RonaldNC Report post Posted June 14, 2009 I bought a 2005 Rexhall with a solar panel already installed on the roof. This may sound dumb ... but how do I determine the wattage of the panel? I can't find any markings on the unit itself, but do have a LCD monitor inside the coach. Any help would be appreciated. Ron Share this post Link to post Share on other sites
tbutler Report post Posted June 17, 2009 RonaldNC, Welcome to the FMCA Forum! I hate to see someone who hasn't gotten some response to their query so here goes. We have a solar panel on our roof, 2' x 4' feet (8 square feet). It puts out between 5 and 6 amps at 14V DC at its peak on a good sunny day. I don't know what that would be in watts but someone likely could make the connection. Is you panel about the same size? If so, you could start from here. There will be variations in the efficiency of solar panels over time but ours is vintage 2003 so they should be similar in output for a similar size. If they aren't the same size, work out the proportional relationship and apply it to the output. I'll keep looking for the equation that would allow me to answer your question. Meanwhile perhaps someone could take my information and apply it to your query. Do tell us what size your solar panel is... Share this post Link to post Share on other sites
Guest 2driftrs Report post Posted June 17, 2009 Ron, the output depends on the type of panel. Crystalline type panels put out at least twice the wattage of amorphous panels, and generally cost about 4 to 5 times as much! TButler is right, you can get an idea of output based on size. His panel is 2' by 4' and puts out 6 amps at 14 volts, or 84 watts. This indicates that he has a crystalline type panel. The same size amorphous panel would only put out around 30 watts. You can sometimes tell the type of panel you have by the color. Most crystalline type panels are a very shiny black or blue/black surface, while amorphous panels are kind of an off white. Share this post Link to post Share on other sites
Guest 2driftrs Report post Posted June 20, 2009 Boy is my face red! I had a couple of folks email me to say the new amorphous panels are also shiny black, so I guess color won't work as a guide. What's really embarassing is that I installed 2 of the 15 watt panels yesterday on our rig and sure enough, they're black. Watch this forum in the near future, I'm wrapping up the solar panel installation today, and I'll have some guidance for those who want to try this themselves. I'll aalso include a way to measure output. Share this post Link to post Share on other sites
larybrandt@aol.com Report post Posted June 21, 2009 To make a 18 volts panel, for example, you connect 36 cells in series (36 cells times 0.5 volt each = 18 volts). And 36 * 1.75 (each cell in theory averages 1.75 watt) watt = 63 watts. The amps you will be getting is 63 watts divided by 18 volts = 3.5 amps. Most cells are 3"X6" Share this post Link to post Share on other sites
larybrandt@aol.com Report post Posted June 21, 2009 To make a 18 volts panel, for example, you connect 36 cells in series (36 cells times 0.5 volt each = 18 volts). And 36 * 1.75 (each cell in theory averages 1.75 watt) watt = 63 watts. The amps you will be getting is 63 watts divided by 18 volts = 3.5 amps. Most cells are 3"X6" Share this post Link to post Share on other sites
jjgnn@yahoo.com Report post Posted July 3, 2009 Buy or borrow a good quality electrical meter. Measure the voltage across the two wires, plus and minus. That gives you voltage. Measure the amperage by interrupting the positive lead with the meter (across a fuse is ideal). That gives you amperage. Multiply the voltage by the amperage, and that gives you the wattage in that sunshine. Careful that you do not exceed the amperage of the meter, as most only go up to ten amps, and a big panel will put out nearly that in good sun. You might measure the amperage in the morning or evening (low sun) to see if it approaches the limit of the meter. Or, you can look at the panel data plate, which should give watts, or voltage and amps so you can figure it yourself, or a manufacturer and serial or model number that can give you wattage. Few solar cells live up to their stated wattage, as they are typically tested in FULL sun, at the proper angle, with clean glass, or in optimum conditions. Most cells in the field rarely have all that going on at one time. Figure roughly 25-30% less in real world conditions. At least that is my experience. I just bought another cell and leave them all flat to make up the difference, and it works very well for boon docking. I have four panels rated at seven amps apiece, and theoretically should have 28 amps in full sun. Best i can get is 23 amps, or 17% less. They are flat (not aimed at the sun) and pretty dusty, so that is pretty good, and expected. Share this post Link to post Share on other sites
jmannett Report post Posted October 10, 2010 You should be able to find a label on the back of the solar panel which describes its specifications. Operating voltage, operating current, and wattage. If you are thinking about adding a panel, you want to make sure your new panel operating voltage is 'close' to your existing one. +/- 1Volt is good. Then the panels are wired in parallel into a charge controller. Be sure the charge controller can handle the amperage of both panels. As far a rated power versus actual power, it is a function of age, angle, altitude, cleanliness, and temperature. Yes, temperature. Contrary to common belief, the hotter a photovoltaic solar panel gets, the LESS power it puts out. (it makes sense when you think about rising resistance with temperature). Panels are tested and speced based on 77 degrees F. which is hardly ever the case. Keep in mind though, a colder panel puts out MORE power. I have a 440 watt system on the roof of my coach. In ideal conditions, it outputs exactly what I designed it to. James Mannett Sun City, AZ Share this post Link to post Share on other sites
