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Old 03-28-2017, 11:35 PM   #1
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Solar Panels, Real World Power Output

There has been a lot of discussion about how much you power you can get out of a given panel. I think lots of people think you can get what the manufacture rates the power at but unfortunately your highly unlikely to ever get that amount of power from the panel. Why? It's due to how the panels are rated. Let me explain the two ratings.

The first rating is Standard Test Conditions (STC). For STC tests the light source is calibrated to a defined set of wavelengths and so that precisely 1,000 watts per square meter fall on the front glass of the solar panel. Temperature is the other key test condition – everything is at 77°F (25°C). The solar cells, glass, aluminum frame, and back-sheet are all at 77°F.

The second is PVUSA Test Conditions (PTC). Under PTC, everything is heated up as if it were in the sun. The solar cells within the panel are raised to their “normal operating cell temperature” which is typically around 113°F (45°C). The ambient temperature is set to 68°F (20°C), and a 2.2 mph (~1 meter/second) breeze blows across the panel. I believe they also use 800 watts per square meter but I'll need to double check that.

Solar Cell Temperature: STC= 77°F (25°C), PTC= 113°F (45°C)
Ambient Temperature: STC= 77°F (25°C), PTC= 68°F (20°C)
Cooling” wind speed: STC= None, PTC= 2.2 mph (1 m/s)

I'm sure someone will notice that the PTC conditions are only 68°F which isn't that high. True but remember in the summer you can see higher than 800 watts per square meter so the output should go up even though the temp is up.

See more at: STC vs. PTC: Why Solar Panel Testing Matters | The Energy Miser

California and the IRS only looks at the panels PTC rating when issuing solar credits since that's the "Real World" rating and not an inflated rating you get by using the STC. So how do panels stack up when you use PTC rather than STC?

To find out I took the CA PTC rating table and sorted it by STC then PTC so I could see the "Real World" amount of power you can get from a given panel. On 160W panels it ran from 134W on the low end to 152W on the high end. That's 84% on the low end and 95% on the high end.

One thing I've noticed is that none of the RV Solar installers show what the PTC rating on the panels they sell. Why? You'll have to ask them but without the PTC rating you have no idea how the panels will preform. AMSolar and other installers should publish the PTC ratings for the panels they sell so we can be better informed consumers.

So what does that mean for us? For now when figuring out how much solar you'll get use 80% of the STC rating as a starting point. This will give you a more realistic value to figure out how much you'll need. If you have the PTC values then use those numbers.

One last point, tilting. A lot as been said about tilting but let me explain in a simple way. If the sun is at a 45° angle a flat panel will only see two thirds of the sunlight. So that 800 watts per square meter hitting the panel just dropped to a little over 500W. Or to put it differently, you just effectively took away one third of your panels.

Happy solaring.
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Old 03-29-2017, 05:10 AM   #2
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But in real time the numbers come in much higher, all you need to do is watch your solar controller and the gain, mine maxed out often.

So before you think you may have this loss of power (don't believe everything you read)try it for yourself.

As far as tilting panels there are pros & cons, one being climbing up there, a total PIA, if you add another panel it likely works out the same.

the cost of the mounting brackets, high wind etc.

Mine are flat on my last two rv's like 9 years, my new one will be flat as well.

I think if you want worry about tilt you should worry more about tracking

two-axis tracking (or "full-tracking" - when your solar panels constantly follow the sun, maximizing the solar radiation gathered) gathers about 20% more solar energy than an optimal annual tilt does.

So what you need to do is every hour turn you RV a few degrees to follow the sun??

How about just having a beer check your SOC and enjoy


.
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Old 03-29-2017, 08:21 AM   #3
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Agree with Jimbo on this one. I am receiving better than expected results out of my install.
One thing to consider when someone says just add another panel. Depending on your design sometimes you can't just add one panel. Like me, I have 2 panels in series, so I can't just add one panel, I would need to add 2 or redesign the whole darn roof thing. I am so glad things are way better then expected.
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Old 03-29-2017, 01:56 PM   #4
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I'm not saying either of you are wrong just that when it comes to calculating panel output you need the PTC numbers to really know how any given panel will perform.

When it comes to flat or tilted, tilting will always increase output. If mounted flat you'll have to contend with shading if the panel is mounted close to anything more than a couple of inched high. Example a 160W mounted within 10 or so inches of the north side of an AC unit can easily get shaded during most of the year. If tilted so that the South side of the panel gets raised it will get out of the shadow of the AC as shown below using longer tilt bars with the current AMSolar mounting feet. This can be done either with manually or powered lift.

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Old 03-29-2017, 04:51 PM   #5
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Quote:
Originally Posted by Timon View Post
I'm not saying either of you are wrong just that when it comes to calculating panel output you need the PTC numbers to really know how any given panel will perform.

When it comes to flat or tilted, tilting will always increase output. If mounted flat you'll have to contend with shading if the panel is mounted close to anything more than a couple of inched high. Example a 160W mounted within 10 or so inches of the north side of an AC unit can easily get shaded during most of the year. If tilted so that the South side of the panel gets raised it will get out of the shadow of the AC as shown below using longer tilt bars with the current AMSolar mounting feet. This can be done either with manually or powered lift.

John unless you're going to track the sun east & west thru out the day, the tilting panels are only getting half baked.

How about this wake up it's raining/snowing any lousy weather.

you want to drive all you need to now climb a ladder and slip and fall from 12'

how much do those tilting bracket cost about $100 a panel plus labor.

