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Old 30-01-2012, 07:49   #16
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Originally Posted by Dreaming Yachtsman

When I first came across the word "heat" I momentarily blanked out and wondered, "What is that for?". Just last week I was complaining to someone that it was so cold in Puerto Rico that I actually had to close the hatches and pull a sheet over me at night to keep warm.

Eat your heart out.
I just came back from the pool... didn't go in, too cold for my taste even though it's 32C out here

ciao!
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Old 30-01-2012, 09:24   #17
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Re: How Do I Know How Many Batteries I Need ?

Thank you all for the info it helps ALOT! I am still a little fuzzy on how the inverter plays in. Would i figure out the amp hours of the inverter (3000W) based on the 12 volt input or would I use the AH from the output 120v?
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Old 30-01-2012, 09:38   #18
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Re: How Do I Know How Many Batteries I Need ?

Quote:
Originally Posted by jakegator View Post
Thank you all for the info it helps ALOT! I am still a little fuzzy on how the inverter plays in. Would i figure out the amp hours of the inverter (3000W) based on the 12 volt input or would I use the AH from the output 120v?
Based on the 12v input, of course, since this is what your batts supply.

A 1500 watt tea kettle used for 10 minutes a day would use 1500/13 = 115 amps * say 1.2 for a generous margin for inefficiency of conversion = about 140 amps /6 = about 23 amp/hours.
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Old 30-01-2012, 09:58   #19
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Re: How Do I Know How Many Batteries I Need ?

Long reply deleted as Dockhead beat me to it, but in his example I get slightly different numbers:

1500W demand (the kettle)
adjust for conversion (in)efficiency -> 1500*1.2 = 1800W required from battery
refer power to voltage, get current: 1800W @ 12V -> 150A

If used for an hour this would require 150Ah

With assumed total use time of 10 minutes (1/6 hour) -> 150/6 = 25Ah

Both numbers are close enough, but watch those Amps. Some serious cabling required.
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Old 30-01-2012, 11:06   #20
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Re: How Do I Know How Many Batteries I Need ?

I'm in agreement on the 400 ah battery bank, but think you will need more like 300-350 watts of solar to keep up with your 1 kwh per day load. You can save quite a bit of consumption with a 12v refrigeration system.
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Old 30-01-2012, 13:14   #21
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Re: How Do I Know How Many Batteries I Need ?

I understand that a dc marine/rv fridge would be more efficient but they cost so much more, with so little space. I did read on the link below (section 18-19 i think) about how to convert an AC fridge to DC. It seems pretty strait forward just replace the motor with a comparable one and the same for and lighting and maybe some wiring. Any thoughts on this?

Home Energy Conservation
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Old 30-01-2012, 14:46   #22
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Re: How Do I Know How Many Batteries I Need ?

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Originally Posted by jakegator View Post
I understand that a dc marine/rv fridge would be more efficient but they cost so much more, with so little space. I did read on the link below (section 18-19 i think) about how to convert an AC fridge to DC. It seems pretty strait forward just replace the motor with a comparable one and the same for and lighting and maybe some wiring. Any thoughts on this?

Home Energy Conservation
err no, most modern fridges are integrated units with the motor and compressor sealed. The best way would be to run a domestic AC fridge via an invertor, but its quite inefficient.
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Old 30-01-2012, 18:28   #23
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Re: How Do I Know How Many Batteries I Need ?

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Originally Posted by jakegator View Post
hello i tried to search for this topic but couldnt find anything, so here it goes. Im designing my electrical system that i want to be powered as much by wind and solar as possible, it will have an AC circuit that will run a fridge and some other appliance that i figure will come to around 1000WH per day? How do i know how many batteries i need and how many solar panels? I will be a liveaboard, but would prefer to use as little shore power as possible. If someone could explain how I do the math that would be great! for this example just throw a random number in for the DC side for now.

Thanks!
I don't see a 110v fridge drawing 1kwh/d, even a tiny cube. In addition to the actual draw of the fridge there will be an additional draw for the inverter to convert DC to AC, they aren't 100% efficient.

You should probably get one of those power monitoring devices (P3 - Kill A Watt) and watch the draw on your fridge for a week or so then divide the number of watt by the number of days to get a per day use. Early summer or very late summer would probably be the best time for the test. It will under-report for the height of the summer but be conservative for most of the year.

A fridge has a VERY high starting load when it kicks in so your inverter will have to be sized to match that load. Under-sizing the inverter will lead to early failure of the both the fridge and the inverter.

What you need to do is a comparison calculation:

Figure the cost of 110v.fridge, inverter, batteries, solar panels, etc for an AC system. Count on needing more batteries and more solar panels to make this system work compared to a 12v system. So you save on the fridge but pay more for batteries, panels and inverter.

