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Old 18-10-2012, 15:00   #1
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Be Honest, What do Your Panels Really Put Out...

I'm wondering if any of you have a real world guesstimate as to what your panels are actually putting out. Here is my experience.

We took a 10+ week trip in Florida with about 3+ weeks in Nov/Dec. and the remainder in Mar/Apr with a lot of sun. The fridge ran on the average about 10 hours a day at 3 1/2 amps and normally there was about 4 hours of computer time at about 2 1/2 amps I figured that on the average we were using about 45 amp/hours a day. Lights were battery powered so the only real users were the two things above. The amp readings were via a digital amp meter.

We had...



...180 watts of solar at the time on the boat and a good Blue Sky MPPT controller. During that total time period we ran the gen-set about 5-6 hours, so over 90% of the power came from the batteries. So I'm guessing that we were getting 40 to 45 amp/hrs a day from the solar into the two 12 volt (new) batteries in parallel.

So in our case if we divided the panel wattage (180) by 4 we would of had a number of 45 which is very close to what I think we were putting in. If this is a trend that matches what others have found then someone looking for the magic panel wattage for their needs could do an energy audit of what their needs were in amp/hrs per day and multiply by 4 and have a fair idea of how large of an array they needed in watts.

I think a lot of people are optimistic on what they are going to get from a panel. A 12 volt panel runs at about 17 volts and the wattage is figured on that. If you aren't running an MPPT controller you loose a fair amount of the advertised wattage right there since the panel is drawn down to battery voltage. As has been mentioned the panel output is usually lower since our panels are operating at a higher temperature than the temp they are rated at. Also who has the panel orientated to the sun at 90 degrees all day or ever in the day. Take all of that into consideration and you aren't ever going to get advertised wattage. Then what about shading on some days or from the boat itself?

My 'own' formula for figuring what I might expect as real output is as follows.

((Panel advertised wattage/17 (typical operating voltage))/2) X 7 (hours of fair sun) = amp/hrs per day

In our case that would work out to ((180/17)/2) X 7 = 37 amp/hr per day

The 37 is a little lower than what I think we were getting but then we did run the gen-set a little also.

One other thing is that as the batteries come up and can't accept as much charge you aren't getting it even if the array can put it out.

If our batteries go below 12 volts I'll run the gen-set and it will usually start with about 22-24 amps to the batteries and withing 15-30 minutes be down to about 10 amps. I'll shut it off and let the solar array finish the job.

So be honest guys what do you think your arrays are putting out not what you see for a few minutes at peak sun,

Sum
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Old 18-10-2012, 15:24   #2
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Re: Be Honest, What do Your Panels Really Put Out...

I sure don't know where or how you put your formula together, really don't want to go through the analysis. But I ran my two DM 145 watt panels through it, the result was in the ball park....maybe. I am not using a battery charge monitor just a digital current meter. My result was 60 ampere hours.

The highest current I have seen so far is 12 amperes with the two panels operating in parallel. For that reason, I ordered another pair from Amazon that now sit in my garage where they will stay until next spring's launch. OH, I am using ProStar 30 ampere for my controller.

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Old 18-10-2012, 16:09   #3
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Re: Be Honest, What do Your Panels Really Put Out...

On a full sun day, we average 150-170 Ahrs from our 4 120W panels mounted flat on a hardtop bimini. If there is some cloud coverage throughout the day, we average 110-130 Ahrs. Today was thin clouds until 1pm, then rain. We got 100Ahrs.

We are in Panama with 90F+ air temps, intense sun (when sunny) and 12hrs between sunrise and sunset. We got much better output in Connecticut USA during the summer time when the temperatures were cooler and the days were longer.

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Old 18-10-2012, 16:38   #4
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Re: Be Honest, What do Your Panels Really Put Out...

Edit Note: Since I can't edit my original post I just wanted to add that in the paragraph under the picture there is a sentence with...

"so over 90% of the power came from the batteries."

that should read....

"so over 90% of the power came from the solar array."

Thanks guys for the input,

Sum
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Old 18-10-2012, 18:42   #5
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Re: Be Honest, What do Your Panels Really Put Out...

A good rule of thumb, confirmed by the data above, is that a panel of x watts will put out x/3 amphours (at 12 v) on a full sun day in moderate lattitudes in the summer.
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Old 18-10-2012, 18:44   #6
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Re: Be Honest, What do Your Panels Really Put Out...

You are needing to put at least 10ahrs each day as lost power into lead batteries & next time they need replacement look at the LIFEPO4 of which a lot has been supplied on this forum. That might just tip the balance for your installation to give all you need. Yes panels lying flat is about .5

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Old 18-10-2012, 18:53   #7
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Re: Be Honest, What do Your Panels Really Put Out...

