Newby solar questions... Overwhelmed

ctindogctindog Registered Users Posts: 2
So I've been researching on how to make a small solar and possibly wind system. Off grid, remote, in Arizona with lots of sunshine. Initially for occasional 110v ac inverter, but mostly just for lighting and recharging USB devices and a laptop. My budget is low. Like under a grand. They have hundred watt panels for a hundred bucks on Amazon. Wally world has 105ah deep cycle batteries for $100 each. I was thinking 3 panels and 2 batteries with a 40 amp controller, would that be a good start? Would that be a big enough controller to add another panel in the future? Does anyone have experience with cheap controllers off Amazon they could recommend? Thanks.


  • Chris11Chris11 Registered Users Posts: 126 ✭✭
    edited August 2020 #2
    I'm not one of the smart guys on here but take what you want from what I suggest.

    First, the price of panels is always pre freight.  Guys on here used to suggest certain panels at a certain price but once you added the freight they got expensive..  I don't like to suggest somewhere else other than our host but..... Home Depot has them right now for $79 including freight to you house.  Type in Grape Solar on the HD website.

    Second .... That 105 amp hour bat sounds like it a marine battery.  Not as good as a real deep cycle.  I suggest you go to Costco ... their true deep cycle, 208 amp hours,  are Interstate Golf cart bats and are just under 100 smackers plus a $15 core charge if you don't have a couple of old dead bats to trade in.

    Third, our host has Morningstar controllers, the small ones like you want and I happen to like because of Morningstar's great customer service ( I had to phone them and got great results )  and have a few of them for the few of my mini systems.  40 amp would allow you to expand in the future since your 3 panels in series would put out well under 20 amps.

    I'm using cheap chinese inverters ... and have had no problems with the ones from a company on Ebay called Reliable.  I did try some other cheapy ones, NOT from Reliable, and they quit real soon after I started to use them.

    As I said, take what you want from what I wrote ... perhaps the smart guys have some other suggestions.

  • BB.BB. Super Moderators, Administrators Posts: 32,606 admin
    edited August 2020 #3
    Chris has given you a good start.

    I suggest looking at your load requirements / power exceptions. Even though a lot of sites like to call these systems "Solar Generators"--They are not what most people expect, power/energy wise, from a genset. One gallon of gasoline and a Honda eu2200i, and you will get close to 10+ hours of power running a refrigerator, LED lighting, cell/laptop chargers, etc... Or upwards of ~3,000 to 4,000 WH of energy (power is a rate, like 400 Watts, and energy is an amount, like 400 Watts * 10 hours of use = 4,000 WH).

    Since we don't have a "load" or expectations starting point, we can start with your hardware definition. I like to call the Battery Bank the "heart" of your system. As Chris suggestions, a pair of 6 volt golf cart batteries (6 volt @ ~210 AH in series for a 12 volt @ ~210 AH battery bank--We use the 20 hour discharge rate for battery AH capacity--Works for most of our designs) is a very good place to start.

    You have a few things you need to do... 1) check the electrolyte level once a month--If you over charge/let the plates get exposed, the Flooded Cell Lead AC Batteries are quickly ruined.

    And 2), make sure you do not run the battery bank "flat/dead". That also kills them quickly. If you run them between 50% and 100% state of charge, they will be very happy (and avoid going below 20% SoC ever--Again can kill battery cell(s)). (fixed--Thank you littleharbor2. -Bill)

    For a full time off grid system, suggest that you use the batteries for 2 days without sun and 50% maximum discharge--That means your "stored energy" supply would be:
    • 210 AH * 12 volts * 0.85 (AC inverter eff) * 1/2 days storage * 0.50 max discharge = 536 WH per night
    • Assuming 5 hours per night for 2 nights: 536 WH / 5 hours = 107 Watt average AC load (for 5 hours)
    Your loads are different that what I suggest above--But the math is easy for you to plug in your numbers. If most of your loads are DC, then you can avoid the 85% AC inverter Efficiency/losses.

    Next, two sets of solar panel calculations. The first is based on charging current for the battery. Nominally 5% rate of charge works OK for a weekend/sunny weather system. 10%-13%+ rate of charge is better for a full time off grid daily use system (battery bank is "happier", and you don't have to watch the system nearly as closely):
    • 210 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 198 Watt array minimum
    • 210 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 395 Watt array nominal
    • 210 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 514 Watt array "typical" cost effective maximum
    And then we can also size the array based on your daily loads and where you live... For example, say your daily loads are 536 WH per day (battery loading), and you are around Mesa AZ, fixed array facing south:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 57° angle from vertical:
    (For best year-round performance)


    Lots of sun... Worst case average is December at 4.78 hours per day (long term average):
    • 536 WH per day * 1/0.52 end to end AC off grid solar eff * 1/4.78 hours of sun per day = 216 Watt array "nominal" for December
    Now--Realistically, you should only use around 50% to 65% of daily forecasted harvest (i.e., running a refrigerator and LED lights every day--Put off water pumping, cooling fan, etc. washing machine use during cloudy weather).... 