Then figure the cost of 12v fridge, inverter, batteries, solar panels, etc for a strictly DC system. You will probably still want an inverter, but it won't need to be nearly as big.

I expect the cost comparison will be close to a wash. Then other considerations come into play: the added weight and space requirements for the extra batteries and solar panels, extra maintenance for 2 different power systems and decreased cost of replacing the 110v fridge.
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Old 30-01-2012, 19:27   #24
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Re: How Do I Know How Many Batteries I Need ?

I may have done the math wrong, the energy star rating was about 360KWH per year, so i divided that by 365 and got aproximatly 986 watts per day. I have done the cost comparison, and it is overall cheaper for the 12volt system, but what I get out of it is not enough and personally not worth the savings. I was planning to have an AC system either way. Please do correct me if I am wrong on the method for finding watts per day for the fridge, the number did seem awfully high to me... but I am by no means an electrician .
Thanks!
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Old 30-01-2012, 19:56   #25
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Re: How Do I Know How Many Batteries I Need ?

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Originally Posted by jakegator View Post
I may have done the math wrong, the energy star rating was about 360KWH per year, so i divided that by 365 and got aproximatly 986 watts per day. I have done the cost comparison, and it is overall cheaper for the 12volt system, but what I get out of it is not enough and personally not worth the savings. I was planning to have an AC system either way. Please do correct me if I am wrong on the method for finding watts per day for the fridge, the number did seem awfully high to me... but I am by no means an electrician .
Thanks!
You did the math right. The energy star rated refrigerators cost more, but you can get a 19 cu ft refrigerator which will run about 1000 watts per day. That might cost you $50/year to run at home, but its going to cost a whole lot more on the boat.
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Old 30-01-2012, 20:26   #26
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Re: How Do I Know How Many Batteries I Need ?

OK I have the DC and AC mostly worked out to run off of solar and wind more so wind as my areas average monthly wind speed is about 10mph as a low but usually higher, of course there will be days when neither will work and I may have to use either shore or a generator. I have looked into several options from rv and marine DC refrigerators to buying an old AC fridge that is belt drive and replacing the AC motor (which I really like its just im thinking those are getting pretty old), just what will work best for my needs is an AC one, I just cant make use of a DC marine fridge because of the size.
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Old 30-01-2012, 20:49   #27
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Re: How Do I Know How Many Batteries I Need ?

Good posts, just a couple things to add;

Whatever your battery capacity, you have to be able to replace what you use....

... extra capacity is good though as cloudy days (solar) and light wind days (wind) will happen.

Sounds to me like you are going to want something like 250w of solar (if you are right about 1000wh)... couple of 135w panels would be a good start... and a couple of 8d batteries ought to do it...

... might want to use a monitor, it will really help you know when to charge. I have a Link 10 which I love.... (link lite now).
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Old 30-01-2012, 21:01   #28
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Quote:
Originally Posted by jakegator
hello i tried to search for this topic but couldnt find anything, so here it goes. Im designing my electrical system that i want to be powered as much by wind and solar as possible, it will have an AC circuit that will run a fridge and some other appliance that i figure will come to around 1000WH per day? How do i know how many batteries i need and how many solar panels? I will be a liveaboard, but would prefer to use as little shore power as possible. If someone could explain how I do the math that would be great! for this example just throw a random number in for the DC side for now.

Thanks!
Two. that's how many batteries you will need.

Get the best two batteries you can afford.

My daily energy budget is slightly higher than yours. I use a combination of wind and solar to charge when on the hook. I'm able to run that system nicely with two AGM 4D cells.

You will be able to do the same.
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Old 30-01-2012, 21:03   #29
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Re: How Do I Know How Many Batteries I Need ?

Really, I didnt expect only two thats good news, I still have to calculate some of the DC needs for a grand total, but I suppose its looking good so far!
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Old 30-01-2012, 21:04   #30
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Re: How Do I Know How Many Batteries I Need ?

Quote:
Originally Posted by jakegator View Post
I may have done the math wrong, the energy star rating was about 360KWH per year, so i divided that by 365 and got aproximatly 986 watts per day. I have done the cost comparison, and it is overall cheaper for the 12volt system, but what I get out of it is not enough and personally not worth the savings. I was planning to have an AC system either way. Please do correct me if I am wrong on the method for finding watts per day for the fridge, the number did seem awfully high to me... but I am by no means an electrician .
Thanks!
Sounds like you did the math right to me.

How big a fridge/freezer is it?

I calc the demand for the AC unit (1kwh/d) at about 100-110amp-hr/day, meaning you will need 300-400ah of battery capacity dedicated to the fridge plus capacity for your other demands. Thats 6-8 golfcart batteries for everything. (2-6v cart batteries give you about 200ah at 12v.
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