I don't agree with your plan here, but if it works for you "more power to 'ya"
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Old 03-29-2017, 07:35 PM   #6
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The STC rating is always overly optimistic. I usually figure about 80% of rated output at latitude tilt, less if the module is flat and it's wintertime. The conversion efficiency if primarily affected by the sun angle and the temperature. The sine of the sun's angle relative to the surface of the module renders the maximum efficiency possible. In essence, the sine of a 90 deg angle is 1 or 100%, 80 deg = 98%, 70 deg = 94%, 60 deg = 86%, 50 deg = 76%, 40 deg = 64%, 30 deg = 50%, 20 deg = 34%.

To calculate the solar angle at solar noon on either the first day of spring or fall, subtract the latitude from 90. Example: Detroit is at 42.8 deg north, so the sun will be 47.2 degrees above the horizon at solar noon on March 21, giving a maximum conversion efficiency of 73% of the namplate rating, at noon. Temperature is another issue, and silicon modules will lose about .45% of their power output per deg C above 25 deg C cell temperature.

The moral of the story, your 400 Watts of solar will rarely put out 400 Watts. I was charging a battery with an 80 watt module and a Victron 75/15 today, under slightly hazy skies, and the maximum it could muster even with a decent tilt angle was 63 Watts.
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Old 03-29-2017, 07:45 PM   #7
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Lightbulb

Quote:
Originally Posted by Timon View Post
...This can be done either with manually or powered lift.

I've just received my new 2x100w panel kit and am installing them right now on my Palazzo....THOUGH, I have had the same concerns for TILTING, or more accurately, HOW to provide an easy, safe, and secure method to provide a Tilt option for each panel without climbing on the roof(twice, of course)... especially if it's just for a day or two of drycamping.

BUT, I also did not want the 'permanent' mounting required on the roof, and the space required(though it's not a big issue for only two 27x40" panels)...

SO... I devised a new and better method... inexpensive, asl well.
I'm engineering it now, and starting the build as we speak(or type!)...

I'll share in a few days, with photos... : )


surprise!
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Old 03-29-2017, 07:55 PM   #8
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The STC rating is always overly optimistic. I usually figure about 80% of rated output at latitude tilt, less if the module is flat and it's wintertime. The conversion efficiency if primarily affected by the sun angle and the temperature. The sine of the sun's angle relative to the surface of the module renders the maximum efficiency possible. In essence, the sine of a 90 deg angle is 1 or 100%, 80 deg = 98%, 70 deg = 94%, 60 deg = 86%, 50 deg = 76%, 40 deg = 64%, 30 deg = 50%, 20 deg = 34%.

Technically, I should have said "percentage of nameplate output" not "conversion efficiency".
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Old 03-29-2017, 10:13 PM   #9
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John unless you're going to track the sun east & west thru out the day, the tilting panels are only getting half baked.

How about this wake up it's raining/snowing any lousy weather.

you want to drive all you need to now climb a ladder and slip and fall from 12'

how much do those tilting bracket cost about $100 a panel plus labor.

I don't agree with your plan here, but if it works for you "more power to 'ya"
Doing a two axis tracking mount is impractical on an RV but doing a one axis sun track is doable. If the power mount I'm working on turns out to be practical and can be made at a reasonable cost then adding single axis tracking wouldn't be that difficult.

I agree that $100 for 4 brackets and a couple of tilt bars seems high but I've not looked at metal fabrication cost for a long time. That's why once I have enough of my design done I can get some estimates. If it's too expensive then I'm only out my time.
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Old 03-30-2017, 01:51 AM   #10
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John you never commented on

How about this wake up it's raining/snowing any lousy weather.

you want to drive all you need to now climb a ladder and slip and fall from 12'
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Old 03-30-2017, 03:20 AM   #11
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My experiences with two RV PV systems. The first 600w with Morningstar PWM controller. The second 1500w with Midnite Solar MPPT controller. Both flat mounted. Both with additional 200w portable deployed as needed. Most geographic usage in the Southwest USA, much in the socal and Arizona desert areas. Typically ~50% to ~80% of rated output for the flat roof mounted panels. Depending on latitude and time of year.

The PVwatts program can be used to model various system senarios. Flat, tilted, time of year, location, etc.

I also have 28 count of 250w panels on our sticks and bricks home in San Diego. 3 degrees off south and on a 3/12 pitch roof. Minimal shading. Grid tied. Negative annual electric bill for now 4 years. Meeting expected performance per PVwatts modeling.

PV rocks. But ones expectations must be realistic.
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Old 03-30-2017, 11:29 AM   #12
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John you never commented on

How about this wake up it's raining/snowing any lousy weather.

you want to drive all you need to now climb a ladder and slip and fall from 12'
Bad weather is bad weather and you just have to deal with it. That said, I sure wouldn't want to go on the roof when it's wet. In fact I wouldn't want to go on the roof for just about any reason while traveling especially to manually tilt 6 to 10 solar panels. This is why I'm trying to design a powered tilting system that's economical so one can just push a button ann up or down they go.
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Old 03-30-2017, 11:39 AM   #13
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Old 03-31-2017, 10:07 AM   #14
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I am getting ready to install a Magnum MS2012 inverter/charger in my 2009 Winnebago Chalet. Does anyone have any experience with the new folding panels in a suitcase that can be attached to the batteries? I do not want to go through the hassle of no sun in a shady rv park. This way I can move the panels to where the sun is located. I understand that they just have battery clamps with which to attach the panels.
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