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Solar panel output simplified??


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Looking at adding solar, considering a Sunpower 170w panel.  I am old school and can rig, fix, service almost anything but don't really get electricity.  I am charging 2 wet cell 4D's, biggest draws are my refer (3.5amps when running) and a Dometic portable freezer (about the same draw).  All light bulbs are LED.  I understand it depends on hours of sun and intensity of sun but can someone give me a simple way to determine how many amps of output I might get?

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14 minutes ago, zenmasterfred said:

Looking at adding solar, considering a Sunpower 170w panel.  I am old school and can rig, fix, service almost anything but don't really get electricity.  I am charging 2 wet cell 4D's, biggest draws are my refer (3.5amps when running) and a Dometic portable freezer (about the same draw).  All light bulbs are LED.  I understand it depends on hours of sun and intensity of sun but can someone give me a simple way to determine how many amps of output I might get?

Your maximum Amperes with a premium MPPT type controller will be Watts divided by battery terminal Voltage: 170W / 12V = 14.2A for example. But that is midday, under a crystal blue sky, with no shadows, and panel pointed exactly at the sun.

On a good day you will charge about 60 Ampere-hours with that setup. Maybe more. On a bad day zero. So i would guess the panel will not keep up with your draw. Not even close.

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Even if it won't keep up with your draw, it's still helpful.

When I first installed solar on my boat (in Seattle), I put 400W of cheap flexy panels on my hard dodger oriented almost horizontally but with just enough tilt to shed the rain. Between shading, flat orientation, etc, I found I could get the equivalent of Rated Power/2 x 4 hours of output per summer day. In this case, the output would be 800 Watt-hours. At a nominal 12V, that's about 65Ah. That was nowhere near enough to keep up with my draw of 200Ah per day but it bought me an extra day at anchor without having to run the engine, which is really the figure of merit I was going for.

Those shitty flexible panels started dying within two years so I replaced them with some big-ass 435W commercial panels for a total of 870W. Now I can get 140Ah or more out of a sunny summer day and even though it's still not quite enough to keep up with my usage, it's extended the duration of my battery bank to the point that it's immaterial. By the time I need to charge my batteries, I'm typically starting the engine to get on or off the anchor anyway so I just keep it running a bit longer to top them up. 

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10 hours ago, El Boracho said:

Your maximum Amperes with a premium MPPT type controller will be Watts divided by battery terminal Voltage: 170W / 12V = 14.2A for example. But that is midday, under a crystal blue sky, with no shadows, and panel pointed exactly at the sun.

You forgot "with the panels at 68 deg which will be true for about 3 minutes on a sunny day".

The loss of output with heat is one of the largest deratings, and often forgotten. The panels often run way too hot to touch, maybe 135 +.

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7 minutes ago, DDW said:

You forgot "with the panels at 68 deg which will be true for about 3 minutes on a sunny day".

The loss of output with heat is one of the largest deratings, and often forgotten. The panels often run way too hot to touch, maybe 135 +.

True. And if you've got a small bank of lead acid cells that are in absorbance charging and/or a lot of panels, you won't get rated output either because the batteries just won't accept the current. When I replaced my AGMs with Lithium, I got a lot more usable power from my panels when charging that last 20%.

FWIW, my rule of thumb of rated power/2 * 4 hours includes every derating factor at play, at least for my particular boat cruising in the summer at 48N.

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11 hours ago, El Boracho said:

Your maximum Amperes with a premium MPPT type controller will be Watts divided by battery terminal Voltage: 170W / 12V = 14.2A for example. But that is midday, under a crystal blue sky, with no shadows, and panel pointed exactly at the sun.

 

 

and the glass clean...

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 My rough rule of thumb is: Output in A.hrs / day = 1/3 x panel rating in watts

So a 170 W panel might get you 56 A.hrs on a truly sunny day.  But clouds, shading, etc. all reduce that number.

Hmm, I've touched my panels on a hot day in the tropics (sun overhead, panels horizontal). They get hot but not too hot to touch. When you wash them, the small amount of fresh water on them does get warm though.

