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  • I need some help

    I need some help here. You guys could do this in your sleep so I figured I ask. I'm sick off the small solar charged garden lights, not bright enough, don't stay lit long enough. So here is what I propose....

    A string of 6 20 Watt spot lights that stay lit for 6 hours/night. So that's 720 Wh right? Which would be 60 Ah at 12 volts correct? I'd have to use a 12 V timer for this.

    So what type and size battery, charge controler and panel would best suit a system like this? I should say I'm considering packaging this and selling it localy here. There is a great market for this where I live.
    I really would like to hear your opinions on this project.

    Thanks,
    Green
    Last edited by green; 08-20-2012, 08:14 PM. Reason: Changed wording

  • #2
    Originally posted by green View Post
    I need some help here. You guys could do this in your sleep so I figured I ask. I'm sick off the small solar charged garden lights, not bright enough, don't stay lit long enough. So here is what I propose....

    A string of 6 20 Watt spot lights that stay lit for 6 hours/night. So that's 720 Wh right? Which would be 60 Ah at 12 volts correct? I'd have to use a 12 V timer for this.

    So what type and size battery, charge controler and panel would best suit a system like this? I should say I'm considering packaging this and selling it localy here. There is a great market for this where I live.
    I really would like to hear your opinions on this project.

    Thanks,
    Green
    1. Don't even consider trying this with other than LED spot/flood lights. The inefficiency of incandescent will kill you and CFLs will not work reliably under outside temperature conditions.

    2. So with LEDs, you will be using a lot less than 20 watts per light (I hope!) Calculate accordingly. Below, I will assume that you really do need 720.

    If you really need 720 WH, that would take a battery of 300 AH at 12 volts because you do not want to repeatedly discharge the battery by more than 20% of its capacity. Unless you use LiFeP04 or other exotic battery.
    That would take about 400 watts of solar panel to charge, which would not be easy to find an unshaded mounting point for in a typical garden setting.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      Ok I found an LED that is equivelent to a 20W incandesant that uses 6W. So that would make it 216 WH or 18 AH. Right?
      So about 94AH worth of battery?
      How much panel? About 120 Watts. That's more like it.

      Any advice is greatly apprecaited here. My goal here is to make landscape lights that are bright like they are plugged into AC, but in reality the run on solar. And before someone comes on here and says I'll be paying ten to twenty times more for the electricty, that's not the point here. I know that the people I would be selling these to wouldn't care and probably wouldn't understand it if I explained it anyways.

      Green
      Last edited by green; 08-20-2012, 09:28 PM. Reason: can't spell

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      • #4
        Originally posted by green View Post
        Ok I found an LED that is equivelent to a 20W incandesant that uses 6W. So that would make it 216 WH or 18 AH. Right?
        So about 94AH worth of battery?
        How much panel? About 120 Watts. That's more like it.

        Any advice is greatly apprecaited here. My goal here is to make landscape lights that are bright like they are plugged into AC, but in reality the run on solar. And before someone comes on here and says I'll be paying ten to twenty times more for the electricty, that's not the point here. [B]I know that the people I would be selling these to wouldn't care and probably wouldn't understand it if I explained it anyways.[/B]

        Green
        You are getting close, and doing the math correctly. The biggest missing thing is that between the panels, the charge controller, the charging process itself and the discharge, you should allow for a factor of two in loss. So 532 watts needed to recharge the 216 watts used. Between that and the need to fully recharge the battery with only 4 or 5 sun hours in summer, the 120 watt panel is about right for summer, but it is too small to handle the same job in the winter. (You get less sun for recharging and yet maybe want the lights to run longer than 6 hours staring at an earlier time of sunset.) Where are you located, to run PVWatts for your location?

        Are you willing to design the system so that if there is a rainy or cloudy day, the lights just will not turn on that night or will shut off sooner? Or would you like to be able to run the battery down farther to cover the occasional cloudy day? Going down as far as 50% will not be too much for occasional use. If you want to run through cloudy days, you will need about twice as much panel to be able to catch up again when the sun shines.
        SunnyBoy 3000 US, 18 BP Solar 175B panels.

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        • #5
          Originally posted by green View Post

          A string of 6 20 Watt spot lights that stay lit for 6 hours/night. So that's 720 Wh right? Which would be 60 Ah at 12 volts correct? I'd have to use a 12 V timer for this.

          So what type and size battery, charge controler and panel would best suit a system like this? I should say I'm considering packaging this and selling it locally here. There is a great market for this where I live.
          I really would like to hear your opinions on this project.
          Steve first thing is forget Amp Hour and use Watt Hours as it simplifies thing quite a bit

          You are correct 6 lights x 20 watts x 6 hours = 720 Watt Hours which is a huge number, as your panels would need to generate roughly 1400 Watt Hours to replace the 720 Watt Hours so if your winter insolation is say 2 hours would require a 1400 wh \ 2 hours = 700 watts.

          Back to the battery. Regardless if you go with Incandescent, Fluorescent, or LED to find the right battery size is [Daily Watt Hours x 5] / Battery Voltage = Amp Hours. So using your 720 WH number with a 12 volt battery is [720 wh x 5] / 12 volts = 300 Amp Hours. That is a pretty big and expensive battery weighing in at some 200 pounds.
          MSEE, PE

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          • #6
            How about 90AH with the 261 WH/day. How much panel, what kind of controler for this type of application. What type of battery would you guys recomend for strictly lights on a 12 volt system. Oh I didn't add my location, right on the border of Illinois and Wisconsin. I'm sorry I did search and read here at the forum but I didn't find exactly what I was looking for.

            Oh I noticed you Bolded a quote of mine and I should explain when I said...
            " I know that the people I would be selling these to wouldn't care and probably wouldn't understand it if I explained it anyways."
            There is alot of people with a lot of money where I live (Not me of course). Certainly they would understand the cost aspect, I just see an explosion of green efforts and investments in properties(remodeling instead of selling).
            I think if I could package something like this it would fit this area nicely and sell like crazy.

            Green

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