Solar panels

[hoddinr]

I've seen Jack Dirkes' Solar Panels mounted behind his boom gallows.

Where else have you mounted your solar panel(s), and what kind did you buy. Defender has a 100 watt system flexible system on sale right now with controller etc. for about $500.

Wouldn't a 20 watt system be plenty for charging a battery? 

Thanks!

Ron

[marc]

I have my boat on a mooring and this is what works for me. I use electricity to power my gps and occassionally, nav lights. I use a 10 watt panel. I Keep it in a cockpit locker, stored on edge in a protected spot and pull it out when my battery is about 12.4 volts. The panel is hardwired to my battery. As I'm leaving, I just place the panel on the cockpit floor. The controller prevents it from overcharging. When I come back (usually a day to two) the battery is all charged up.
Marc

[FireDrill]

According to Don Casey's book "Complete Illustrated Sailboat Maintenance Manual"(McGraw Hill), the output of a maintenance panel should be .3% of the total battery capacity. (maintainence means fixing capacity that deteriorates w time)   His example is a 220 amp hour battery bank you need 0.66 amps, about what a 10 watt panel would produce.  On my Suncat I have a group-24 battery with 65 Amp Hour capacity.  That means my solar panel should be .19 amps for maintenance purposes only.  At 100 deg F a 30 cell solar panel should be about 14.5 Volts (per Casey).   Volts x amps = watts.  Therefore I only need about 2.8 watts (.19x14.5)  for maintenance.   I  use a 5 watt solar panel.   

Now if I want to replace lost charge from using current on an overnight cruise,  lets say (using Casey's numbers p 442 for device amp hrs - - anchor light 9.6 -12 hrs-, cabin lite 1.8. 8 hrs of depth meter for 1.6 and GPS for 4.0,) a total of 17 amp hours, then my solar panel at 5 watts (5/14 Watts/volts= amps) = .36 amps/hour = 47 hours at full output to replace the used charge. Because of sun angle inefficiencies, a fully sunny 12 hour day typically produces only 4 hrs of rated output (Casey) so it would take me approx. 12 fully sunny  days (47/4) to replace that charge. (around central NY probably 3 weeks) For day sailing on the other hand, (my usual experience)  I'd only use 5.6 amp hours or less (depth & GPS) and 15.5 (5.6/.36) hours of solar charging would work nicely, i.e. 4 days of full sun.  These calculations appear reasonable and readily verified by my daysailing experience using my 5 watt panel.  I usually use 4-6 hrs of sailing per week and in our area during the summer,  there are usually 3 or 4 days/week of full sun.  (Not so the rest of the year!) 

I am happy with the 5 watt panel because  I mostly day-sail since owning my SunCat, and a couple of weeks to recover once or twice a year from a one nighter is OK. A 5 watt 30 cell panel does not need a regulator but the voltage must stay above 13 volts.  In hot southern climates, more cells and more voltage (Therefore more watts) is a good idea because of heat build up which causes voltage loss.   

You can plug in your numbers and do the arithmetic for your situation but you need a good inventory of what you actually use to do this. OBTW solar panels should have plenty of ventilation to prevent overheating, an inch below them is recommended and a shadow on only one cell can shut the whole system down so placement is critical (cells are in series)!

Hope this helps.                 

[hoddinr]

Yes, Don, thanks very much for the thoughtful answer.  I'll be adding up amps needed as I get closer to a purchase.

Ron