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Old 26-02-2014, 09:09   #1
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solar output question

We've just added 100 watts of solar panels to out boat for an upcoming cruise from Victoria BC to Hawaii in May-June. Now working out the amp budget based on our gear. Is there someone who can give me an idea of how many amp hours we can expect, on average, per day for 100 watts?
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Old 26-02-2014, 09:20   #2
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Re: solar output question

Others way smarter than me on panels... Stab? 35ah?

bump.....
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Old 26-02-2014, 09:20   #3
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Re: solar output question

Should be about 8 amps per hour of useful sun light for a 12v system.
100 watts divided by 12 volts = 8.333 amps x (hours of useful light) = Amp Hours
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Old 26-02-2014, 09:29   #4
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Re: solar output question

It mixes up unit, but figure approx 30% of the wattage rating as daily amp-hour gain. So a 100 watt panel might put in 30ah a day back into the battery. With some luck you can get a little more. You never get them compeletely shade free and pointed to the sun all day, so the 8.3ah rating is only for a few hours a day.
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Old 26-02-2014, 09:33   #5
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Re: solar output question

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Originally Posted by ontherocks83 View Post
Should be about 8 amps per hour of useful sun light for a 12v system.
100 watts divided by 12 volts = 8.333 amps x (hours of useful light) = Amp Hours

That's only if the Vmp of the panel coincides with 12v

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Old 26-02-2014, 09:46   #6
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Re: solar output question

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That's only if the Vmp of the panel coincides with 12v

Dave
Bingo... Which it doesn't....

100w/14+Vx12hx50%max outputx80%efficiency=34ah x fudge low... =30ah
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Old 26-02-2014, 09:47   #7
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Re: solar output question

I have an iPhone app called "sun seeker." It gives you the angle (elevation) of the sun throughout the day based on your locale. Since the maximum output of a panel is rated with the panel perpendicular to the sun, if the panel is horizontal (most common case on a boat), the sun will almost always be at an angle. To calculate the maximum "potential" power of your panel, take the elevation of the sun from the horizon and subtract from 90 (the perpendicular) - this gives you the angle off of the perpendicular (still with me?). Now, using your iPhone calculator, take the cosine of this angle. The result can be multiplied against your max rated panel power - assuming the cosine is .75, then the maximum potential of your panel is .75 X 200 or 150 watts. Of course, each hour of the day will result in a different potential. Then you have the losses of your controller/charger - an MPPT will maximize the charge to the battery.
This does NOT take into consideration overcast or rainy days or dirty panels. If you get a meter, you can read the sun's power at any given time - since panels are rated at 1,000 - if your meter reads 800 when pointed at the sun, you have another discount of 20% to deduct. Or you can save the math and hold the meter pointed straight up (same orientation as panel), and assume it reads 600 - then you're getting 60% of rated power before losses from controller/charger (no need to deal with cosine, etc.). Of course, your controller should have a screen showing the amount of charging amps.

I find I get about 50% through the 6 hours in the middle of the day on average actually getting to the battery. But I'm in Florida.

It is a good idea to have the panel voltage higher than charging voltage - Mine are nominally 40v - wired in parallel to 80v - charging a 48v battery.
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Old 26-02-2014, 09:54   #8
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Re: solar output question

The panel output is more likely 18-19 volts before getting to the controller, so more like:
100 watts divided by 18.5 volts = 5.4 amps per hour x (hours of useful light) = Amp Hours

I guesstimate 5 good hours of sunlight equals about 25 amp hours back to the batteries.

That seems to be about what I get using 2-50 watt panels, sunsaver duo controller into a bank of 2 12 volt batteries wired in parallel.

Following general recommendations from MaineSail I also added an ACR so the house bank and starter battery share charging from solar and/or alternator as needed.
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Old 26-02-2014, 10:37   #9
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Re: solar output question

Quote:
Originally Posted by goboatingnow View Post
That's only if the Vmp of the panel coincides with 12v

Dave
Quote:
Originally Posted by HappyMdRSailor View Post
Bingo... Which it doesn't....

100w/14+Vx12hx50%max outputx80%efficiency=34ah x fudge low... =30ah
I was assuming 12volts. Are you seeing something I'm missing? Not sure how we know voltage yet? I figured he could deduce the rest if he had the basic math.
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Old 26-02-2014, 10:50   #10
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Re: solar output question

Ignore the wattage rating. What is the current rating of the panel (Isc)? Under optimum conditions this is near the current the panel will put out regardless of voltage (unless battery voltage is higher than panel voltage, in which case you get nothing). For a decent installation you should be able to get 3-5 hours equivalent to the rated output. For a 100W panel presume the rated Isc is 7-7.5A, so 21 to 35Ah/day is likely output with a decent installation.

