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Solar Powered Light for Shop Display

  1. Aug 6, 2014 #1
    Hi All,

    Please forgive me as its a few years since I did my A-Level Physics. However I am now a retail designer and just trying to figuring out how to light a shop window using Solar Power.

    I'm pretty sure its not possible but I am determined to do the maths to prove that its not. I need to light a shop window, however it is in a mall. The window will be lit from large lights above, however I was wondering whether these large lights would be able to power smaller spots using solar panels. The idea being that if we used solar panels we could put certain spotlights in locations that are unreachable by wires. I understand that this is highly inefficient, but as the light is shining from above already the question was posed in the office as to whether it could be done?

    So I have the spot lights I would like to use and an example of a solar panel. I can't find an equation to calculate the power of light that would be needed above to power the smaller spotlights. Some of the initial calcs (Rusty physics knowledge from my school days) are laid out below.

    LED Spot light
    4w @12vdc

    4/12= 0.34amps
    0.34amps x 8hours = 2.67amp-hours

    Solar Panel

    Output voltage: 18V
    Output current: 277mA
    Output power: 5W

    5Wx8hrs = 40Wh s
    40whs/12 = 3.3amp-hours

    Thank you in advance for your help.

    Kind Regards,

  2. jcsd
  3. Aug 6, 2014 #2


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    hi there

    I think you will find that your main problem will be that the stated output of your panel is what it will achieve
    (at maximum) under full sunlight.
    Its going to be significantly less than that when illuminated by a small spotlight

    It would be worth experimenting with a panel just to see what you may get.

    another thing to keep in mind is that the ratings that are usually given for a panel
    are its open circuit voltage. It will drop somewhat under load and the amount of voltage drop will vary
    depending on how "heavy" the load is

  4. Aug 6, 2014 #3
    Hi Dave,

    Thanks for the reply. I think you are right, I'll just have to do a bit of trial and error and see what happens.

    These are what I was thinking of using:

    LED Light: http://www.beamled.com/4w-biard-led...69WviMRS5D997lqUJD8MGw1UzE56uZPzo_BoC6V_w_wcB

    Solar Panel: http://www.ebay.co.uk/itm/18V-5W-27...01239984745?pt=UK_Gadgets&hash=item46234d6269

    Would you recommend a Solar Panel that runs at a higher voltage to account for the drop?

    I'm thinking it might be better to use mirrors and focus light from spots above.


  5. Aug 6, 2014 #4


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    I am pretty sure that the PV solution is a non starter. PV performance is based on 1kW/sq metre. In a mall,the illumination will be much less than that. Outside is always significantly brighter and internal lighting comes from just one direction to avoid shady spots in the shopping areas. If you want a 'source' of light to appear in the shop window, you would probably be better to do it optically - using mirrors to reflect and focus light from the external lamps onto parts of the window display. For a given area of light receptor (PV or mirror) you will get the same amount of incident light flux and the efficiency of a parabolic mirror (or large Fresnel Lens) plus sub reflectors could well be a lot higher than a PV system. Using a large mirror for the floor could be good value (but the colour would be a bit boring).

    However, I don't think any system could ever be 'startlingly impressive'. Using parabolic mirrors would actually be very much like a Camera Obscura and they are usually viewed in a darkened room. The brightness of any images projected on the display would be a lot less than that of the outside lights. The reflectors / lenses would need to be pretty huge and obscure the display, reducing the ambient light. I guess mirrors could be positioned on the upper wall of a building opposite.

    If you want inaccessible (by wire) places to be lit in the display, why not use mains powered projector lights and have mirrors near where you want the illumination to reach? Did you consider optical fibres?
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