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Old 22-05-2016, 19:36   #1
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Wattage Usage on a 30' sailboat

I have just bought 4 30 watt/12 volts panels and a 20A controller for my Bristol 29, which I plan on living in and cruising full-time. I chose the 30 watt panels because I didn't want to spend a lot of time installing my first choice(2x100watt). I also have 2 690AH starting batteries and a 800+ AH battery(left it at home) ... all wetcell batteries. I might add another large house battery later(along with 2 40-80 watt panels) My alternator delivers 40 amps when the engine runs.

I cook on an alcohol stove, use minimal lighting and am operating on a 2A notebook computer. My fridge is a Frigoboat which draws 3A per hour of running, which is about 15 minutes per hour(daytime), and about 5-10 minutes each hour at night. Beginning last years I've learned to "enjoy", heat and tolerate cold. My 7"TV draws about 1/2A and my mp3 player(with speaker), draws about 1/6A.

My computed amperage, daily is expected to be about 35 or so amps on average.

For summer, I expect the 120 watts should suffice and deliver about 40+ amps daily.

I'm figuring that rainy/cloudy days will reduce wattage, but these days might also reduce fridge running time and hopefully keep me covered.

I know there are lots of sailors with hundreds of wattage, and with wind vanes to boot, but I'd like to hear from those who have operated in the 100W to 200W range ... and hear what they think of their system setups.
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Old 22-05-2016, 20:16   #2
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Re: Wattage Usage on a 30' sailboat

You say you have 2 starting batteries of 690 amp hours each. Are you sure about that? A 690 amp hour battery would be huge. Also why do you call them starting batteries? Are you using one for starting the engine and one for house loads?

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Old 22-05-2016, 20:30   #3
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Re: Wattage Usage on a 30' sailboat

I think you're confusing the Cold Cranking Amp (CCA) rating on your batteries with the AH ratings.

A good Group 29 or 31 marine battery has about 180-210 AH rating, discharging at about the 20 hour rate.

Such a battery will be cycled between about 50 and 85 percent capacity when operated properly, which is about 35 percent of capacity, which is about 70 AH between charges.

Your four 30W panels should give you 2 amps or so each under good conditions, so for the five hours a day you can expect good sun and a good output, that's about 10 AH per panel...about 40 AH per day total.

Your two most significant loads, your computer and your fridge, can easily burn all that the panels put into your batteries or provide to loads.

You'll need to run your engine periodically to charge...top up...your batteries. Perhaps every 3rd day.

I recommend you buy a good battery monitor.
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Old 22-05-2016, 22:14   #4
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Re: Wattage Usage on a 30' sailboat

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Originally Posted by fryewe View Post
I think you're confusing the Cold Cranking Amp (CCA) rating on your batteries with the AH ratings.

A good Group 29 or 31 marine battery has about 180-210 AH rating, discharging at about the 20 hour rate.
Yes he is confused but you are as well. A group 31 battery will have approximately 100 to 130 AH at the 20 hour rate. To top 200 AH you need an 8D.
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Old 22-05-2016, 23:32   #5
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Re: Wattage Usage on a 30' sailboat

The biggest issue is you are all over the place with your units of measurement:
- Watts = Instantaneous power usage regardless of voltage
- Amps = Instantaneous power usage at a particular voltage (12V in this case)
- Amp-hr = Total power usage at a particular voltage (in theory you could use watt-hr but it's not commonly used when talking about 12V boat systems)


(feel free to look of the official definitions, I've simplified them a bit for ease of understanding).


Batteries: They have two key measurements.
- Cranking Amps: This is really only of concern for starting the motor. It's almost unheard of for anything else to use even half the cranking amps the starter needs.
- Amp-Hr Rating: This is the total storage capacity (think gal of gas) except you can't use all of it. If you empty the tank on lead acid batteries you damage them. Also, how fast you discharge them (amp draw) also impacts how many amp-hrs you get out of the batteries. A high amperage will result in less amp-hrs available.


Solar Panels: Divide the rated wattage by 12V and multiply by 5 or 6hr to get an estimate of amp-hrs produced in a clear day.


Alternator: The 40amps will do well until the batteries get near full and then the batteries simply won't take all the amps available.


Usage: Take the amps (or wattage/12) and multiply by how many hours the device will be in operation. Add up all the devices to get your total amp-hr usage.


