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Old 24-05-2016, 00:46   #31
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Re: Wattage Usage on a 30' sailboat

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Originally Posted by SURV69 View Post
and ... if a person talks about a TV drawing 4 amps, it can be generally assumed to be per hour, since 4 amps per minute or per 24 hours is obviously, each grossly incorrect.

.
Actually, this is a perfect example of why the use of proper units are important.

Technically 4amps per hour implies, the TV starts out drawing some unknown amperage (X) and increases to X+4 amps draw after an hour, so assuming amps per hour makes no sense. Technically, even this isn't true as the TV could be on for 1/2 hour and the ending amp draw X+2 amps never reaching X+4 amps. This is the one option, where it is safe to assume the person writing it simply doesn't understand the units.

But in your example there are actually two very realistic possibilities for what you meant and they differ by a factor of 2 on the resulting demand on the battery bank:
- If you TV draws 4 amps, we need to know how long it is in operation to know how many amp-hrs you are using. Without that we have no idea how big of a battery bank to suggest nor the charging system.
- Or it could mean you an efficient TV that draws 2amps and you typically watch 2 hrs of TV per day, thus using 4 amp-hrs but that's not clear.

If it was just the TV drawing 4 amp-hrs vs 8 amp-hrs, it would be no big deal but assuming you made the same error on all your calculations, you could wind up with a battery bank half the size of what you need.
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Old 24-05-2016, 04:04   #32
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Re: Wattage Usage on a 30' sailboat

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Originally Posted by SURV69 View Post
while I understand the perceived need for proper terminology, keep in mind that if a person is talking about 12 volt panels, it can generally be assumed that a 12 volt system is being discussed.

and ... if a person talks about a TV drawing 4 amps, it can be generally assumed to be per hour, since 4 amps per minute or per 24 hours is obviously, each grossly incorrect.

Typically, if a person's target is to operate certain items that will somewhat approximate what 4, 12volt, 30watts panels might deliver, the general premise could be that someone wants to make use of items would require something in the area of 500-600 watts ... most likely at 12 volts and therefore at something approximating 30 or so amps, and probably per day, as in 24 hours. With no more specificity than just given, most people would know the time frame and could continue the discussion without basically stating that without more specificity, the given figures are, almost contemptuously worthless.

If someone stated that Hank Arron hit 45 homeruns ... virtually nobody would have to ask if it was in one game or one lifetime.
You are still missing the point. Maybe this will make it clear:

120 Watts of solar panels will deliver around 600 Watt hours per day (on the common accepted figure of 5 hours equivalent solar input)

To a first approximation, that is 50 Amp hours.

Which means that if your average current draw is more than about 2 Amps, you are in deficit.

If your TV draws 4 Amps, it will use 2 of those 50 Amp hours if you run it for 30 minutes per day. If you run it for 3 hours per night, it will use 12of those 50 Amp hours.

You said earlier that your fridge draws 3 Amps and averages maybe 12 minutes per hour. So it will draw an average of 0.6 Amps or about 15 Amp hours per day.

It really is important to understand the difference between Amps and Amps hours.
Tell a marine electrician that your average usage is 30 Amps, and he will grossly overspec your systems - because that means you will be using 720 Amp hours per day and will need a huge battery bank and lots of generating power.
Tell him that your average usage is about 30 Amp hours and you will get the system you need.
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Old 24-05-2016, 07:43   #33
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Re: Wattage Usage on a 30' sailboat

Coastal Climate Control has some informative blogs regarding solar panels on boats, like this one about clarifying some general mis-informtion http://coastalclimatecontrol.com/index.php/blog/165-10-myth-busters-solar-panels-on-boats.html
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Old 24-05-2016, 11:46   #34
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Re: Wattage Usage on a 30' sailboat

If instead of amps, we used the term "coulombs per second" it might simplify things a little!

Then it would be more obvious how little sense (none at all) it makes to refer to amps per hour - coulombs per second per hour is obviously nonsense.

It makes as much sense as "fahrenheit per hour" or "RPM per hour".
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Old 24-05-2016, 13:13   #35
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Re: Wattage Usage on a 30' sailboat

It is not amps per hour but amp hours. It makes as much sense as foot pounds to anybody who survived a course in physics or basic engineering. It is a very useful measure approximating the peak capacity of a battery or for comparing different batteries of the same voltage. It is the basic tool to determine the required size of a house bank given the requirements (current required and hours per day used) of the various appliances on board.
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Old 24-05-2016, 13:47   #36
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Re: Wattage Usage on a 30' sailboat

Kmac, your 60 w panel puts out maximum 5 a at 12 v. Using the typical 5 hours of sun per day that 25 AH feeding into your batteries (actually a lot less as yor batteries approach maximum charge). I find it hard to believe your battery is rated at 20 AH. A more usual number would be 100-130.
Your problem is not your panel but your battery bank Ignoring the small draw of the anchor light and limited use of the cell phones you seem to have 2 devices you need for sailing, the computer (for navigation) and your depth finder. Typical numbers for a depth finder are 7a, my laptop at 12v draws 10a. If you sail 5 hours a day you are burning 17x5=85 Ah as your panel is generating something less than 25 AH. Your battery capacity at end of day will be down 60 AH and unless your battery bank is at least 200 AH you have a problem. Of course, if you only use your navigation program and depth finder for limited periods things will be fine. I think it helps to think of it as a budget problem. If you don't watch your spending you can use up your income and deplete your savings. Hope this clarifies things for you.
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Old 24-05-2016, 14:24   #37
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Re: Wattage Usage on a 30' sailboat

Can;t add much to the other responses already, just ancedotal. I'm in the caribbean sun right now with 7 36W panels (250W) on a 27ft. With full midday sun I see 13A coming thru sometimes. A good day will be just north of 80Ah (just over 1Kwh). I power a tiny little engel fridge which takes about 1.8A on average (say 2A x 24hr = 50Ah). That already most of I get even on a good day. That along with lights, fans and possibly an autopilot / chartplotter, I'm lucky to come out ahead. No way with a computer! It would be great to have another panel or two, or, even better, a windgenerator to compliment the panels.