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If you are just looking for a bump and to cut engine charging down probably be ok, but to sit on the hook for days in quiet I am guessing you probably want double that.  The theoretical on component usage is a starting number only most times unless you are by yourself and very minimal.  Best thing is if you have a smart meter of some sort that logs amp hrs, keep a daily log over your next cruise.  Once you have a several day log of real power consumption you can make a decision on the panel size.  

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Not expecting 170W will do it all but hope to cut engine charge time if we are sitting for a few days.  where to put the panels is the question and I need to see if I can perhaps get 2 smaller ones w/ the 170W panel on my dodger.  They would curve down the side slightly but that would make one of them effective as the sun angle changes.  Thanks Zonker, a simple rule of thumb is exactly what I need for my simple mind to get wrapped around.  1/3 rated output is about what watts / battery voltage x 4 hours calculates to.

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On our catamaran we started with 325 W. That was pretty good but if we got a few cloudy days or my wife was writing for 8 hours on the computer (12V x 3A continuously) we would start to get behind. The fridge was super insulated - 6" closed cell extruded polystyrene with a 4" thick door so it didn't draw toooo much power, even in the tropics. Never had an a.hour meter or anything fancy. Just kept an eye on the voltage.

When we increased solar to 500 W we almost never, ever, ever had to run the engine unless we had many rainy and reallly overcast days in a row. Even on moderately cloudy days we had enough power.

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My take on solar is that you can't have too much. Best case scenario, it covers your needs and puts some electrons in the bank. On cloudy days, it still helps out. It's silent. It's reliable. As boat systems go, it's not even expensive. It's perfectly suited to the mission of a sailboat: use the sun (which makes the wind) to provide locomotion and pleasure.

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Yes, the cost has come so much it's a great addition to any cruising boat that has the space. Our first 50 W panel in 1995 was about $250 ($425 in today's dollars).

For $425 I can get ~350 watt panels these days.

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On 2/21/2020 at 11:42 AM, zenmasterfred said:

I have a Link battery monitor, I see Victron recommended by a lot of people, any suggestions on upgrading?  House bank is 2 4D's and starting is a group 28 so pretty simple monitoring needs.

No real difference between a Link & a Victron.  

Keep in mind you typically charge at 13.5-14.5V depending on whether it is absorb voltage or float.  So for the amperage that matters (absorb), the theoretical maximum is 170W/14.5V = 11.7A  

If you are looking for more SunPower flex panels, I can make you a screaming deal on 50W or 100W panels.  Brand new, on Orcas Island.  We bought a pallet of them.  Sold most of them, but we've been so busy installing residential and commercial solar that we haven't been marketing them.  We aren't an online retailer, but more of the local shop for the San Juans.  

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11 hours ago, Roleur said:

If you are looking for more SunPower flex panels, I can make you a screaming deal on 50W or 100W panels.

I'd be interested in buying some but my PM doesnt seems to work, lt me know if you have a pair of 50's and we can go from there.

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  • 4 weeks later...

Thanks to help from Roleur and others I got all the stuff, 2 100watt panels going on my dodger, separate controllers and I actually understand how to wire them up.  Came up with a fantastic plan for running the wires and mounting the panels without having to penetrate the dodger thanks to another sailmaker friend.  Now just need to be able to cross the Cheese Curtain into Canada this summer to try everything out up north.

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On 2/18/2020 at 10:07 PM, El Boracho said:

Your maximum Amperes with a premium MPPT type controller will be Watts divided by battery terminal Voltage: 170W / 12V = 14.2A for example. But that is midday, under a crystal blue sky, with no shadows, and panel pointed exactly at the sun.

Actually, panels can produce more than rated output.  Panels are spec'd based on Standard Test Conditions, but in some environments the STC's are exceeded and modules can routinely exceed the STC rating if they get more insolation  and are in cooler temperatures than defined in the STC.  For example, oriented panels near the equator (high insolation) at high altitudes (less atmospheric attenuation) in winter (low panel temps) with the ground covered in snow and white puffy clouds with the sun shining through a clear spot (high reflected insolation)  routinely produce 20% more power than the STCs would predict.  Of course not a concern at sea level in temperate 'climes on boats,  but it's wise to over-spec your charge controller by at least 20% to handle those rare especially favorable situations where your panels are actually performing better than the STC numbers.