Now you get to the fuzzy math:
- I would figure on using only 30-40% of the battery amp-hr rating.
- The Cranking Amps must meet the starter requirements not much room to play with there but small sailboat engines usually don't take a lot so your deep cycle bank may have plenty.
- Are you willing to run the motor regularly? If not, you need to decide how many days of overcast before you are willing to run the motor and then multiply your battery bank amp-hr by that amount.
- Do you regularly stay at docks with shore power? If so, you should be good.


I suggest starting with a spreadsheet to list out and calculate the amp-hr loads as your starting point.
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Old 22-05-2016, 23:45   #6
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Re: Wattage Usage on a 30' sailboat

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Originally Posted by valhalla360 View Post

Solar Panels: Divide the rated wattage by 12V and multiply by 5 or 6hr to get an estimate of amp-hrs produced in a clear day.
Dividing wattage of the panels by 12 is too optimistic. 12 volts will not charge a battery - 14.4 is typical for solar controllers set for Fla batteries. I divide by 15 for a slightly conservative estimate.

120 watts divided by 15 is 8 amps. Figure about 5 hours at full output. The 40 AH estimate is accurate for a full sun day with a PWM controller. About 15 to 20% better with an MPPT controller.
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Old 23-05-2016, 01:17   #7
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Re: Wattage Usage on a 30' sailboat

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Originally Posted by mitiempo View Post
Dividing wattage of the panels by 12 is too optimistic. 12 volts will not charge a battery - 14.4 is typical for solar controllers set for Fla batteries. I divide by 15 for a slightly conservative estimate.

120 watts divided by 15 is 8 amps. Figure about 5 hours at full output. The 40 AH estimate is accurate for a full sun day with a PWM controller. About 15 to 20% better with an MPPT controller.
I missed the last part about accounting for losses and I would consider actual vs nominal voltage part of that. A 12v system almost never runs at exactly 12v but close enough.
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Old 23-05-2016, 08:09   #8
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Re: Wattage Usage on a 30' sailboat

I was way off base ... the batteries are starting batteries of the EVERSTART variety, being the worse of the Walmart brand batteries(24MS) and the best of the worse(24DC), with respective AH of about 80+AH & 125AH. My third battery(house), is a size 29(about 210AM)

I spent half the year timing my Frigoboat(with keel cooler), as running no more than 15 minutes per hour(daylight) and what seemed to be about half as many cycles at night/early morning. Frigoboat manual/papers indicate a draw of 3 amps, which generally requires 4 cycles to add up to an hour's runtime. By my conservative computation, I expect 25-35 Amps at 12 volts to be needed daily ...

ANYWAYS ... Now that I've cleared my little fog about the batteries ...

I'm trying to do two things ... use panels that are "easily", installed, and with enough wattage to get by(without overkill), on cloudy days, even if it requires me to down-draw my batteries by 20 amps or so on rainy/snowy days(which I assume is the worse of all conditions).

I'm not a "power" user ... I am just wanting to have a "little" more than I generally have when camping ... and I'd like to hear from others who are also getting by on 120-150 or so watts per hour-solar.
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Old 23-05-2016, 08:34   #9
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Re: Wattage Usage on a 30' sailboat

Should you head south where it is hot all the time your fridge will probably run much more than your estimate. At least 60 amps hours day.
Refrigeration is your biggest use of electrical power.
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Old 23-05-2016, 08:36   #10
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Re: Wattage Usage on a 30' sailboat

Like you we survive on a modest 31 ft yacht with 185w of solar which will give 10AH in strong mid day sun and not a lot in the pouring rain of a UK summer.

Fridge, laptops and lights etc use about 30-40 amps per day on average, but running the heating another 25 amps easily. At sea GPS, auto pilot VHF and nav lights a little more so don't have a problem running the engine for a few hours on a night time passage.

If you can find the space another 60 or 80w panel which is a suitcase portable type is very useful when anchored as it can easily be moved to face the sun several times a day. We haven't noticed any change in the fridge power usage between hot and cool weather. Like you its on for 3 mins and then off for 15 or so.

So I think you are nearly there and summer sailing will provide you with the answer, if not another portable large panel will do nicely. Add two and you can cook with a small slow cooker

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Old 23-05-2016, 08:57   #11
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Re: Wattage Usage on a 30' sailboat

Here's a few things to keep in mind. With a bunch of small panels that are not mounted up on an arch it will be very hard to keep them all from being shaded. Shade on a small part of a panel significantly reduces output. Also remember that you never get 'full' sun in Ohio b/c you are too far north. So it is unlikely you will see rated output on your panels, ever, at that latitude.