    The math says that 198 to 514 Watt array would work. If you want to have a system that supports more energy (fan, working in the shed, etc. during non-Winter), a 10%+ system would be nice. A 395 Watt array with 5.55 hours of sun per day average (February though October) would produce:
    • 395 Watts * 0.52 AC system eff * 5.55 hours of sun per day = 1,140 WH per day harvest
    • 1,140 WH per day - 536 WH "night usage" = 604 WH for "day time usage" and 536 WH for night time.
    A 40 Amp solar charge controller--The maximum array for PWM would be:
    • 40 amps * 17.5 volts Vmp array * = 700 Watt array maximum for a "cheaper" PWM solar charge controller (or less-check manual)
    • 40 amps * 14.5 volts Vbatt charging * 1/0.77 panel+controller deratings = 753 WH typical cost effective maximum
    And there are always the "Tools of Trade"... For flooded cell lead acid batteries, a hydrometer is pretty much a requirement:

    And an AC+DC current clamp meter-DMM. Makes it very easy to understand and diagnose problems (both AC and DC side--There are AC only current clamp meters, work great, just not for DC): (cheap "good enough") (nice mid priced meter)

    For solar panels--140 Watt "12 volt" panels (Vmp~17.5 to 18.x volts) are cheaper/easier to ship and move around. Work well with PWM type charge controllers. But these panels cost more $$$/Watt.

    And there are >~200-350+ Watt panels that have to ship truck/pallet (shipping one or two panels can be very expensive). And their Vmp~30-38 volt or so) output needs a more expensive MPPT controller (take high voltage/low current from array and efficiently down converts to low voltage/high current for charging the battery bank). Ends up many times with "cheap" $/Watt panels and more expensive (and usually more capable) MPPT charge controllers.

    If you need a little bit of AC power... A 300 Watt or smaller PSW/TSW AC inverter is nice. They don't waste much power. And some these days come with remote on/off, and even search mode (inverter runs in low power standby until >~8 Watts of AC power is needed--Then turns on to run AC loads).

    MorningStar makes a nice/rugged 300 Watt PSW AC inverter with remote on/off and search mode... May be more than you want to spend, but at least you can see what other options are out there:

    Our Host Northern Arizona Wind & Sun does have a bunch of reference articles if you want to do some more reading:

    And one of our members has some nice real world photos and evolution of his DIY solar power systems:

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • littleharbor2littleharbor2 Solar Expert Posts: 1,692 ✭✭✭✭

    And 2), make sure you do not run the battery bank "flat/dead". That also kills them quickly. If you run them between 50% an 10% state of charge, they will be very happy (and avoid going below 20% SoC ever--Again can kill battery cell(s)).

    Not trying to be the spellcheck police here but I think Bill Meant to say 

     :If you run them between 50% and 100% SOC they will be very happy.

    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.

  • BB.BB. Super Moderators, Administrators Posts: 32,606 admin
    Thank you Littleharbor2. I have fixed.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 5,805 ✭✭✭✭✭
    With Amazon the shipping is included in the price, you might get a little better deal, but I would rather you do the Home Depot and  have a local place to deal with if you have nay issues, $79 is a pretty good price for 100 watt panels.

    I too would suggest looking at your loads and usage. If you are uncomfortable trying to figure out, you might post here... I want to run... A laptop for 4-5 hours, 2 - 9 watt LED lights on 120 volt AC using a 200 watt car inverter...

    I ran a similar system for several years with 200-300 watts of panels (array) and 2 golf cart batteries using an old PWM charge controller and a Modified sine wave inverter made for auto use but hard wired. I typically ran a laptop for 3 hours a night, a fan and a 11-13 watt Compact florescent. I even ran a Video tape player and tiny 5" TV a bit.  I think you should shoot for build around $6-700 with similar components today. I, personally, wouldn't bother with a combiner box for 3 - 12 volt nominal panels, even with a short they would be hard pressed to maintain an arc and start a fire (and not much to burn on the back of a panel). 
    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, 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.
  • mountainmanmountainman Registered Users Posts: 439 ✭✭
    edited August 2020 #7
    Since home depot and Amazon was mentioned. Amazon sells a 100 watt panel $78. 
    I'll agree with @Photowhit deal with 
    Home depot.
    They have a 200 watt kit with a 40 amp grape solar controller and connectors for $239. Free shipping 
    You can add up to 3 more panels
    1 at a time or as your budget allows.

    My first system. 4 100 watt panels
     and a 30 amp grape solar pwm contoller.  (3 years old and works) 
    came from ebay.
    Within 3 months a panel stopped working.
    Renogy said they had no record of my purchase.
    Then I had a 300 watt system.

    2kw array 6 345 q cells  make sky blue 60 cc
     6 230ah GC @36 volts 
    18 amp accusense charger. 3650 champion 
  • ctindogctindog Registered Users Posts: 2
    You are all so helpful I'm blown away. I found someone local that has 250w 39v solar panels from a previous on grid install for under a hundred bucks each. I'm thinking of picking 3 up (1 as a spare) and a 40a mppt they have for $135, and writing as well as 2-3 of the golf cart batteries several have mentioned are available at Costco. I've really got to think about the inverter because I have a 3200 has generator, and I don't want to blue $300 on an inverter that is over rated since I could just for up the generator if I need to run a big draw power tool. Is 8guage awg h heavy enough for like a 2to4 foot run to the batteries? Thanks for all your help!
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