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Old 10-04-2010, 18:25   #1
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My New Boat Has Solar Power - Help ?

I wondered if maybe one of you fine people could explain to me, in laymens terms how solar power works? The previous owner of our boat installed 2 [max 46watt] solar panels, adding two additional batteries, as well. So we have a total of 4 batteries. I haven't the foggiest idea how solar works...He was fantastic, everything is well labelled as far as getting it all hooked up again when we're ready to sail, but I would like to have a bit of an understanding as to how it works.

He told us he had enough power for the fridge [icebox conversion], but I didn't ask if it would power anything else. Will our batteries die if the fridge is running over night and there is no sun. I don't want to sound like and idiot, I'm really not...this is just completely new to me. Thanks for any help you may have.

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Old 10-04-2010, 19:02   #2
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The solar panels are just a battery charger. Once the batteries are charged, you can run anything in the boat until the batteries die. You will learn through use what will kill the batteries fast and what will do it slowly. The batteries are not limitless in their ability to store electricity, but four batteries, fully charged should run several lights, refrigeration, and other accessories simultaneously. If you aren't wasteful, you will have more than sufficient power to make it to the next day to replenish you supply of power.


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Old 10-04-2010, 19:27   #3
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You need to learn some basic electricity and then it's not so hard.

Volts X Amps = Watts (Ohms Law - very big deal)

If you learn this you'll be able to compute the raw numbers. Not so hard is it?

On land you think in 120 volts (other parts of the world are 220 volts) but on the boat it is 12 volts (maybe 24). So you see your shore amps are ten times more powerful than your boat amps but a watt is a watt!. We also need to think DC volts instead of AC volts too but that is another lesson. for now you can convert between them but it's not perfect.

If you have 2 panels that can under ideal conditions generate 46 watts that translates into 92 watts or 7.67 amps at 12 volts. Your fridge we don't know but maybe it draws 5.5 amps and runs 50% of the time. If it's dark 50% of the time you might be close to running that fridge from the solar panels with enough for electric lights at night.

Batteries measure power the same way but battery capacity is measured in Amps X Hours. The idea is how many amps can you run the batteries for how many hours.

On shore you have a meter and you use what you want and get a bill. On the boat you use what you want but you have to repay the batteries for what you used. Your solar panels won't run the fridge but they will charge your batteries. The idea is you can charge your batteries when you choose so you can use power when you want even in the dark or when not running the engine. There needs to be a balance between what goes in and what comes out. To make it a little more complicated you want to not draw the batteries below 1/2 full and you want to recharge them back 100% regularly. Doing this means the batteries last a long time. Not doing this means the batteries eventually go dead sooner or they don't last a long time.

There are some additional complexities in that charging is not 100% efficient and batteries can't be charged instantly. They take time to absorb the power being added back. This limits how quickly you can charge or the charge rate.

In the end it is less about how much battery capacity you have and more about how much charging you can do and really does not matter how you charge but only that you recharge fully.

On the power calculation you are both the consumer and the power company. You need to look at power from both sides at the same time. So, how much can you run is at the end of the day related to how much you can charge back the batteries. You are fortunate in having some solar power but it's not a huge amount. You'll need to run the engine once in a while and less often is usually desired.

To get into the serious numbers you need to figure out how much power you will need and when. On a very conservative budget the fridge is about 1/2 of the electrical power you need. You measure usage in watts over time and from the voltage you can compute amp hours. If you look at how much power things take you find that the 1200 watt microwave oven at 120 volts now uses 100 amps in 12 volt power and that assumes a 100% efficient conversion from 12 volts to 120 volts (see above). Your fridge may only need 60 to 80 amp hours per day so this microwave is going to use all the power it takes to run the fridge all day in about 1/2 hour. The use of electricity is about "lifestyle choices".

You start with how much power things take and then look at how long they run, then you can compute a daily energy load that needs an equal energy supply. The problem is easy when you plug into shore power every night and have an almost infinite supply to recharge with and the crew is asleep, but not so easy in the cool quiet anchorage you just can't stand to leave for 3 days.

Start by thinking in these terms and then you can get started thinking in more technical details of how this really does work. Ohms law is the truth everything else is just the ramifications and the cold reality of stuff isn't perfect nor are people. When you have to be the power company too it does get a little more complicated. It's about thinking in different ways and looking at what things cost in terms of work, engine noise, convenience, and enjoyment. Oh yes, you have to pay for it all too.

I suppose you hoped it could be easier. It really is once you get your head straight on how it really works. Start with thinking about the ideas and less about the details until you start to see the bigger picture of it all. It's less about the magic in a solar panel and more about so what amount of power can it make? Power is everything. Given enough amp hours you can make everything work so long as you repay the short term loan with the battery bank.
Paul Blais
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37 15.7 N 76 28.9 W
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Old 10-04-2010, 22:27   #4
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We had four solar panels that were slightly over 70 watts each. In the tropics, I found that we could reliably get about 50 amp hours per day out of the combined panels. That was not enough power on our boat to cruise full time with all our electronics, refrigeration, autopilot, and 12 v watermaker. We ended up installing two Aerogen 6 wind generators, and they gave us the electricity we needed as long as we were in the trade winds. If we were out of the trade winds, then it was necessary to charge our batteries with the engines.
Dave -Sailing Vessel Exit Only
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