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No real difference between a Link & a Victron.  

There are a lots of differences.   Victron is much more programmable, has a Bluetooth user interface including live data, historic, etc.   There than the latest LinkPro all of the other Xantrex monitors only did tenths.....   not hundredths on the voltage, amps, etc.

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  • 2 weeks later...
On 3/18/2020 at 11:55 PM, zenmasterfred said:

Thanks to help from Roleur and others I got all the stuff, 2 100watt panels going on my dodger, separate controllers and I actually understand how to wire them up.  Came up with a fantastic plan for running the wires and mounting the panels without having to penetrate the dodger thanks to another sailmaker friend.  Now just need to be able to cross the Cheese Curtain into Canada this summer to try everything out up north.

Can you share more details and detailed photos of attachments, Zen? I'm about to add a Sunpower 110W flexible on my flexible canvas dodger to supplement two hard panels on top of the bimini.

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I'll take some photos tomorrow, I am tying my panels together w/ the grommets and holes in the center of one edge of each, the ends will tie to hand grip bars on the sides of the dodger, the wires run inside a sleeve that attaches the dodger (2 wires for each panel + and - ) to it's frame at the aft end, then wires down the dodger support on the side and through the deck.  I installed two controllers so each panel will work independently avoiding compromising both if one is shaded. From the controllers the + side goes to breakers and from there to the + battery switch.  The - side goes directly to the ground battery switch.  Next step is to install the panels on the dodger so I can get the lengths to the controllers, holes are in the deck etc. to now just the finishing touches.  I'll get some photos and would be happy to pm them to you if you pm me w/ your e mail.  It is taking me quite a while but we are sheltering in place on our little island in the PNW so it keep me out of the tavern as my mother would have said.  Happy to answer any questions.

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Made the last connections as electrically challenged as I am, turned the battery switches on and threw the breakers from the controllers on and amazingly no sparks.  Got some sage advice from Roleur before throwing the switch, turned on my refer and with it running still was charging at 3+ amps.  W the F didn't I do this while I could still go sailing instead of finishing it now when we can't cross the cheese curtain into Oh Canada?

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Don't know why I waited so long, ran the hydronic furnace at anchor last night for a couple of hours, refer on, misc. lights, ran the heater this morning for a couple of yours, sailed off the mooring and 5 hours sailing today with refer and intstruments on, auto pilot steering most of time and ended up home only down 7 amps.  If you don't have solar do it NOW.  Will do more local stuff until we can cross the cheese curtain into Oh Canada again, hopefully this summer.

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  • 4 weeks later...

I haven't installed one of the 170 but have done many of the 110W & 50W.   Great panels and price-point.   At a minimum mount them on a twin-wall polycarbonate panel.   I usually do that and a foam cored composite panel for support if it's on a rail, etc.  

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On 5/7/2020 at 2:25 AM, solosailor said:

I haven't installed one of the 170 but have done many of the 110W & 50W.   Great panels and price-point.   At a minimum mount them on a twin-wall polycarbonate panel.   I usually do that and a foam cored composite panel for support if it's on a rail, etc.  

Solosailor, are you saying to use the rigid backers in addition to canvas (if its a Bimini)?  Or was that just for placement other than on canvas?

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I would never, ever ever install a solar panel on canvas.  They are not designed to continuously flex.   They are designed to flex slightly for mounting on non-level surfaces.   So yes, I would still use the polycarbonate twin wall on canvas to stiffen the panels.

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32 minutes ago, solosailor said:

I would never, ever ever install a solar panel on canvas.  They are not designed to continuously flex.   They are designed to flex slightly for mounting on non-level surfaces.   So yes, I would still use the polycarbonate twin wall on canvas to stiffen the panels.

Even Coroplast will do the job. It's not strong enough to support the panel adequately in mid-air but as a backer on canvas it's plenty good enough and is about as thin and light as you'll get in a support.

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