PS Advisor: 08/15/03


Solar Panel Power
Your review of solar panels left me with some “solar panels for dummies” type questions. For trickle-charging a battery, how do you determine how many watts are needed? My boat uses a single battery for the engine, running lights, compass light, radio and GPS. I assume the engine causes the biggest drain. The lights see only occasional service, and the radio and GPS stay on for extended periods now and then. Would a 5W panel be sufficient? Could a trickle charge panel power up a completely depleted battery? Over what period of time? Why do the amorphous panels get only a limited three-year warranty, compared to the 10 and longer warranties for the rigid panels?

-Bruce Caldwell
Riverhead, NY

It’s worth getting familiar with the relationship of volts, amps, and watts in a 12-volt DC system, and with the number of amp-hours you can pull out of and put back into a battery. The arithmetic is easy: volts times amps equals watts, and watts divided by volts equals amps. In this case we’ll assume the voltage is always 12.

To calculate what you’re taking out of the battery in terms of amp-hours (Ah) , multiply the amperage of each device by the hours used. For example, a 15-watt incandescent bulb will use about 1.25 amps (using rule-of-thumb numbers). If it’s on for 2 hours, that’s 2.5 Ah. Starting the engine shouldn’t be a big drain, because even though the starter might demand 150 amps, it only demands them for a tiny fraction of an amp-hour-and presumably it recharges the battery anyway, once it’s started.

For solar panel specifics, we turn to John Gambill of Hotwire Enterprises (mentioned in Mailport this month):

“To keep the battery charged, you have to put back whatever you take out, plus a little extra (maybe 10%), as the process is not 100% efficient. Depending on your latitude, you will get between 1/4 and 1/3 the rated wattage of the solar module in Ah per day, so to replace the 2.5 Ah taken out by that light bulb, you’ll need a solar module that puts out about 10 watts.

“Trickle-charging usually refers to the process of maintaining a battery’s charge. A battery that is kept fully charged will last a lot longer than one that stays discharged all the time. A rough rule of thumb for sizing a solar module for trickle charging is to use one that has a rated amperage of one percent of the capacity of the battery (at the standard 20-hour discharge rating). A 100 Ah battery would need about 12 watts, a little less in the southern latitudes, a little more up north. Gel and AGM batteries need significantly less because they have a very low self-discharge rate, and they are more likely to need a charge controller. Batteries that have neared the end of their useful life will need more. A charge controller is not needed on wet-cell batteries until you get over two percent, though you may have to add water more frequently. It sounds as if you have a battery about the size of one used in a car (about 50 – 70 Ah), so a 5-watt module should keep it charged in the summer-but it might take a month or more to recharge a significantly discharged battery.

“One nice thing about solar is that you can add modules if you find you need more power. Just hook them up in parallel. If the boat is only occasionally used overnight, it’s cheaper to add another battery and leave the dock with more power rather than add more solar to keep up with relatively high demand for a short period of time.

“The warranty on solar modules includes a performance guarantee that the modules will still be producing nearly the rated power after a stated period of time. What eventually degrades solar modules is the sun! It was thought that amorphous would degrade more rapidly, but they are holding up better than expected. The rigid amorphous modules from Shell (formerly Siemens) and Uni-Solar are warranted for 10 years, and the flexible ones are warranted for 5 years, probably because the plastic frame degrades. Most modules will make power for a long time. I saw some early Arco modules more than 30 years old and still working.”

Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida.


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