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Old 17-04-2019, 16:42   #1
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How Many Watts of Solar?

How many watts of solar is need for a 34'-38' sailboat? I'm trying to figure out what I need to run lights, laptop, micro wave etc.


Thanks.
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Old 17-04-2019, 16:43   #2
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Re: How Many Watts of Solar?

Have you done an energy budget?
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Old 17-04-2019, 17:13   #3
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Re: How Many Watts of Solar?

I would recommend 200 to 300 watts. 200 Watts will get you there on sunny days. 300 watts will be better for the cloudy days. One thing to remember is that you're only going to get about 65% of the rated capacity, due to various issues.


I have two 100 watt panels and I can get up to 70Ah a day with a victron smartsolar controller. This under sunny skies.
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Old 17-04-2019, 17:27   #4
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Re: How Many Watts of Solar?

Search "energy budget solar sailboat." Otherwise it is a trick question.



  • How much you use.
  • How much you store (do NOT use the battery Ah rating).
  • How many days of clouds, which brings you to...
  • How many watts.
Since you save on both storage and generation by economizing, start there (LEDs etc.).
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Old 17-04-2019, 18:13   #5
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Re: How Many Watts of Solar?

Quote:
Originally Posted by Razoo View Post
How many watts of solar is need for a 34'-38' sailboat? I'm trying to figure out what I need to run lights, laptop, micro wave etc.


Thanks.
The length of your boat is irrelevant. What is relevant is the "etc" part. You need to figure out how much energy you're going to use in a given day. Then you need to ensure you have enough storage for that energy. Then you need to figure out what you need to recharge that storage on an ongoing basis.

Do you have refrigeration? That is usually the biggest draw on a boat, followed by a laptop and/or other significant draws. Do you have an inverter? You'll need one to run a microwave.

Are you living on the boat? Do you want a fully solar solution? How do you use your boat? What exactly are the the other draws, in terms of amps and duration, that you have?

You can have a 35' boat that uses considerably more power than a 65' boat. It's all a matter of what's running on it.
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Old 17-04-2019, 18:36   #6
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Re: How Many Watts of Solar?

here is the real story install as many watts of solar as you can fit on your boat. you always find used for energy if you have excessive solar you can make hot water .
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Old 17-04-2019, 18:41   #7
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Re: How Many Watts of Solar?

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How many watts of solar is need for a 34'-38' sailboat? I'm trying to figure out what I need to run lights, laptop, micro wave etc.


Thanks.
600W assuming you also are going to run your refrigeration
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Old 17-04-2019, 18:51   #8
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Re: How Many Watts of Solar?

It is more complicated than a simple energy budget.

Not only will you have to calculate your typical usage at anchor, you will find the demands are usually higher underway.

Usage is merely one factor. The other factor is your battery bank capacity, state of charge or drawdown per day, average sun hours per day, any deficits per day, and charge rates and/or profile.

http://emarineinc.com had a energy budget spreadsheet on their site, but seems to be a bad link now.

http://custommarineproducts.com has additional worksheets.

I also customized some of the sheets for my calculations. If you PM me I can send you examples of what I put together for my boat.


But...if you don't want to do the actual work of going through your equipment's energy consumption in a detailed manner, and calculate usage and other factors...600W seems like a good answer.
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Old 17-04-2019, 19:09   #9
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Re: How Many Watts of Solar?

Hereís a simple answer: Assuming weíre talking a 34-38í monohull, just maximize the solar wattage you can reasonably manage. The most Iíve heard of is 1000 watts on a 38í monohull. Iíve squeezed 400 watts onto my 37-footer. Somewhere in between is likely what youíre looking at.

Given your very rough needs that you sketched out, I suspect youíll be a heavy amp user. So you will need lots of charging capacity if you plan to be off the dock (and not relying on dino charging).

Of course, the best answer is to do as others have said. Do an energy audit. Figure out what your amp-hr demand will be. Add a considerable fudge factor to this (double it to be safe). Build your battery bank to match this demand, and then match your charging system to this bank.

Ö then, just maximize your solar . Iíve never heard of a smallish sized mono cruiser who said they had too much solar.
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Old 17-04-2019, 19:19   #10
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Re: How Many Watts of Solar?

I have 200 watts and struggled to keep up through the doldrums and at anchor in the Marquesas. While underway, my tow generator made up the difference. I believe with 400 watts I'd have been okay. We were running:

Engel fridge
Vesper AIS
Iridium Go!
Stereo w/ Amp
Portable devices
Pur 40E Watermaker (quite often, as we had 4 people on a 33 day crossing with ~30 gallons of tankage)
Lights
Macbook

We had ~215 AH capacity on the house bank (for whatever that is worth).

Yes, we ran a tight budget
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Old 17-04-2019, 20:30   #11
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Re: How Many Watts of Solar?

Quote:
Originally Posted by Mike OReilly View Post
maximize the solar wattage you can reasonably manage
+1 this

And have other energy sources, do not depend on just the solar.
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Old 17-04-2019, 21:49   #12
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Re: How Many Watts of Solar?

Quote:
Originally Posted by Razoo View Post
How many watts of solar is need for a 34'-38' sailboat? I'm trying to figure out what I need to run lights, laptop, micro wave etc.


Thanks.
to many variables to even hazard a guess yet .
What is your actual estimates usage daily ?
What is your battery chemistry you want to install?
What are all of your expected normal charging sources?
Solar , wind, water, and dino juice as well as shore power at the dock.
Also would be nice to have an idea as to your boat usage and cruising grounds.

Here's some info to help in calculating your power budget
https://astrolabesailing.com/2017/06...-power-budget/
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Old 17-04-2019, 21:50   #13
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Re: How Many Watts of Solar?

Quote:
Originally Posted by john61ct View Post
+1 this

And have other energy sources, do not depend on just the solar.
all I have is solar but I have plenty of it . Also have spare controllers
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Old 17-04-2019, 23:11   #14
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Re: How Many Watts of Solar?

Location of your cruising grounds and the season(s) in which you will be depending upon the solar power are very important. For instance, we're in Tasmania, about 43 south and while during the summer months we were well ahead of demand with 400 w, as the sun retreats northward and the days get rapidly shorter we're about breaking even, and if we stay much longer, we'll be back to supplementing with alternator amp-hours. These are critical factors in your planning.

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Old 17-04-2019, 23:49   #15
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Re: How Many Watts of Solar?

Quote:
Originally Posted by Mike OReilly View Post
...Build your battery bank to match this demand, and then match your charging system to this bank.
This ^^^^^^^^^^^^^!

No point having a tiny battery bank that is filled in moments from the solar, only to have it run out two hours after sunset.

Also, as I learned the hard way, battery capacity is not JUST about Amp Hours of capacity, the bank needs to be able to deliver adequate CURRENT, without too much voltage drop, to run your heaviest load. So my original (temporary) house bank of 2 x 105 AH batteries were more than adequate in terms of storage capacity, but really struggled to run my coffee machine, and would not have had a hope of running the smallest microwave I could find. (750 watts)
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