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post #1 of 6 Old 09-22-2006 Thread Starter
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solar panel choices - overwheming

I want to charge my 12v battery with a solar panel. How do I know I am selecting one with adequate wattage to keep it charged? I have a solar anchor light on the mast, but the rest of the lights-- cabin, running etc.and a couple radios along with gauges rely on the battery.
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post #2 of 6 Old 09-22-2006
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Pelicani...you need to know what your typical use of AMPs is in order to get the right size panel and the right size battery. Here are some typcal draws:


Typical Current Draw in AMPS


Device
Anchor light0.8 Anchor windlass150.0 Autopilot0.7 Bilge blower6.5 Cabin fan (efficient)0.2 Cabin fan (oscillating)1.2 Cabin light (8w fluorescent)0.7 Cabin light (25w incandescent)2.1 CD player0.2 Chart light (10w)0.8 Compass light0.1 Computer (laptop)2.5 Spreader/deck light (20 w)1.7 Depth sounder0.2 DVD player0.3 Gas detector0.3 Gas solenoid0.6 GPS0.5 Ham radio (receive)2.5 Ham radio (transmit)30.0 Inverter—standby0.2 Microwave (700 w)115.0 Pump—bilge15.0 Pump—freshwater3.0 Pump—shower sump2.0 Pump—washdown6.0 Radar4.0 Reading light (10w)0.8 Refrigerator5.5 Running lights2.5 Running lights—Tricolor0.8 Spotlight10.0 Television (13-inch)3.5 Toilet40.0 Speed log0.1 SSB (receive)2.5 SSB (transmit)30.0 Starter150.0 Strobe1.0 VCR2.0 VHF (receive)0.5VHF (transmit)5.0 Watermaker (6 gal)20.0 Weatherfax1.0 Wind indicator0.1

Once you know your draws...figure out how many hours (or parts of an hour)a day you'll use the item. Say your running lights draw 2.5 amps...and you use them on average 2 hours a day...you need 5 amps a day for them. Add up all your daily uses and see how many total amp hours you need. Then check your battery capacity...it should be at LEAST twice this number and preferably 4x this number since that will let you go 2 days without sun once fully charged.
Lets say that your needs are a modest 20 amps a day...that means your solar panels have to AVERAGE that per day. Since wattage equals AMPS times VOLTS...an 80 watt solar panel in a 12V system can put out 4 AMPS per hour in IDEAL CONDITIONS. Figuring 6 hours of direct sunlight in a day means that an 80 watt panel will put out 24 amps...enough to cover your 20 amp / day needs under best case scenario...and sunny days.
With me so far?
Since best case never happens...it is a good idea to exceed your needs by a bit which will also give you excess amps to store in your battery for that rainy day. We had 160watt panels on our last boat and averaged a 30-40 amp input in the sunny tropics.
Our daily needs were much greater so we supplemented with wind power and running the engine every couple of days for a bit. We found that wind power generated around 80-100 amps per day since it runs 24x7 but of course...there are windless days too. Worst of all were cloudy windless days!
This is probably more info than you were looking for since your needs appear minimal but hopefully it will help out others with similar concerns.

Last edited by camaraderie; 09-22-2006 at 10:20 AM.
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post #3 of 6 Old 09-22-2006
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Quote:
Originally Posted by camaraderie
This is probably more info than you were looking for since your needs appear minimal but hopefully it will help out others with similar concerns.
Hey camaraderie, I don't even have a REAL sailboat yet, but the answer you provided is very helpful for my future need. Thanks for taking time to write.

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post #4 of 6 Old 09-22-2006
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Something to consider with solar panels, is that the "ratings" are for an ideal sun-dweller, i.e. summer in Arizona. More typical users further from the equator may see half the ratings, and by the time you get to New England in the winter, you'll be lucky to see 1/8-1/4 of the "daily" output. You can figure eight hours of dim northern winter sunlight might only equal 1-2 hours of full output, so the rule of thumb is probably more like "How many panels can I fit on my boat? How many amps can I afford?" unless you're in a very sunny place.
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post #5 of 6 Old 09-22-2006
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cam, actually, the units you should be using are amp-hours, not amps. You've dropped a unit somewhere...

There are three types of solar panels out.

Monocrystalline and polycrystalline panels which are rigid, with a glass face and aluminum frame usually. These have the highest efficiencies, with monocrystalline panels being a bit better but more expensive than polycrystalline panels.

The third type are amorphous or thin-film silicon panels, which have about a third the output of the other two types, but are available in either flexible, or semi-rigid panels. They do not have a glass covering panel, and the semi-rigid panels are often designed so that you can actually walk on their surface without damaging the panels within reason...stiletto heels will damage them.

All of the solar panels are fairly hard to get right now, as there appears to be a greater demand than the current manufacturing capacities can handle.

Some things to consider if you're planning a solar panel charging system.

1) You should probably get a MPPT-type charge controller to maximize the current accepted by your batteries. A charge controller is recommended if your panels can supply more than 10% of the capacity of your battery bank IIRC.

2) You should either install blocking diodes, which will incur a voltage drop, or a switch, to prevent the solar panels from draining the battery bank at night...as the panels act as a electrical drain at night. If you have a charge controller, this isn't normally needed, as most solar-panel charge controllers have blocking diode functions in it.

3) You may want to mount the panels in such a way that you can remove them for bad weather, etc. I have mine setup to plug into a weather-proof electric trolling motor type connectors, and can put the panels below in the case of bad weather or heavy seas.

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post #6 of 6 Old 09-22-2006
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SD...You're right of course but I just use amps for short as most people get it OK. I actually made a simple math error above on the 80W panel...it will actualy output 6.5amps/hour in direct sun.
Ditto on your 3 points though we had a regulator that diverted excess voltage into the hot water heater. I like the charge controller idea bette as we rarely go hot water! We mounted ours off the stern pulpit bars in adjustable clamps, they swung down for storage and we could secure them in heavy weather (stood up to Isobel!) and up for anchorage. Just another option.
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