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Old 10-12-2006, 17:18   #1
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how many amp hours from a solar panel or wind generator

I am curious to know how you work out how many amp hours you can get from say a 100w solar panel or a 400w airx wind generator. Now i realize it will only be a rough average but there must be some sort of formula otherwise how do you know haw many panels etc you need???? Oh if it makes any difference i live in sunny QLD Australia.
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Old 10-12-2006, 19:36   #2
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For solar panals the usual rule of thumb is

0.3 x Rated watts = Amp-hours/day

So, for a 100 W solar panel expect 30 amp-hours per day.

Better if you have a good system to point the panels to the sun, worse if you have lots of shady spots on the panels.

Bill
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Old 10-12-2006, 20:16   #3
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The 0.3 x Rated watts formula that Bill mentioned is a good estimate, in my experience. This formula assumes a 12V system.

If you are travelling far north or south, or the skies are overcast, the power delivered will be less. It isn't a bad idea to do some calculations assuming a couple of cloudy days in a row, and see how your batteries and other charging sources hold up.
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Old 10-12-2006, 22:08   #4
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Viking, if you go to the Construction Forum above, you will see a post by GordMay titled "Ohm's Law and boats". This will give you the formulae for calculating power, voltage, current etc etc.
The factor that Bill has put into the equation is the loss of efficinecy due to Sun angle, cloud, shadows etc. this brings down what could be expected as a 100% charge rate to a more realistic charge rate of only 30%. So of course, YMMV depending on the sun angle more or less cloud and maybe or maybe not having shadows cast upon the array.
For wind, that can be a little more difficult. It depends on dependability of wind and wind strength.
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Old 11-12-2006, 01:02   #5
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Alan,

The 0.3 in the formula isn't really an efficiency derating factor, but a conversion from watts to amp-hours (@ 12V), taking into account the amount of sunlight a panel will see over 24-hours. It goes something like this:

A 100W panel in full noon sun puts out about 8A at 12V (8A x 12V = 96W)
If the full sun was hitting the panel 24-hours a day, the output would be 8A x 24H = 192AH.
Since during a 24-hour day, the panel will only put out full power for a couple of hours, the output will be much less. It looks like the 0.3 number assumes the equivalent of 3.7 full-sun-hours per day. I haven't done the trigonometry for the sun angle effects, so there may be some efficiency factor thrown in, but the bulk of the equation is for the Volts - Amps - Watts conversion and the solar power throughout the day.

If you are able to orient the panel to track the sun throughout the day, that will definitely improve things (but who actually does that???). Using one of the new MPPT regulators should also help. Clouds and shadows remain a big factor, of course this depends on where you are sailing.

On VALIS, I have three 100W panels on top of the dodger, pointing straight up. Given the effects of rigging and sail shadows, I figure that one of the panels will be useless, on average, and assume 300W * 0.2 = 60AH per day output from the panels. I usually get more, sometimes less.
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Old 11-12-2006, 03:26   #6
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Viking, I'd encourage you to reflect on what your realistic plans are for your boat when evaluating the windgen's output. Your current location would be generally favorable for the use of an AirX, altho' if you are berthed up a canal or in the cove of an estuary, its output won't reflect what's possible when coastal passagemaking. If you have sailing plans N of Queensland, you'll get a steady output and find a windgen to be very productive. Southwest Windpower does offer an output chart and, if you make some assumptions about your location and talk to a few sailors who've been there, you can estimate what to expect from it.