The altestore has several good solar calculators here..
https://www.altestore.com/howto/calculators-c36/

Using 420w daily use based on your est. (35Amp x 12V = 420W) and Cleveland as the location, It says you need 290W of unshaded solar panel. I also think you are going to draw more than 35A as a live a board.
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Old 23-05-2016, 09:00   #12
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Lightbulb Re: Wattage Usage on a 30' sailboat

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Originally Posted by SURV69 View Post
I was way off base ... the batteries are starting batteries of the EVERSTART variety, being the worse of the Walmart brand batteries(24MS) and the best of the worse(24DC), with respective AH of about 80+AH & 125AH. My third battery(house), is a size 29(about 210AM)

I spent half the year timing my Frigoboat(with keel cooler), as running no more than 15 minutes per hour(daylight) and what seemed to be about half as many cycles at night/early morning. Frigoboat manual/papers indicate a draw of 3 amps, which generally requires 4 cycles to add up to an hour's runtime. By my conservative computation, I expect 25-35 Amps at 12 volts to be needed daily ...

ANYWAYS ... Now that I've cleared my little fog about the batteries ...

I'm trying to do two things ... use panels that are "easily", installed, and with enough wattage to get by(without overkill), on cloudy days, even if it requires me to down-draw my batteries by 20 amps or so on rainy/snowy days(which I assume is the worse of all conditions).

I'm not a "power" user ... I am just wanting to have a "little" more than I generally have when camping ... and I'd like to hear from others who are also getting by on 120-150 or so watts per hour-solar.
I regard anyone who has and on board refrigerator as a "Power User" because it's the biggest overall near constant power load 24/7. Everything else is minor like your VHF radio (AIS) which will run all night also along with your LED lights.New 3G radar uses less power then a cell phone. I suggest more panels with MPPT controller. Maybe one of those 3rd party Prius Hybrid batteries($2,500)would be nice. I may look into that myself one day. A floor scrubber battery is good very deep cycle also.You can dig into them 85% before you do any damage to them. You have to do a lot of research to get whats right for your boat and your needs. I never had a walmart deep cycle battery last more than a year. you get what you pay for. Make sure you size your wire according to your amperage. Most of the tips from the other posts are exceptionally good.
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Old 23-05-2016, 09:02   #13
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Re: Wattage Usage on a 30' sailboat

I agree with those who question your battery data. 100-120 AH per battery sounds much more likely. Adding up your AH you can get a maximum bank capacity. My guess is that your 2 battery bank is about 200 AH. Assume you may use no more than 40% of that without significant damage. That gives you a budget of 80 AH before recharging.

The calculation of 40 AH solar output on a nice day sounds reasonable. Your usage estimates seem strange. If the frige manual says 3A I would use that as an average over a 24 hour day or a draw of 72 AH daily. Your TV and MP3 player depends on your usage but these are low consumption items. You need to go through everything including on board lighting, navigation and anchor lights.

You also need to figure that if you run your batters down to 60 or 70% it will take some time to get near capacity on engine or shore power. You might be best off doing more research before buying anything else. Marine electronics can be very confusing.

Good luck with your venture.
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Old 23-05-2016, 10:12   #14
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Re: Wattage Usage on a 30' sailboat

Yup I agree that fridges are the heaviest consumers of power but a basic necessity in my books.

I battle to draw less that 120 amps a night !


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Old 23-05-2016, 10:22   #15
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Re: Wattage Usage on a 30' sailboat

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Originally Posted by SURV69 View Post
I was way off base ... the batteries are starting batteries of the EVERSTART variety, being the worse of the Walmart brand batteries(24MS) and the best of the worse(24DC), with respective AH of about 80+AH & 125AH. My third battery(house), is a size 29(about 210AM)
210AM ? What is that?

I agree - if you have refrigeration you are a power user. Measure the amp draw when the compressor is running with a clamp meter. The manual will not tell you accurately. That and run time per hour x 24 will give you real usage where you are located. As posted multiply that by 1 1/2 or 2 times if you go south.

Best battery bank for the dollar is 6 volt golf cart batteries - 4 in series/parallel ideally for a total AH of 440 to 480.

Make sure you have an MPPT controller for best results from the panels. Try to avoid any shading, although that may be hard to do with 4 small panels.
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