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For the math experts..

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Posted by merlin-7 on August 7, 2007, 11:28 pm
 What would be needed tp keep a single LED light on 24/7 ?

 Lets say it needs 10 days back up power (battery back up)

 solar panel gets 4 hours of sun per day on most days, but lets say for this
question that you get 2 days of sun (4 hours per day on the solar panel)
followed by 10 days with no sun and the pattern repeats itself.

 what would you need to do this as far as battery storage and solar output?

 Could you double whats required to run 2 led's ?

 mulitiply by 8  to run 8 led's? etc.

 It is a simple question.Lets count in the losses etc.


Posted by Ron Rosenfeld on August 8, 2007, 12:32 am

How much power does your LED require?  Given the proper circuitry, you
could be drawing as little as 3ma @ 9V from a single LED.

Are your LED's AC or DC?  With DC bulbs, you won't have to deal with
inefficiencies of AC-DC conversion.

Do you really have four hours of effective sunlight per day?  Is that
during your worst month for sunlight?  If your on the west coast of
California, that's probably an adequate estimate.

For the sake of argument, let's assume your LED draws 1 watt and you are
setting up a 12V system.

Here is the math:

DC load in watt-hours/day        24    Wh
Total load                24.00    Wh
System nominal DC voltage         12    volts
Total load in ampere-hours         2.00    Ah
System losses/safety factor multiplier    1.2    
Adjusted total daily load         2.40    Ah
Average insolation            0.67    ESH
Required array current (Line 8/Line 9)    3.60    amperes

Note that your Average insolation is 2/3 hr/day.  That is computed from
your specification of 8 hrs full sun over 12 days.

So to supply the required power, you need a nominal 12V PV panel capable of
putting out 3.6A at peak sun.

BP Solar has a 65W panel (BP365) which has a maximum current of 3.69A and
should be satisfactory.

For 10 days of reserve, the amount of battery capacity is dependent on the
allowable depth of discharge for the batteries you select.  Some can safely
be discharged 50%.  Others can go 80%.

If we assume 50%, then you need storage capacity of 48Ah @ 12V.  MK Battery
makes a 50Ah 12V Gel cell battery that should be adequate for the task.


Posted by Eeyore on August 8, 2007, 8:32 am

Ron Rosenfeld wrote:

You don't mean drawing do you ? It's not a power source.

Show me a LED with a Vf of 9V will you ?


Posted by Ron Rosenfeld on August 8, 2007, 11:00 am
 On Wed, 08 Aug 2007 09:32:58 +0100, Eeyore

Typo:  I should have written "for" instead of "from", but you should have
been able to deduce that.

Again, you should have been able to deduce that "from" should have been


Posted by Eeyore on August 8, 2007, 4:24 pm

Ron Rosenfeld wrote:

9V is still wrong.


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