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Low-Power Solar?

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Posted by (PeteCresswell) on January 25, 2012, 10:50 pm
I'm looking in to putting an IP camera on an outbuilding and
maybe one of those flood lights that is triggered by motion.

Running AC to the building sounds more formidable the more I read
about it.

OTOH, a wireless version of the IP camera's 24-7 draw is less
than 10 watts and the flood light could be set up to only shine
for a minute or so after detecting motion.

Unencumbered by any knowledge or expertise, I am led to thinking
maybe this could be a solar application:  automobile battery and
el-cheapo inverter in the shed, collector panel on the roof, some
sort of box to facilitate panel's charging the battery...

Is the scale/complexity of something like this small enough so it
would compete favorably with a couple thou to hire an electrician
to run 120v to the outbuilding?
Pete Cresswell

Posted by news on January 26, 2012, 12:42 am

The camera is probably powered by a wall-wart supply that delivers low
voltage DC - often around 12 volts.  If so, then you don't need AC at
all - just 12 volts.

You can get LED flood/spot lights that run on 12 volts (check ebay or
the local auto parts store for LED fog/off-road lights).

At a minimum, you need a marine battery - even better is a deep cycle
battery.  And buy big - a battery that is never discharged more than
20% can last a LONG time.  The two specs you're interested in are the
20 hour amp/hour (AH) rating and the reserve minutes (how many minutes
the battery can provide 25 amps).  Bigger numbers are better.  The
cranking amps (CCA, MCA) are not important - you're interested in
long-term output, not the instantaneous power delivered to start an

Whatever the wattage required by the camera, you need 4 to 6 times
that much solar panel capacity (10 watt camera means minmum of 40 - 60
watt solar panel).  Remember that there will be cloudy/rainy (maybe
snowy) days when the solar panel will NOT provide power.  When the sun
does shine again, the solar panel must be able to run the camera and
charge the batteries - and winter days may only provide 4 - 5 hours of

Assume a 12 volt system with a "12 volt" solar panel.

The 10 watt camera requires just under 1 amp * 24 hours = just under
24AH daily and 80AH battery would need to be fully recharged each day
to stay at no more than 30% discharge.  A 40 watt solar panel would
provide 3.5amps * 5 hours or 17.5AH - not enough to keep the battery
charged.  A 60 watt panel would provide 5amps * 5 hours or 25AH -
almost enough to keep up with the load (charging isn't 100%
efficient). A 100 watt panel would be the minmum reliable source
because it would provide charge in fewer hours and thus provide a
cushion for days without sun.  Calculations assume the solar panel is
mounted at the optimum horizontal angle, faces south (northern
hemisphere) and has a clear path to the sun (no trees, etc,
interfering with the sunlight).

Note that 5 hours of sun may optimistic in winter, but the design
should account for the differences in sun time during the different
seasons and the fact that the battery loses some capacity when cold.

Quick estimate: minimum of $00 for low end solar panel, charge
controller, deep cycle battery - could easily reach $00+ with better
quality equipment.


Posted by Winston on January 26, 2012, 12:46 am
 (PeteCresswell) wrote:

Personally, I don't think so.
I would *seriously* consider doing some NEC homework
and trenching and installing conduit myself.
The high - priced talent can do great quality wiring
at both ends afterword.

Photovoltaics are still 10x too expensive for any
reasonable payback, for most on-grid applications.

Battery backup in the shed is still a good idea though.


Posted by (PeteCresswell) on January 26, 2012, 7:40 pm
 Per vaughn:

Are there "code" considerations with 12v wiring outside?

I'm hoping to hear "no", but in other threads I have seen
references to differing electrical potentials in the two
buildings creating some sort of un-named hazard.
Pete Cresswell

Posted by Anon on January 31, 2012, 8:55 pm

If you do go with this suggestion, be aware that many landscape lighting
transformers are 12V AC, so it would need rectification for DC requirements.

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