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Old 21-06-2011, 15:32   #1
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Rule of thumb for solar output

I know there are a lot of variables but is there a general rule of thumb that would allow me to estimate how many amps hours of output I could expect to see under optimal conditions per watt of solar array?

Something like "you should be able to get on a good day in the tropics "X" amp hours out of a 100 watt panel"?

Im looking at assembling a 4 panel array of 135 watt panels and using an outback flexmax 60 chrage controller and Im trying to get a ballpark estimate on how many amp hours on a good day I might get.

Thanks,
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Old 21-06-2011, 15:45   #2
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Re: Rule of thumb for solar output

For the middle 4 hours of the day in the tropics with 2 x 120 watt pannels I get a bit over 10 amps/h sometimes up to 13.
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Old 21-06-2011, 15:56   #3
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Re: Rule of thumb for solar output

Rule of thumb I have seen repeated, which I found to be reasonably accurate for my panel:

take rated watts, subtract 10%, then divide by 2 to get expected panel output.

For example, my 120 watt kyocera panel:

120 watts - 12 watts = 108 watts
108/2 = 54 watts expected output.

that's about right, I see 4-6 amps/hour throughout the day.

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Old 21-06-2011, 16:03   #4
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Re: Rule of thumb for solar output

I find that a good approximation is A output (@ maximum) X 5 hours/day. It is actually an ALL day bell curve, but this comes close.

So, a 5A "max" panel should give you about 5A X 5hrs = 25 Ah/day. If you size your solar array @ 2X the needed A per day, you will be fine for those cloudy overcast days as well.

M.

PS... I don't know the V of your panels, but if they are 18 or 19V, then you should get around 140 Ah/day, on a good day. If your power need is 70 Ah, then you will get to 100% every day, even on very cloudy days. If your power draw is more than 70 Ah, conserve, downsize, insulate... get it down!
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Old 21-06-2011, 16:57   #5
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Re: Rule of thumb for solar output

Rule of thumb, confirmed by tests and numbers from Sandia National Labs and many others, is that if there's any such thing as an "average", ignoring your location, ignoring the weather, ignoring the time of year, it is that you will get the equivalent of four hours of full output, in one whole day.

Since you also lose 10% of the power for every 15 degrees (two hours) that your panels are not directly facing the sun as it shifts around...you can see there's room for variation there. But roughly, four hours times full output, for a "full" day of bright light.
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Old 21-06-2011, 17:23   #6
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Re: Rule of thumb for solar output

Here are some factors that can either increase or decrease the amps you place into a battery with solar.

1. PVM vs. MPPT controller, when dealing with panels over 14v MPPT can give up to a 20% boost.
2. Wide size. Larger ga. wire can make a difference, although not a huge one. Maybe 2-5%?
3. Battery Chemistry. FLA batteries will "deposit" about 1AH for each 1.3AH sent to them down the wire. This is due to the internal resistance of the batteries. Gel, AGM and LIFEPO4 batteries will do much better. Look at their perkert quotient for an idea of their ratio.
4. Fixed vs. Movable. Panels that can aim can make a significant improvement.
5. Shaded vs. open sun. Panels that are even partially shaded can drop their output very quickly.

Chris
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Old 21-06-2011, 20:49   #7
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Re: Rule of thumb for solar output

All that, and the technology is changing so fast. New panels are up to 20%+ efficient and more in the last year. Some of the hybrids even do better at low/diffused light than the panels made just 2 years ago... It is going to depend on angle to the sun, time exposure and technology.
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Old 21-06-2011, 22:18   #8
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Re: Rule of thumb for solar output

According to Beth Leonard's 'Voyager's Handbook', 2nd Ed. pg 235, the rule of thumb is panel's rated wattage divided by 4 = amp-hr per day.

I don't know if this is for fixed or tracking panels. Atom Voyages | Voyages Aboard the Sailboat Atom - The Atom SolarTracker adjustable solar panel mounts for sailboats shows a simple tracking mount. I expect that being diligent about adjusting the tracker, say every hour, will probably bring output up by an appreciable amount.

If you increase panel efficiency you will get more watts from the same sized panel or you will get the same watts from a smaller panel.
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Old 22-06-2011, 12:52   #9
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Re: Rule of thumb for solar output

Thank you all for your posts. Using the info provided Im thiniking I should then expect to get something like 120-150 Amp Hours per day from (4) 135 watt panels given good conditions. Now to figure out mounting solution....

Below is a photo of my boat. Im looking to mount (2) 135 watt panels on either side. Thinking I would like to stick them along the fixed stern rail that runs along the sides of the aft deck. I would like to be able to fold the panels down when not in use and also angle them on a fore/aft axis to track with the sun. (figuring on being anchored in the caribbean facing generally eastward).
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Old 22-06-2011, 13:59   #10
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Re: Rule of thumb for solar output

Demeter,
A good deal of what you got here already is accurate, but not really what you asked for.....
Quote:
Originally Posted by SV Demeter View Post
I know there are a lot of variables but is there a general rule of thumb that would allow me to estimate how many amps hours of output I could expect to see under optimal conditions per watt of solar array?

Something like "you should be able to get on a good day in the tropics "X" amp hours out of a 100 watt panel"?

Im looking at assembling a 4 panel array of 135 watt panels and using an outback flexmax 60 chrage controller and Im trying to get a ballpark estimate on how many amp hours on a good day I might get.