All of the above assumes a very simple installation with no controller or PWM controller, panel a typical 12-14V nominal (18-21Voc), etc. YMMV.
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Old 26-02-2014, 10:52   #11
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Re: solar output question

I've made the Salish Sea to Puerto Vallarta trip (2000 nautical miles or so) three times on boats with solar panels as their primary energy source. The power output of the panels varies wildly as conditions change and as you move south. So I'd say- there is NO single easy or accurate answer to your question.

We have had four 125 watt solar panels since early 2000. Used them extensively in Salish Sea 2005-2010 and two trips from Tacoma to San Diego on to the Sea of Cortez. Also used them in San Diego and Mexico 2000 thru 2004. I have spent a great deal of time monitoring the performance of our solar system.

-Clouds - the slightest cloud shadow kills solar output
-Angle of sun relative to mast and backstay (shadows!)
-Location of panels relative to sails
-Type of sail in use
-Rolling of boat at sea significantly changes angle of solar incidence
-dust buildup on the panels

For example - flying the spinnaker early in the day shadows the front of the panels for several hours when sailing SSE off the Oregon coast.

Using the big overlapping drifter late in the afternoon when sailing WSW off the California coast shadows the panels.

Sailing SE along the Washington coast with a 5' quartering swell from the NW and 4' wind waves from the west makes the boat roll 20 degrees side to side and pitch 10 degrees up and down. The mid-July solar output (two 125 watt panels) in bright sunshine would oscillate from 14 amps @ 13.5V to 2 amps in synch with the rolling of the boat. The sails were not shadowing the panels but the panels angle of incidence to the sun was changing by over 45 degrees. And the mast/backstay was shadowing the panels as the boat moved about.

The good news is that, at least in our case, the panels made so much power that we never bothered to deploy our Ferris Towed generator. The solar panels always kept the batteries at almost full charge. Our underway energy consumption is about 100 amp hours / day and the panels easily made that.

My brother had a Tartan 42 with two 125 watt panels that we also used in the Salish Sea, Puget Sound to Puerto Vallarta, and a year in Western Mexico. The boat had less electronic and electrical stuff that our boat but he still used 75 - 80 amp hours / day while at sea. His solar panels kept the batteries fully charged.
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Old 26-02-2014, 10:52   #12
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Re: solar output question

Quote:
Originally Posted by HappyMdRSailor View Post
Others way smarter than me on panels... Stab? 35ah?

bump.....
Quote:
Originally Posted by ontherocks83 View Post
I was assuming 12volts. Are you seeing something I'm missing? Not sure how we know voltage yet? I figured he could deduce the rest if he had the basic math.
True that... re 12v... BUT... any charging SOURCE MUST BE well over battery potential.... Alts are 14.5+... Panels are way higher... you gotta figure current A directly with the potential V.... There is no other variation... Kirchoff man.... If he gave us V, we'd have a more refined answer, but power is power... No gettin around that either... Higher V=lower A

Effin A...
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Old 26-02-2014, 10:55   #13
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Re: solar output question

PS... I'm impressed with my stabber...
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Old 26-02-2014, 11:03   #14
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Re: solar output question

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Originally Posted by HappyMdRSailor View Post
True that... re 12v... BUT... any charging SOURCE MUST BE well over battery potential.... Alts are 14.5+... Panels are way higher... you gotta figure current A directly with the potential V.... There is no other variation... Kirchoff man.... If he gave us V, we'd have a more refined answer, but power is power... No gettin around that either... Higher V=lower A

Effin A...
Gotcha.....point taken.
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Old 26-02-2014, 11:14   #15
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Re: solar output question

We have this discussion every time a solar question comes up.
Quote:
Higher V=lower A
doesn't work with solar panels. Solar panels are a current source. They put out a specific amount of current. I grabbed the specs for a 100W panel. It has an Isc of 6.2A and a Imp of 5.8A at 17.8V. Between 0V and 17.8V it will put out about 6A regardless of the voltage. The amount of power it puts out depends on the voltage, but not the amount of current.

If the panel is operating at 14V (typical float voltage) it will still put out 6A, for a power of 6x14 = 84W. If it is running at 12.5V (depleted batteries) it will still put out 6A for power of 75W. The only way you will ever see 100W is if you can run the panel at it's "preferred" voltage of 17.8V.

All of the above based on STC, etc. and very simplified. Unless you are using an MPPT controller, ignore the power rating. Look at the Isc and Imp current ratings, you will see that current (at 25C panel temperature with 1000 W/m2 sunlight intensity at the panel surface) regardless of the voltage.
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