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Solar panel charger

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slacker
1st Mate

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USA
26 Posts

Initially Posted - 06/17/2025 : 19:14:46

Can anyone recommend a solar panel battery charger ? I am curious as to what the proper wattage would be and how they are mounted thanks

Paul Schupbach

OLarryR
Master Marine Consultant

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USA
3458 Posts

Response Posted - 06/17/2025 : 22:50:49

Paul,
There is a lot of info to digest before making your decision on a solar controller. There is no detail(s) that you provided to kick-off shopping for a solar controller. I have info on my website regarding my solar panel/controller install and even though it is for a Cat25 versus a Cat250 it should be a good place to start. One issue though - I took a look at my solar panel page on my website and it appears the text is a bit jumbled and I will have to fix that in order to make it easier reading.

What I recall when I started off with my solar panel/controller install was to first get an idea as to what electrical loads/demands I would need to be handling on a daily/weekly/periodic basis. That is, what size solar panel would be needed to regularly charge my two 12 volt batteries?
I was not a heavy load user - Just needed for basic charging of the two batteries that would be used regularly for mainly starting the outboard, operating my depth finder/fishfinder for all sailing hours and occasional nav lights and cabin lights (very infrequently). I read somewhere that a 12 volt battery without even used will discharge approximately .5 amps daily in the summer months and about .25 amps daily during the winter months. Two batteries...well then double the daily discharge. What you do is estimate the daily/weekly loads from accessories and the battery daily discharge. Some individuals may have heavy loads and if daily/weekly, that may then require a larger solar panel. For example, some may regularly use a large boom box or music system, some utilize a large multi-color chart plotter versus a small mono-color fishfinder or depth finder. Some regularly use cabin lights (non-LEDs) versus others that rarely use cabin lights and also utilize LEDs. Some rarely use nav lights while others use them frequently.

Try and calculate the anticipated daily and weekly amperages for all loads. (ie. 4 amps/day for 3 sailing days a week and the other 4 days just a battery drain of 1 amp/day for 4 days = about 16 amps per week. (Use of an outboard that charges will help a bit in recharging the batteries but only if the outboard is run for a significant time.).

Sizing the solar panel first, then solar controller:
Depending on location and normal weather conditions, some areas have sunny days all the time and other areas....not so much sun. Where I am, I figure conservatively that the solar panel will rarely charge equal to its amp rating and during a normal day when the sun is lower it will charge at a very low amps. For example. a 20 watt panel may ideally charge at 1.2 amps/hr. when new and so figure something less. During a normal sunny day you can do your own rough amperage guesstimate: .9amps/hr for 4 hrs and .4amps/hr for 3 hrs = 4.8amps on sunny days.....and for 4+ days a week = 20 amps/week. If weekly discharge rates per above rough estimate is 16 amps/week, then a 20 watt panel is sufficient for those with light loads. A heavy load user may want a 30W, 40W or larger rated solar panels especially for those with coolers. large music systems, etc. Then there are those that may have very minimal needs/loads like just handling the battery self discharges and just want a solar panel to trickle charge the a battery. Be aware, My calculations above are very rough as I am recalling what I did eons ago - The important thing is for you to consider what your own estimate should be conservatively so as to size the panel appropriately and for future years use.

Solar controllers: Those that are just trickle charging a battery utilizing a 5W or less solar panel may not even use a solar controller since the daily charging rate may not be that much higher than the daily summer battery self discharge rate especially when you consider the days during the week when there is no sun. But in most instances, it is best to always utilize a solar controller since there are some very inexpensive ones that will not only provide protection from overcharging but during hours when a solar panel does not charge it may actually contribute slightly to small discharging of a battery. A solar controller usually will prevent the panel from contributing to the discharge during non-use hours.

In the example above, a 20 watt panel that has a max of 1.2 amps/hour charging rate, would need a controller that exceeds that rate. Most controllers are rated much higher. What also may figure into deciding on a solar controller is if it just has idiot lights to indicate charging or fully charged status of the battery or the controller also has a digital display indicating actual charging amp charging rate and battery voltage. More sophisticated controllers will mostly be rated at 10 or 15 amp controllers. Your choice.

I installed my solar charging system eons ago...probably back around 2006 or so. I have a 20 Watt Kyocera panel and a 15 amp digital display Morningstar Pro15M controller. My system has been working fine all these years - No changes/replacements! These days, the controller is still sold but apparently there are different models of it in an idiot light version but also in a digital display version. There are also plenty of other controllers to consider.

Hope this helps in getting you started. Below is one photo off my website of my controller. did a rather minimalist mounting for it - Mounted on a board and slides onto my cabin shelf. The board is held in place by the shelf and the above cabin liner. Other photos on my website show the install.

