Users Posts: 4

    Bill,Here's the info on the panels I found.

    Vmp 27.4

    Imp 7.21

    Open circuit voltage 35.9

    Isc 7.75

    Pmax 195w

    Are these an oddball? They don't seem to match up with 12 or 24volt.

    DD,thanks for the input. I'll check out your links. I need all the help I can get.


  • littleharbor2littleharbor2 Solar Expert Posts: 1,231 ✭✭✭✭
    edited April 5 #7

    Sounds like 52 cell panels. Definitely NOT 24 volt panels. These would likely be considered 18 volt nominal. Definitely going to need an MPPT controller and a series string of panels bringing the voltage up if you are going with anything over a 12 volt system. They would have high enough voltage for a 12 volt bank but still with an MPPT controller. Be sure you get a real MPPT. If in doubt run the model by us here before you buy for assurance it isn't fake.

    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, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • EstragonEstragon Registered Users Posts: 3,408 ✭✭✭✭✭

    This one panel would be to float a pair of 6v ~220ah flooded golf cart batteries in winter, right?

    The panel is really designed to be used in larger arrays with higher voltage strings feeding an mppt controller. Ordinarily, a single panel could be a problem with an mppt controller (not enough voltage "headroom"), BUT...

    You only really need float voltage, and maybe an occasional lowish voltage absorb. In winter, panel voltage will be a bit higher in the cold as well. An mppt controller might work ok in this application. That said...

    You don't need much current to float, Imp at ~ 7a would be good on a 220ah bank. An mppt controller would give you more amps (roughly 195w x .75 ÷13.5 = ~10.5a), assuming the actual voltage to the controller is high enough after line losses.

    As Bill said though, you only need ~3-4a to float. A cheaper pwm controller (provided it can handle ~40+v) may work fine in this application. Besides being cheaper, they also tend to have lower self-consumption (which can be a problem if winters tend to be gloomy).

    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
  • DaangeroussDanDaangeroussDan Registered Users Posts: 23 ✭✭
    edited April 5 #9

    I swear by these things https://www.ebay.com/str/MakeSkyBlue

    I been using the fake v114(got ripped by an American dealer, I ordered the make sky blue) for over a year now at way past its max recommended for a 12 v battery bank, and it floats my stuff as high as 15.1 volts, which makes my inverter beep ;). I still don't get why anyone would recommend a pwm, its antiquated technowlogy, and for 99 bucks, this Make Sky blue can run at 12-48 volts into a 12 v battery bank, and can handle 2,800 watts of panels at 48 volts battery bank. You can buy 4 or 5 for the price of a comparable "good" controller, and have 3 or 4 backups to experiment with. I swear, China is kicking every-ones butts in the tech these days,, and its so affordable you can buy backups and still save money. But nothing I bought from them has died yet.

  • KenthKenth Registered Users Posts: 4

    morning guys,sounds like these panels aren't suitable for me.It sounded too good to be true.If anyone is interested they are advertised on craigslist Williamsport,PA for $100 a piece and they have a lot of them--about 275.Sounded cheap to me.

  • littleharbor2littleharbor2 Solar Expert Posts: 1,231 ✭✭✭✭

    Keep shopping. That price actually isn't cheap for, (I presume) older, odd cell count, panels. CL in So Cal I see has surplus unused commercial 310 watt panels for $100.00, for example.

    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, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • PhotowhitPhotowhit Solar Expert Posts: 4,833 ✭✭✭✭


    Your exuberance is interesting... "...can handle 2,800 watts of panels at 48 volts battery bank. You can buy 4 or 5 for the price of a comparable "good" controller,..."

    Did you notice it can't handle being 'over paneled'? Might also note it hasn't been through testing...

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • TecnodaveTecnodave Registered Users Posts: 205 ✭✭✭

    I personally have thrown out Chinese controllers that failed in my system , sold them for cheep, they came back and bit me in the butt, had to refund money to avoid an unjustified lawsuit. I took the sledge hammer to them......