Thanks,
So, here, in a nutshell is the "general rule of thumb, for determining how many amp-hours per day, you'd get out of a solar array in the 'tropics'...."
No formulas, no "variables" to fudge, etc....

[Please note: I do love Beth Leonard, and do respect her well-earned reputation.....but most of her and Evans' voyaging is/has been in higher latitudes, and I'm not sure if she's had much experience with modern charge controllers (either PWM or MPPT).....]

1) The commonly accepted "Rule-of-Thumb" is:

In the tropics (assuming you don't have much, or any, panel shading)..... Take the total number of watts of your solar array and divide by 3 (or multiply by 0.33)....and that number equals the approx. number of Amp-Hours per Day (A/H-day) that you can expect from you array.....

(The above "rule-of-thumb" does not apply to high latitude sailing.....)



2) In addition to keeping the panels from being shaded, if you mount the panels in the clear, allowing for plenty of airflow under and around the panles (and/or spalsh some sea water on them occassionaly), keeping them cool.....And, if you use a good quality MPPT controller (Blue Sky, Outback, etc...), You can increase that number of A/H's per day, that you can expect from the array....

This is a controversial point, since it does rely on the subjective variables of "shading", panel temps, battery state-of-charge, etc.....
BUT, it is clear that if you have good sun angles (summertime or in the tropics), and have plenty of airflow under and around the panels, using a good MPPT controller, you CAN get significantly more than the old "rule-of-thumb"......
Although, I'm hesitant to post the number here, since there are many that discount it, and for certain you should NOT plan on this output.....but, if you do it right, you can get as much as 40% - 45% of your panel's wattage number out in amp/hours-day......

Do NOT discount the affect of panel temps on their output.....
If you can keep them cool, they'll output above their "specs"/"ratings", with high sun angles,,,such as in the summertime Bahamas, etc...



3) FYI, in addition to my current 520-watt solar array on-board, I've been using / insatlling solar on board boats on/off for over 15 years.....and have been using / installing solar on-shore in far remote loacels since the early 1980's....almost 30 years now.....and I've taught seminars on alt. energy, solar power, batteries, etc.....I've seen a lot of new stuff (PWM controllers were a great boost....and then came MPPT controllers, etc...as well as new panel designs, etc....)



4) Have alook at an article I wrote (w/ lots of photos) of my current 520-watt array on-board....
Solar Panels



5) Demter, as long as you can rig/insatll your panels in the clear, free of shading and with plenty of airflow.....sailing in "the tropics" (within the tropics of Cancer and Capricorn) you should see 200+ A/H-day average from your 540 watt array year long......and probably closer to 225 A/H-per day, average year long....(assuming you can keep the panels cool...)

Since shading really cuts down your daily average output, as does having hot panels, etc.....use the "rule-of-thumb" above as your guide, and try to mount them in the clear....
That's the best advice I can give...




I hope this helps...

John
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Old 22-06-2011, 14:35   #11
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Re: Rule of thumb for solar output

Wow I was not thinking I would get anywhere near 200 AH out of this array but if you say its doable under the right conditions I'll take your word for it. I probably need to post the mounting questions in a different thread. Im hoping to put the panels on the sides of the boat and be able to angle them but the mounting will be tricky.
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Old 23-06-2011, 08:46   #12
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Re: Rule of thumb for solar output

Quote:
Originally Posted by SV Demeter View Post
Thank you all for your posts. Using the info provided Im thiniking I should then expect to get something like 120-150 Amp Hours per day from (4) 135 watt panels given good conditions. Now to figure out mounting solution....

Below is a photo of my boat. Im looking to mount (2) 135 watt panels on either side...
... and also angle them on a fore/aft axis to track with the sun. (figuring on being anchored in the caribbean facing generally eastward).
Which might place your 2 port side panels on on a Northern exposure. This might cut your calculated production nearly in half.
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Old 23-06-2011, 09:31   #13
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Re: Rule of thumb for solar output

Yep, that is the trouble with mounting panels on the rails. Before you spent the $$$ on them, it might make sense to cut some cardboard the same size as the panels, and mount then where you think you might, go anchor, and watch what happens.

Access to the south is what panels need to get the most out of them. Your mizzen rig is what is going to cause you headaches. I don't think you will see anywhere near 200ah a day on your boat.

Thinking out loud, I wonder if you could rig them up with quick connectors so that you could move them to the front of the boat when at anchor?

Quote:
Originally Posted by GordMay View Post
Which might place your 2 port side panels on on a Northern exposure. This might cut your calculated production nearly in half.
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Old 23-06-2011, 09:48   #14
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Re: Rule of thumb for solar output

Quote:
Originally Posted by witzgall View Post
mount then where you think you might, go anchor, and watch what happens.
Then go to the Marina and fuel dock and see what happens.

Rail mounted panels can be ripped off by pylons etc.
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Old 23-06-2011, 10:10   #15
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Re: Rule of thumb for solar output

Not worried about pilings as my boat has quite a bit of tumble home making the stern side rails several inches inboard of the max beam in that area. I also dont frequent marinas or fuel docks very often. Had thought about bimini mounting but like to be able to stow the bimini. Either way this thread is wandering so I am going to move this to another that is looking for solar panel mounting suggestions thanks.
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