Forum Discussion

D_E_Bishop's avatar
D_E_Bishop
Explorer
Mar 08, 2017

Solar Questions

I am trying to figure out what I need to recharge a battery in an average 8 hours of sunlight per day with a maximum load of 2.52 amps.

This is an exercise in building a system which hopefully will lead to building a system for my rig. I am building a 12 volt system for my S&B entry and landscaping in order to gain experience and to dump the big transformer I am currently using.

I currently have five fixtures using 90 lumen #921 LED bulbs, one fixture using one 240 lumen #921 LED bulb. They are all self contained and placement is very limited due to aiming of the panels.

I would like to be able to expand the usage by 100 percent and have one single battery and one 12vdc PE switch.

So with the max load at 3.78 amps (30.24 watts), and with the controller and PE both using some power, how do I pick a controller, solar panel and battery?

I can or at least I could when I retired over twenty years ago, repair a lot of microprocessor based equipment and I still can wire a house and all the day to day stuff. What I have trouble doing is figuring out the formulas from the stuff I've found on line.

I am what is considered moderately to severely dyslexic and really need line to line drawings and written out formulae.

I know about making searches here and all that but sometimes that is far more involved and hard for me to understand that asking for help, so please keep the level of "figure it out for yourself, it's already been asked and answered" to a dull roar.

7 Replies

  • Almot's avatar
    Almot
    Explorer III
    Mar 09, 2017
    BFL13 wrote:
    However, the OP chickened out before we could sell him on a grid-tie 500w solar, MPPT, built-in 48v inverter, and a Trimetric monitor to keep track of it all, for his garden lights. Drat! :)

    Note that the thread was about solar for house entry and landscaping. The need arose because they wanted to stop using a big transformer - not because there was no grid power available. With the goal to learn eventually (on this garden lights system) how to built something for their rig, yet without having to learn anything. Well, it was fun while it lasted :)
  • BFL13's avatar
    BFL13
    Explorer II
    Mar 09, 2017
    Thanks for the confirmation of the ballpark I was thinking of. However, the OP chickened out before we could sell him on a grid-tie 500w solar, MPPT, built-in 48v inverter, and a Trimetric monitor to keep track of it all, for his garden lights. Drat! :)
  • KD4UPL's avatar
    KD4UPL
    Explorer II
    Mar 08, 2017
    If you're using 30 AH each day your battery needs to be a minimum of 60AH. However, you have to account for cloudy days. A good rule of thumb is 3 cloudy days in a row so now you need 180 AH.
    Low solar insolation in your area is around 5 hours per day. This likely occurs in December. So if you need to make 30 Ah x 12v per day that's 360 watt/hours / 5 hours a day = about a 75 watt panel absolute minimum. In reality, due to losses and inefficiency this really wouldn't be big enough to cover one day. I'd go 100 watt minimum. However, if after a cloudy stretch you really need to fully recharge the batteries after being drained for 3 days of use you will need more like 300 watts.
  • D_E_Bishop's avatar
    D_E_Bishop
    Explorer
    Mar 08, 2017
    Thanks BLF13, it was just an idea and a fun way of learning a lot. No messing around with ideas and no learning about the unknown.
  • BFL13's avatar
    BFL13
    Explorer II
    Mar 08, 2017
    Ok 30AH a day so how many days do you want to go keeping the lights on if the sun doesn't come out for a few days? That is your battery bank reserve needed. So you really want more battery than you are thinking for replacing the daily AH every day.

    If you are home I suppose you could always throw a simple battery charger on it from household 120 power if there is not enough solar. so you want to cover for when you are not there.

    I think at least one 100AH AGM might be a good solution. You can go below 50% with those which gives you more margin. Say that gives you two days without any sun and you are at 40%

    A single 100w panel gives you 6.2a mid day. You aren't going to get 5 hours a day at 6.2 amps though to get your 30AH. Most of the day it is under 4 amps. You might be looking at 200w worth of panels, I haven't done the math --don't know the annual insolation for there.

    This is getting out of hand for a few garden lights? :) Maybe forget the solar and just put one battery on the job with a trickle charger on it from the house. But that's no fun.
  • D_E_Bishop's avatar
    D_E_Bishop
    Explorer
    Mar 08, 2017
    Considering the short winter sunlight hours, I would guess lights will be on 12 hours a day, 2.52 amps X 12 hours = 30.24 AHs. Yes it will be used 365 days a year. We are on top of a hill and no trees to block the sun. I haven't decided on tilt but most likely the 3-1 pitch of the roof. We are about 25 miles in a straight line across the Los Angeles basin from the beach, with the CL of the house running East West and a large roof that is South facing, so we have good exposure.

    My figures are my current usage plus additional 100 percent. I currently have 5, 921 LED at 90 lumen or 0.84 amps and one 921 LED at 240 lumens or 1.68 amps. I am designing with that 100 percent fudge factor in case the DW wants more lighting.

    Am I looking at a full size 12 volt WLA battery or a motorcycle battery? If I remember the two GC2's in the MoHo are 210 AH. Do I just look up for example Interstate and find a 70 or 75 AH battery or is a 35 AH battery in the ball park?
  • BFL13's avatar
    BFL13
    Explorer II
    Mar 08, 2017
    Not enough info. How many AHs do you need to restore each day? Is this all year round? Panels tilted? You can use Macslab for your latitude for optimum tilt all year and other sites for how much sun you get over the 365 days where you are.

    It is is all averages anyway, so you will overshoot to be sure of having enough. You should start with how big a battery and what kind that is going to be run down no more than 50% where that AH per day asked about above is 50%. The panel size and controller needed then follows from that.

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