    EP Solar Tracer......one of China's biggest manufacturers of controllers...stone copies of Mstar products....pure junk

  • TecnodaveTecnodave Registered Users Posts: 205 ✭✭✭


    I think I've seen those......Oddball 48 cell panels....I passed......hard to mix those with anything else. I would stick with standard "12 volt 36 cell panels" about $100 an Amazon for Grape Solar or Renology 100 watt and MidNite Brat controller. The Brat is PWM type and 30 amps 12 or 24 volt Brats are about $90 at wind-sun.

    I have 1 Brat charging 2 105 a.h. AGM's (24 volt system) with two Sharp NE-170 72 cell "24 volt" panels in parallel running a couple of 24 volt LED flood lites way up a light pole......the whole thing...for security lighting. Thieves can't get up there....Brat hasent been touched in a year Very good hi efficiency PWM controller outdoor..waterproof...doesn't need enclosure........but must be closely matched to panel........I.e. 12 volt battery.. 36 cell panel.......24 volt battery 72 cell panel better performance than a Chinese MPPT controller

  • DaangeroussDanDaangeroussDan Registered Users Posts: 23 ✭✭

    Over paneled? Why would you do that? Besides mine is "max paneled" and has been cranking max panel output for a year now. I'm interested in helping people learn how easy and affordable it is to begin creating their own clean energy, for the planet and future generations, not personal profit. Is this an online store forums and I'm stepping all over your business toes? If so, sorry I didn't realize, I'm only concerned about CO2 emissions, not hurting anyone's business.

    But for the record, I now have 3 Make Sky Blue 60 amp because the original system I purchases already nearly paid for itself, (in 1.5 years) so its time to expand. I'm just trying to save people money and get their feet wet doing it themselves. Perhaps I'm in the wrong place doing it. But I'll try to remember to come let you know when or if one of my chargers or inverters goes out, none has yet, so I'm a believer, and good enough at math to realize my savings, which is about 50% total cost compared to anywhere I have found. But then I have worked with electricity for 20 years, it comes easy to me.

    So is this forum a store support page I should avoid? Don't want to upset anyone.

  • BB.BB. Super Moderators, Administrators Posts: 28,768 admin

    No, this is not a NAWS support forum. It was created and hosted by them, and they pay all of the bills for us here.

    Pretty much everyone here is volunteering their time (as is me... I do not work for NAWS, am not paid by NAWS).

    If you like Make Sky Blue controllers, just link to their website or the product you are talking about:

    And if people have questions about their controllers--Feel free to answer them to the best of your ability (I am guessing you don't work for them).

    And avoid posting EBay links to the same two or three items 15x per day as the answer for every thread/set of questions on the forum.

    I get it, You found a Mfg you like. Create a new discussion (thread) and tell us about your installation and how it works for you (pictures, text, video link to youtube, etc.). And folks can ask you questions about it.

    No problem.

    -Bill "moderator" B.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 4,833 ✭✭✭✭

    @DaangeroussDan If you are really interested in an answer to "Over paneled? Why would you do that?"

    It's because solar panels are rated at Standard test Conditions (STC) but they actually produce closer to Normal Operating Cell temperature values.(NOCT) Which ae about 75% of STC.

    So a 2800 watt array charging a 48 volt battery bank that is at 20-30% SOC, might be putting in 58.3 amps at 48 volts at (2800/48=58.3 amps) But since the charge controller has limited you to 2800 watts. You are more likely to be putting in 75-80% of that. Where quality charge controller is normally spec'd 60 amps charge controller to use as much as 25% more panels so it can produce 60 amps it's rated at... Most from Midnite, Schneider, Outback, Morningstar, can happily handle quite a bit more. This make the best most cost effective use of the charge controller. I'd argue a 60 amp that limits the array size is more comparable to a Morningstar 45 amp controller.

    A year isn't really a very long test period. My first large system is happily chugging along on a Pulse Energy 60 amp charge controller built about 1999, Only failure was when a Japanese beetle shorted out a FET after a 4-5 years. I found a replacement board here (or perhaps another forum) and it's been fine. I sold it to some friends so I still hear about how it's doing. I did take some actions to prevent bugs from entering the power center, spray foam and steel wool! I retired the Specialty Concepts 30 amp charge controller (MKIV) built around 1982 when I installed that system.

    If you would relax a bit you could learn from a lot of us "old farts".

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
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