Solar Panel Output

harrygee

Member
391
121
Tasmania
I don't know where you shop mate but take a look at the real world.
They are not exaggerated at all. The outputs are guaranteed and if they cannot achieve the rated output under standard test conditions you can get your money back. However the test conditions are generally not representative of the real world. The rated power can only be obtained with 1000 W/m^2 irradiance while the panel is at 25 C. That is equivalent to aiming perfectly at the sun on a clear day while the panel is somehow maintained at 25 C (which will be difficult on a clear day, unless the panel is in a cold place).
 

mckenzie.keith

Aspiring Anarchist
859
287
Santa Cruz
And even if the panel is in a cold place. On a 10 deg day, with 1000 W/m^2, the black panel will be too hot to touch. Unless it is a 10 deg day with 50 knots of wind and the back of the panel is well ventilated.
Yes. Maybe in a place with clear cold winters. Nowhere that I would like to visit with a boat will it be possible to keep the panel at 25 C with 1000 W/m^2 incoming. But I would not say there is no such place on earth. Maybe somewhere on the eastern side of the rocky mountains somewhere there is a solar panel that sees full direct sunlight without getting hotter than 25 C.
 

mckenzie.keith

Aspiring Anarchist
859
287
Santa Cruz
I don't know where you shop mate but take a look at the real world.
Virtually all solar panels sold for installation on roofs or in solar farms have guarantees that they will meet their rated power UNDER THE TEST CONDITIONS I MENTIONED. They also have guarantees that the power production will only decrease by a certain amount over time. Nowadays there are temperature coeficients given for Vmp and Imp so you can calculate power at other temperatures. The panels will meet these specs. Count on it.
 

DDW

Super Anarchist
6,708
1,236
Most places that cold aren't that sunny. I have a 4.5KW rated array on the house. On a cold but sunny winter morning it starts out at 25 deg F (-4 C) and I can get 4.8KW out. That lasts about 1/2 an hour before it begins to drop even as the sun gets higher. By 11 it will be down to 2.5WK with air temp barely changed. So sure there is probably someplace cold enough and sunny enough to practically meet "standard conditions". Nowhere I've been though.
 

JC522

Member
96
22
This will seriously hamper your travels to the foredeck!
And, as always, try for the shortest possible wiring run to the batts. Or a large part of panel output will disappear into wiring
I don't see a problem. I can put both panels on one side or just step up to the coach roof to go forward. The panels are only out there while I'm at anchor. If I use a reasonable wire gauge (offhand maybe 14 gauge), the 20 foot run shouldn't be an issue.
 

slug zitski

Super Anarchist
7,097
1,467
worldwide
I don't see a problem. I can put both panels on one side or just step up to the coach roof to go forward. The panels are only out there while I'm at anchor. If I use a reasonable wire gauge (offhand maybe 14 gauge), the 20 foot run shouldn't be an issue.
Always think about shading

I’ve seen small at anchor panels on the bow pulpit , looks like the ideal location
 

SCARECROW

Super Anarchist
6,017
705
Melbourne, Aus
Make the wires as short as possible and a lot thicker than you'd normally spec for the current and keep the MPPT as close to the panel as practical. The MPPT algorithm uses the panel output voltage as a data point and any voltage drop between the panel and MPPT will effect the efficiency. Normal wire sizing for DC boat system usually result in a voltage drop of around 2%.
 

El Borracho

Verified User
6,833
2,790
Pacific Rim
Make the wires as short as possible and a lot thicker than you'd normally spec for the current and keep the MPPT as close to the panel as practical. The MPPT algorithm uses the panel output voltage as a data point and any voltage drop between the panel and MPPT will effect the efficiency. Normal wire sizing for DC boat system usually result in a voltage drop of around 2%.
True about the wire size, but the typical MPPT tracking algorithms are insensitive to the absolute panel voltage. Like tuning a radio by ear. It’s better that way because the designer doesn’t know the panel or wiring characteristics.
 

mckenzie.keith

Aspiring Anarchist
859
287
Santa Cruz
I don't see a problem. I can put both panels on one side or just step up to the coach roof to go forward. The panels are only out there while I'm at anchor. If I use a reasonable wire gauge (offhand maybe 14 gauge), the 20 foot run shouldn't be an issue.
14 awg wire is about 2.5 mOhms per foot. You can look this up in a table online. If you have 20 feet that is 20 x 2 x 2.5 = 100 mOhm total round trip wire resistance.

If you run 10 amps through 100 mOhms, that is 10 Watts of loss (P = I^2 x R).

I am not sure if 10 Amps is accurate. It depends on which panels you use. If the current is 5 Amps, that is only 2.5 Watts of loss in 14 AWG wire. I don't think I would want to lose 10 Watts of solar panel power in my wiring. But 2.5 Watts seems OK.

Anyway, 20 feet will likely be no problem if you just use the same wire diameter as what comes out of the panels (or go one size up).
 

221J

Member
200
75
CT
Somehow I think that moving panels and panels on lifelines are eventually going to cause a problem.

I have three semi-rigid panels screwed to the deck on the centerline of my cabin top. They do the job. May through August there is enough solar that I don't recharge with engine. I usually run the engine for ten minutes a day to heat shower water and I tend to move every 2-3 days which means another ten minutes of engine while anchoring etc. No sharp corners, no moving parts, nothing to trip over, nothing hanging off of lifelines and it's good enough.
 


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