Your energy production will not go as far as you think it will, but whatever you got is better than nothing.
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Old 24-05-2016, 15:09   #38
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Re: Wattage Usage on a 30' sailboat

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Originally Posted by Dave22q View Post
Kmac, your 60 w panel puts out maximum 5 a at 12 v. Using the typical 5 hours of sun per day that 25 AH feeding into your batteries (actually a lot less as yor batteries approach maximum charge). I find it hard to believe your battery is rated at 20 AH. A more usual number would be 100-130.
Your problem is not your panel but your battery bank Ignoring the small draw of the anchor light and limited use of the cell phones you seem to have 2 devices you need for sailing, the computer (for navigation) and your depth finder. Typical numbers for a depth finder are 7a, my laptop at 12v draws 10a. If you sail 5 hours a day you are burning 17x5=85 Ah as your panel is generating something less than 25 AH. Your battery capacity at end of day will be down 60 AH and unless your battery bank is at least 200 AH you have a problem. Of course, if you only use your navigation program and depth finder for limited periods things will be fine. I think it helps to think of it as a budget problem. If you don't watch your spending you can use up your income and deplete your savings. Hope this clarifies things for you.
It's a 20AH lithium battery. A depth finder does not draw 7a. More like 0.1 amp. Your tablet doen not draw anywhere near 10 amps at 12V. Not even a tenth of that. I only use about 4 or 5 amp hours a day. I have a Victron Energy BMV700 Precision Battery Monitor and will get actual numbers this weekend. This is the battery: Elite Power Solutions
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Old 24-05-2016, 15:53   #39
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Re: Wattage Usage on a 30' sailboat

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You are still missing the point. Maybe this will make it clear:

120 Watts of solar panels will deliver around 600 Watt hours per day (on the common accepted figure of 5 hours equivalent solar input)

To a first approximation, that is 50 Amp hours.

Which means that if your average current draw is more than about 2 Amps, you are in deficit.

If your TV draws 4 Amps, it will use 2 of those 50 Amp hours if you run it for 30 minutes per day. If you run it for 3 hours per night, it will use 12of those 50 Amp hours.

You said earlier that your fridge draws 3 Amps and averages maybe 12 minutes per hour. So it will draw an average of 0.6 Amps or about 15 Amp hours per day.

It really is important to understand the difference between Amps and Amps hours.
Tell a marine electrician that your average usage is 30 Amps, and he will grossly overspec your systems - because that means you will be using 720 Amp hours per day and will need a huge battery bank and lots of generating power.
Tell him that your average usage is about 30 Amp hours and you will get the system you need.
Why try and teach Kindergartners Calculus?
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Old 24-05-2016, 15:56   #40
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Re: Wattage Usage on a 30' sailboat

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Originally Posted by Anna Sail View Post
Coastal Climate Control has some informative blogs regarding solar panels on boats, like this one about clarifying some general mis-informtion http://coastalclimatecontrol.com/index.php/blog/165-10-myth-busters-solar-panels-on-boats.html
As soon as I read "To calculate daily 12v amp/hour yield:" I discounted what they are saying. If they can't even understand the basic units, they shouldn't be in the business.


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Old 24-05-2016, 16:00   #41
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Re: Wattage Usage on a 30' sailboat

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Originally Posted by kmacdonald View Post
Why try and teach Kindergartners Calculus?
It's not Calculus, it's simple arithmetic. A x B = C.
Amps x Hours = Amp Hours.

And those Kindergartners are playing with electrickery. That's dangerous if you don't understand what you are doing and miscalculate current and wire sizes.
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Old 24-05-2016, 16:11   #42
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Re: Wattage Usage on a 30' sailboat

It has been explained here and elsewhere countless times. If they don't understand it by now they never will. That's why most old boats electrical systems need to be redone.
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Old 24-05-2016, 16:22   #43
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Re: Wattage Usage on a 30' sailboat

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Canada has their language police and in America we have home-owner's associations.


In Germany, in the 1940's they had the nazis.

Give a little latitude to those of us(me), evidently too stupid to understand the proper usage of A, Ah, wattage and voltage.
Why be too stupid to simply learn the proper terms? StuM has suggested this for many many electrical threads, and he's right.
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Old 24-05-2016, 16:30   #44
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Re: Wattage Usage on a 30' sailboat

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If someone stated that Hank Arron hit 45 homeruns ... virtually nobody would have to ask if it was in one game or one lifetime.
Rubbish. 45 homers would be in one season. NOT one game, and certainly NOT lifetime. Geez....

Please, please, please, listen to what we're trying to tell you and stop arguing with the great input you're getting.
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Old 24-05-2016, 18:03   #45
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Re: Wattage Usage on a 30' sailboat

The best way to answer the original question is it takes very little energy if you don't have refrigeration and do have all LED lights. Of course cooking with electric is out of the question. If you need refrigeration, microwave, coffee pot, etc, plan on spending an extra 5 to 7K on batteries, solar, wind generators, alternators, regulators, etc.
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