To give you some real-world examples, we spent this last summer in the W Med, hardly known for its windy weather. We have a 1st generation Air Marine 303 wind gen (2 models earlier than the AirX you are considering)and the output we saw - using a Link 2000R to tally cumulative windgen output in amp/hrs - ranged from 0 to 100 amp/hrs/day, very much a function of the weather pattern. (Friends we cruised with for a while have an AirX and they were seeing more output, perhaps by 25%). In the mild summer weather of the Baltic, perhaps 35 amp/hrs/day was a fair average. On a relatively windy cross-Biscay passage, we could run the boat 24/7 on the windgen's output even tho' at-sea amp consumption is usually higher than coastal sailing. We were perhaps drawing 80-100 amp/hrs/day on that run. In the Caribbean, which does have quite varied winds over the course of 4 seasons, I think it's fair to estimate that we saw an average output of 50-70 amp/hrs/day for most days over a period slightly less than 2 years.

Because there's a formula for solar panel output, it invites one to think of panels as a more reliable and therefore more usable form of electricity, but I think that's a perception at odds with the reality that a good windgen will in cruising mode usually outperform several large panels over any extended period of time. (The more relevant issues seems to be mounting geometry, available space for panels and noise, vibration removal for windgens). And FWIW, after relying on our AirMarine unit for the last 6+ years - and thinking that nothing lasts forever - we just bought an AirX to be installed when we return to Malta, even tho' our current unit still performs normally. Even in the alleged 'windless Med', we would hate to be without a windgen.

Hope this is of some help to you.

Jack
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Old 11-12-2006, 10:29   #7
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The 0.3 factor is not really a "conversion factor". It is simply an empirical number that lumps together all of the various unit conversions and efficency losses. It can't be derived from "first principles" rather it is a number just based on experience.

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Old 11-12-2006, 11:09   #8
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Thanks guys for all you information it has been realy helpful.
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Old 11-12-2006, 12:39   #9
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I almost bought an Air-X, brand new in the box over at e-bay..Got it for $610.00, but before I pusehd the button, did some Google searching and found the out-put was quite a bit less than advertised.

Here is one of the links I found;

Paul Gipe's Air curves

Due to plenty of cloudy days every time I go sailing, a wind generator would be on my list, but so far I have not decided. KISS seems to put out and be popular among cruising folks, but $400.00 more than the Air-X.

As for solar output: The .3 factor mentioned above seems to be quite accurate based on a few years experience with 2 Siemens 75W panels in Florida and the Bahamas.
On a perfect day, in the Exumas in June, at noon with the sol overhead and no air pollution, I have seen 10.3 amps being produced, quite a bit over the rated power of 8.8 amps. Unfortunately, those days are rare....
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Old 11-12-2006, 13:00   #10
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High output = expensive. Kiss and DUOGEN d400 are about the same price/output, and cost about double that for a Rutland 913 (which only has half the output)
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Old 11-12-2006, 18:10   #11
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Quote:
High output = expensive.
Yup, ya get what ya pay for..My beef is with the claims from Air-X: They claim high output, but don't deliver.

Called false advertising. Pretty serious stuff in most civilized countries, there is law-suits and jail cells to follow.
I would be happy to buy an Air-X for half price and half output, but not if some used car salesman is lying into my face claiming double the horsepower for half the price.
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Old 12-12-2006, 03:14   #12
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CSY Man suggests that ”... false advertising. Pretty serious stuff in most civilized countries, there is law-suits and jail cells to follow ...”

I just wish it were so!

You’ve got money, I’ve got pockets: Canadian humorist Stephen Leacock said, "Advertising may be described as the science of arresting the human intelligence long enough to get money from it."

Most jurisdictions have false, misleading, and/or deceptive advertising laws, usually in the form of consumer fraud or deceptive practices statutes. Under these laws, officials can seek injunctions against unlawful ads, and take legal action to get restitution to consumers. Some laws provide for criminal (misdemeanor) penalties (fines and imprisonment), but criminal proceedings for false advertising are rare, unless fraud is involved.
I cannot recall a case where a jail sentence was actually imposed for false advertising.

Here's a fact that may surprise you: candidates have a legal right to lie to voters just about as much as they want.
False Ads: There Oughtta Be A Law! Or -- Maybe Not. - FactCheck.org
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