Question about solar panels and batteries.

[madmax]

Hello, I'm new here. I plan to get a sailboat, my first one, in Spring 2025. I have some questions about the electricity on a sailboat.

I know how the electricity works in a car (battery and alternator), but I'm not familiar with the sailboat's electricity stuff. I understand that most sailboats came with two batteries, one for engine-starting only (inboard engine) and another for powering the instruments (radar, AIS, some interior lighting, etc).

To keep the batteries charged while sailing (not using the shore power), the solar panels will keep the batteries charged, is that correct? If the engine is running, it will charge the batteries but I don't plan to keep the engine running all the time while sailing.

My #1 concern: After I purchase a sailboat, I'll need to move it from a marina to my "home port" marina which is about 65-70 nautical miles. That means I'll have to sail for about 10-12 hours. I want to keep the instruments running all day (GPS, AIS, etc) while sailing. I'm concerned that I might run out of battery juice. Do you think the solar panels will keep the battery charged (maybe 80% or higher)? What's the minimum wattage (solar panels) to keep the battery fully charged?

THANKS!

 

[kappykaplan]

What is the size (amp hours) for the house bank? How many solar panels and what is their wattage?

 

[DArcy]

That's a very open question with a lot of variables. The fist thing, maybe the most important for your specific case, what is the size and condition of your battery bank? List the loads and duty cycles to determine your consumption in Amp-hours then compare this to your battery capacity. In addition to the navigation equipment you mentioned, you likely have a refrigerator, autopilot, cabin lights, navigation lights.

Many boats still have the 1/Both/2 switch when allows use of either battery or both. These setups generally have two batteries of the same size and they are used for both house and engine starting. Other setups have a dedicated engine battery with a separate house bank. The house bank would generally have higher capacity in this case. If you have a separate engine battery that is not used for house loads you have low risk running out of juice since you can always start the engine to charge the batteries.

I have 250 Watts of solar and it keeps up, without autopilot use, such that I don't need shore power. I still have incandescent cabin lights, but navigation lights are LED. My refrigerator is not very well sealed so it runs about 50% duty cycle in the warm days of summer.

If you don't have a battery monitor then you are guessing at the state of charge. A volt meter helps here, I installed one so I can always see battery voltage. If you keep the battery voltage above 12.2V you are ok. Keep an eye on it and start the engine to charge if it drops to 12.2V. It's a good idea to keep an eye on battery state of charge so you don't run them down too low and damage them.

With a large house battery bank you could probably make the entire trip without charging. With a smaller bank you would likely need to run the engine for and hour or two along the way. Old batteries, or newer ones that have been abused, won't be able to hold a charge as well.

 

[JoeWhite]

Before you start your trip, at the dock with no shore power, turn on all the instruments that you will be using while sailing for 12 hours using the house battery with starting battery disconnected. Then measure the house battery voltage. If it is much less than 12.2 vdc, then you need new batteries. But it is likely that the solar panels will supply all the current that is needed for the instrument so the battery will not be discharging most of the day.

 

[dlochner]

As others have said, too many variables to give any kind of a reasonable answer and you don't want to spend a bunch of money on the electrical system without a lot of forethought and planning, that stuff ain't cheap.

Once you find the boat you want to buy, ask about the batteries, how old they are, how long since they have been on a charger, what kind of use they have had, what size are they. This will help ascertain their condition and whether they are up for the trip. If you have any concerns about the batteries and their condition, just replace them. Don't go to West Marine, you'll pay a premium. Try Costco or any national Auto Parts store, think cheap and disposable. It will cost you a couple of hundred dollars for new batteries, but that will buy you some time to research and decide what you will do to the electrical system to meet your needs.

Next, buy towing insurance from TowBoatUS or SeaTow. If the batteries fail, they'll come get you. The USCG won't unless someone's life is at stake (the prior owner and broker don't count if they mislead you and you want to end their existence).

And this winter begin your education on all things marine electrical. For an organized course, check out BoatHowTo.com. This is a site developed by Nigel Calder a guru for all boat systems especially electrical. I have been to his seminars and he can explain complex ideas in a way we mere mortals can understand. It is a pay course, but well worth the money spent.

A second choice is MarineHowTo.com This is Rod Collins' site. He is especially good at dealing with specific issues and questions. Get to know his site and what's on it. Rod's site is free, however, if you can help him out by donation or using his affiliate links that would be beneficial.

Finally, don't hesitate to return and ask questions. Just be prepared for a deluge of answers to sort through.

Good luck with your search!

https://boathowto.com/

Marine How To - DIY for Boaters - Marine How To

For DIY boaters in an attempt to try and keep the sport as affordable & safe as possible. How To articles for sailboats and powerboats.

marinehowto.com

 

[Johann]

Great advice here. Another good resource from Nigel Calder is his Mechanical and Electrical manual. The website seems to focus on electrical… This covers quite a bit for a first time sailboat owner.

Amazon.com

 

[mermike]

Charlie Wing's book is also excellent.

 

[madmax]

What is the size (amp hours) for the house bank? How many solar panels and what is their wattage?

I haven't gotten a sailboat yet, so I don't know.

 

[madmax]

Lots of good advice and suggestions. It's true...a lot of variables. I hope I'll get at least 250 watts of solar panels for my boat.

I'll check the books you recommended. I've never thought of boat towing insurance. Great idea!! I'll get one as soon as I get a sailboat.

THANKS for your wonderful advice/suggestions. Looks like this forum is a great place for me to learn more about my sailboat.