The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.
The rigid panels will provide a smaller footprint for the intended current/capacity compared to semi-flexible or flexible solar panels. Warranties, an indication of reliability, are also longer for rigid panels but flexible panels provide more versatility as far as being able to locate them onboard or putting away onboard, whereas, rigid panels are usually mounted to stay in place unless extremely small such as a trickle charger, a 5 watt panel for trickle charging one battery. Solar controllers are generally needed to prevent overcharging a battery, when one uses greater than a 5 watt charger for one battery or 10 watts for two batteries. Also, to a small degree, solar panels will slightly discharge back thru the panel during non-sunny periods but a solar controller prevents that discharge.
Sizing/capacity of a panel has to take into account how many batteries you have onboard and your average daily/weekly load use, running various electrical devices. Also, you have to account for the panel generally not charging at it's ideal rated capacity. For example, if you get a 20 watt panel that charges ideally at 1.2 amps per hour, anticipate it is going to average perhaps .7-.8 amps charging on a sunny day for perhaps 5 hours and then somewhat lower rate earlier and later in the day for a few addl hours. Then it may be significantly overcast or raining for 2 days or so each week, depending on what is likely in your region of the country. So calculate your weekly loads (current to be used each week) and then calculate charging anticipated from the panel for say 6 hours each day for ~ 5 days. That will then give you an idea as to the size panel you need. A flooded battery will generally discharge, even if no loads put on it, about .4 Amps/day in the summer and .25 Amps/day in the winter. (Gel and AGM batteries discharge less per day.) So, if you just want to trickle charge a battery and not use it for any loads. a trickle charger can produce, ideally, about ~.3 Amps/hr, figure conservatively, .15Amps for 6 hrs per day and for 5 days equals .9 x 5 = 4.5 Amps per week. Even if non-sunny for 3 days, that would be .9 x 4 = 3.6Amps. The battery (flooded battery) discharges on it's own during the summer, approximately .4A/day x 7 days = 2.8Amps. Since a solar controller is not used with a 5 watt panel, there is also some slight discharge at night back thru the panel but I do not believe it amounts to very much....and so a 5 watt panel on a weekly basis is sufficient to maintain a trickle charge on one battery: Between 3.6 - 4.5 Amps charging per week, conservative numbers, compared to battery self-discharge = 2.8 Amps per week. The slight discharge back thru the panel at night is very small but the trickle charger is putting out enough per week to account for that discharge as well. (Maybe it only amounts to about .1A per night x 7 = .7Amps per week addl. (That's a guess.)
I have 2 batteries, generally use them for light duty, running the fishfinder, occasional use of the nav lights and/or fans and same goes for very limited use of a mini-boom box. I have found that the 20 watt rigid panel that I have been using for over 10 years has been sufficient. It has a 20"x14" footprint. Perhaps a better choice for a 2 batteries and moderate loads put on a weekly would be a 30 watt panel which you can get in a 20" x 20" configuration and hang off the transom rail. As far as solar controllers go, you can get a digital readout one like I have or a much less expensive one that has no voltage/current readout - This based on your preferences and spare dough.
Here are a few photos of my setup but there are more photos and details on my website. By the way, my mounting hardware allows me to remove the outboard without dismantling the panel. It is high enough that it does not interfere with sitting close or resting against the transom area and I can tilt it during the winter months when the sun is lower in the sky to maintain it perpendicular to the sun. Using wing nuts facilitates tilting it and if I ever have to dismantle it.
I highly recommend the Morningstar brand of charge controllers. They will treat your batteries right. I have a small one similar to Larry's for my 30W panel on my C250 and a large one that handles 2 160W panels on my travel trailer.
Check the solar panel technology versus "current density", meaning that amorphous silicon develops the least current or power per square inch. But they are cheap!! Like Harbor Freight cheap. Crystalline silicon offers a greater amount of current or power per square inch. It could be like 2:1. For our boats look for panels that generate open circuit voltage between 16 and 22 volts DC. Some panels develop 36 or 48 VDC. These are overkill for your 25 foot boat unless you've decided to use a 24 VDC battery system. Conventional charge controllers assume a 12VDC system. Go for up to the ampere rating of your charge controller, typically 7.0 Amps. This affords around 100W for your boat. More is generally better, but don't exceed the max current.
Yes, my solar controller is a Morningstar. I like it with the digital readout but it is expensive and there are less expensive options....a lot less expensive. One benefit of my controller which was beneficial when I switched out my batteries, it has a switch to select for Flooded, Gel or AGM batteries and my new batteries are AGMs. AGMs and Gel batteries, ideally, are charged slightly different than flooded batteries and believe also at a slightly lower voltage. Flooded batteries is more forgiving as to how they are charged.
One important difference between flooded batteries and AGM and Gel is that the full charge voltage for flooded is 12.6 VDC while the others are 12.8 VDC. Two tenths of a volt may seem trivial but the switching solar charger is set to maximize flooded batteries. The set point for AGMs and Gel needs to be about 0.2 VDC higher to complete the charge and “float” the battery.
This means that if you have the wrong type of charge controller, your expensive batteries will be under charged. Moral of this story is: make sure you charge controller will accommodate your battery type.
If you wire two of the 50W panels in parallel, you can get away with this charge controller for any battery type. It won't work with two 100W panels. I have one of these on my Capri 14.2 and used to have one on my C25.
If you wire the panels in series, then you'll get more charging hours each day, but you'll need a better charge controller to step down the voltage. This is a good unit for any battery type. I have one on my C25.
Last thought for this post: if you're going to run panels in series so that the the supply voltage is significantly higher than the battery voltage, then get a MPPT controller (like the Mastervolt model I recommended above) instead of a PWM controller. MPPT controllers convert excess voltage into current while PWM controllers just waste the extra power.
I'm using a Ganz GSP-12 from Defender ( 15.3 volts, 12 watts, 0.78 amps ) to a controller to a single AGM battery, to a big fuse, to two littler fuses, to two power panels, one switched and one unstitched. Hanging the panel from the aft rail, center.
Im on the lowest end of power usage, I literally could not hook up a regular stereo as it would draw the battery down farther than the essentially "float" charger that the solar provides.
But that being said mine has worked great for 6 years and I expect many more. I've had to clean the terminals on one panel once.
LEDS on all but one if the interior lights, and LED anchor light. If the voltage drops below some level on the battery the led lights just go black, so I've kept my running lights and one interior light incandescent. If the voltage drops they still light even though they dim..
I'm charging an iPhone for my music to a bose portable speaker, so both are lithium batteries, and a handheld marine radio also lithium battery. A depth finder and that's about it. Maybe a spotlight or halogen deck light on occasion.
Im on an inland lake so my needs are simple and limited but there you go.
I made the mount for the rail with some Grill mounts also from defender and some alum angle.
after reviewing I realize it has all lasted 8 years now. Like I said, little use mostly daysailing and overnighters.
Not even a bilge pump... but It works for my design.
Ray in Atlanta, Ga. "Lee Key" '84 Catalina 25 Standard Rig / Fin Keel
Notice: The advice given on this site is based upon individual or quoted experience, yours may differ. The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.