Quote:
Originally Posted by djmarchand View Post
A good rule of thumb, confirmed by the data above, is that a panel of x watts will put out x/3 amphours (at 12 v) on a full sun day in moderate lattitudes in the summer.
Like you said on a full sun day we might of gotten that much a couple times, but for us X/4 is a better average if you take the good, the bad and the ugly,

Sum
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Old 18-10-2012, 19:03   #8
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Re: Be Honest, What do Your Panels Really Put Out...

The MPPT will only continue to charge you in marginal light if the array voltage is at or above the minimum charge voltage. For this reason, we purchased pannels with open circuit voltage of 54 VDC. Our system is 24 volts so we can charge even as the clouds cover or the sun drops. I have considered a switch system to change the three panels form parallel to series to continue to generate in lower light but the gain might be too small to worry about and the risk of damage to the MPPT is real if we forget to switch back. Our MPPT can handle 118 VDC input.

All of this aside, I assume we will probably max out at no better than 50% of the rated output due to angle of incidence, clouds, temperature, sail shading. We have three Sanyo 220 watt panels rigidly fixed above the bimini. The boat is a ketch so there is no other place to mount panels.
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Old 18-10-2012, 19:08   #9
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Re: Be Honest, What do Your Panels Really Put Out...

I've got twin 130s on a horizontal solar bimini, and I can reliably budget 90ah per day from that array.

My panels get a daily scrub, and are shadow-free most days until the sun is too low on the horizon to worry about.
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Old 18-10-2012, 19:54   #10
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Re: Be Honest, What do Your Panels Really Put Out...

You may try the panels in series after checking the max input of the MTTP unit you have. I have two 40V panels like what is used on house solar for 80V @ 5amps
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Old 18-10-2012, 20:46   #11
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Re: Be Honest, What do Your Panels Really Put Out...

i've got one 135 watt kyocera and i average about 45ah per day. so watts/3 seems to work out perfectly.

i have NO controller (hard wired to battery bank with switch and fuse and a ten foot run of 10 guage wire), the panel is mounted on a swivel bracket so, when i'm aboard, i can move it around for a better sun angle, it's up in the airstream so it stays fairly cool, and i'm in florida/bahamas so get pretty good sun angles.
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Old 18-10-2012, 21:00   #12
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Re: Be Honest, What do Your Panels Really Put Out...

Is your fridge and computer running on DC or do you have an AC inverter? There is some loss with AC inverters, and it isn't always small.
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Old 18-10-2012, 21:14   #13
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Re: Be Honest, What do Your Panels Really Put Out...

Quote:
Originally Posted by SunDevil View Post
Is your fridge and computer running on DC or do you have an AC inverter? There is some loss with AC inverters, and it isn't always small.
Fridge is DC and computer is a very low amp DC that I made....



Compurer-Chart Plotter-Navigation Index

....and runs on less than an amp for the computer itself. With 16 inch monitor 20 watts and when the long distance WiFi and router is running about 30 watts. Nothing on inverters.

You know maybe we are closer to X/3 now that I think about it since the controller is usually in acceptance by mid afternoon and float not to much later. If we were using more and the batteries didn't keep getting topped off then the controller would be sending more. Now most days we are just putting in what we are taking out and the array is actually capable of more if that makes sense.

So I'll go along with the x/3 or panel wattage/3 = potential daily amp hours as a good formula. Hopefully your guys insights will help others that are trying to size an array,

Sum
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Old 18-10-2012, 21:16   #14
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Re: Be Honest, What do Your Panels Really Put Out...

There is never really an "average" solar day; it's all over the map.

On a nice sunny day when we're not up against a cliff somewhere we'll see maybe 150ah. But really, there's no average worth noting.
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Old 18-10-2012, 22:40   #15
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Re: Be Honest, What do Your Panels Really Put Out...

Quote:
Originally Posted by Sumner View Post
Fridge is DC and computer is a very low amp DC that I made....



Compurer-Chart Plotter-Navigation Index

....and runs on less than an amp for the computer itself. With 16 inch monitor 20 watts and when the long distance WiFi and router is running about 30 watts. Nothing on inverters.


That is very cool. I like it. It looks like you have the same switches that I have to disconnect items too.

I'm not sure how much the wifi antenna support bars and mast are shadowing your panels (and what effect that will have).

I haven't used this at all, but it might be of interest.
http://www.lightobject.com/Programma...ring-P278.aspx
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