My website photo below and on my website need to be updated as I utilize the same components but many years ago, tweaked my wiring. My solar panel originally hooked up directly to the pos and neg terminals of the batteries but I then moved the pos wire to connect to my switch panel positive terminal off the main circuit breaker. Also, my solar controller and many others also have optional wiring that can be connected directly to the batteries as sort of a sensor for I believe either the temperature or charging feedback to the controller - It assists in dialing in the appropriate charging rate at all times - Not necessary but I decided to add the wiring hookup since it is easy to accomplish. The other thing is that my controller has different settings depending if your batteries onboard are flooded, AGMs or Gel batteries. Looking back when I bought the controller, this may not have been on my mind, I had flooded batteries and probably never gave any thought to replacing them with anything other than flooded batteries. But I now have AGMs and so my controller has a setting for that - Charging rate differs somewhat than for flooded batteries - AGMs and Gel batteries are more sensitive to how the charging ramps up and then rate slows down as batteries approach full charge. These days, controllers may still have the battery setting to be done manually or may sense the type of battery automatically....but I would be leery to buy any controller that does not mention suitability for charging all types of batteries.

Larry
'89 Robin's Nest#5820, Potomac River/Quantico, Va
http://catalina25.homestead.com/olarryr.html

Edited by - OLarryR on 06/18/2025 06:05:09

Voyager
Master Marine Consultant

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USA
5368 Posts

Response Posted - 06/18/2025 : 06:44:13

Hello Paul, this is a great question. I agree with Larry. To sum it up, you have a few ways to go depending on your power requirements.
Stay with a 12V system with two group 27 or group 29 lead acid batteries, or equivalent AGMs, or Lithium batteries. Use a 100 or 200W panel. Renogy Wanderer MPPT charger works well but you really can�t take more than 800-1000 Watt-hours daily from this system. Will run lights, radios, maybe a small fridge.
Go to 24V with four batteries (series/parallel). Maybe run a coffee maker, microwave or toaster for short periods or a refrigerator all day. A few 200W or 400W panels.
If you want to run an AC all summer, this will be a challenge for solar without shore power.

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain � Milford, CT

slacker
1st Mate

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USA
26 Posts

Response Posted - 06/18/2025 : 17:46:04

Thanks guys . Based on my minimal needs I believe I can get by with a 20 watt charger and 10 amp controller. Seems like someone would make a dual system with both . I appreciate the good information. I�m off to start my search

Paul Schupbach

OLarryR
Master Marine Consultant

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USA
3458 Posts

Response Posted - 06/18/2025 : 21:12:50

Addl things to consider regarding choosing a solar panel:
1) Rigid, flexible or semi-flexible panels: Rigid panels are generally more reliable and have longer warranties than panels that have benefits such as able to roll up and/or step on. Also, for a given watt rating (ie. 20W, 30W, etc), rigid panels have less of a footprint (ie. A 20W rigid panel may have dimensions as small as 20" x 14", whereas, non-rigid panels may have dimensions 50% longer/wider.}
2) Consider how you will mount the solar panel and what attachment points exist on the panel you are considering. Sometimes, it is hard to tell from mfrs photos just what attachment points exist on the backside of the panel. For example, on my panel, the backside had a grooved channel slot along the perimeter of the aluminum panel frame. It allowed a bolthead to be inserted in the slotted channel. Then a hinge or fastener could be attached to the bolt and made tight with a nut or wing nut. That is how I attached my support structure for the panel.
3) Not to make this more confusing but not all similar watt rated panels function in the same manner. I do not recall all the info/details but panels are generally sold two ways: Believe they are defined as either Mono-crystal or poly-crystal cell form....or something like that. The difference is that the less expensive one is significantly affected in charging amps if a small portion of the panel is shaded/blocked from the sun. Whereas, the other form/design is also affected by a portion of the panel shaded but not as significantly. I would do a search on mono and poly crystal solar panels to compare their pro/con. Back when I was checking this all out, I seem to recall a significant difference in cost between the two types of panels. I suspect these two types of panels still exist for low wattage solar panels unless they all now default to one type or the other. Many customers are buying much larger solar panels for all sorts of applications, mostly houses. So, that's why I am not sure what is being sold these days for 20W. 30W panels.

Larry
'89 Robin's Nest#5820, Potomac River/Quantico, Va
http://catalina25.homestead.com/olarryr.html

Edited by - OLarryR on 06/18/2025 21:16:42

Voyager
Master Marine Consultant

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USA
5368 Posts

Response Posted - 06/19/2025 : 04:11:55

Paul, a 20W panel will handle a few lights and radios and keep a largely passive system charged up, however, if you plan a few days� passage with actual living aboard, I would recommend a 50W panel. They are inexpensive (~$50) and will help your battery recover more quickly if you happen to have some heavy use overnight, e.g.: anchor light, 12VDC fans, music or video, device recharging.

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain � Milford, CT

Little Wind
1st Mate

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USA
34 Posts

Response Posted - 06/19/2025 : 11:07:47

Easy to find on Amazon...

https://www.amazon.com/dp/B00PFGP0EA?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1

Controller is not waterproof but you can mount it out of the way with the addition of this:

https://www.amazon.com/dp/B079ZTBHYK?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1

Leave the panel in your cockpit while you're not aboard or construct a mount.

I installed one of these on a friend's boat and I use one of these for my VW Vanagon.

Mark

Mark
Little Wind
1996 C250WB #206

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