When is a 100watt panel really a 100watt panel(or 40/40, 60/60, etc)

[calstar]

I'm new here and new to solar for camping(or solar anything), looking to build a system for my camper/trailer. I came across this write up/review of a panel sold on Amazon:

I ordered this to evaluate its possible use in developing a 1kw electrical source for use in RVs and motor homes. My panel was evaluated by using it to charge a 12 volt 100amp hour battery. During the charging the panel delivered 12 volts at 4 amps or 48 watts. During the charging the panel was equipped with an auto tracking mechanism to ensure maximum energy production. The output of the panel remained constant at 12 volts and 4 amps. It did not seem to be effected by temperature. This is NOT a 100 WATT solar panel. This is a 48 WATT solar panel. Renogy multiplied a 20 volt open circuit voltage by a 5 amp short circuit current and declared the panel a 100 watt panel. That is not the way to determine wattage capability and causes me to seriously doubt Renogy technical expertise

How often is this the case? Not being solar literate I thought 100 watt panel is a 100watt panel but apparently the marketing departments, as with most all products, have their say in how the specs are measured for public consumption. This happens to be a Renogy(not picking on Renogy) but most likely others do the same. How can I tell what I'm getting before I buy, comments and info appreciated.

thanks, Brian

 

[PirateMcGee]

The 100w is the max output from the solar panel including the highest possible Voltage and Amp ratings. This is where a good charge controller comes into play as most panels in peak sun will produce voltages well over 12 for a given amperage. Basically you need to read into how the wattage was calculated for a given panel and see how that fits your needs.

 

[228B]

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During the charging the panel delivered 12 volts at 4 amps or 48 watts.

Yikes. With "100 watt" panels, this would be a factory stock Model A Ford's performance output as compared to, say, a 1965 289 Hi Performance-engine Mustang. In other words, pretty dismal performance (although a stock Model A Ford exhibits many other enjoyable dynamic attributes).

Coupled to a good solar charge controller placed in clear, high sunlight, a good 100-watt panel will output well over 5-1/2 amps and at least! 16 volts. Whatever that works out to be, wattage-wise, is good enough for my small camp trailer system. (although since I now have a second panel I'd purchased to add during days of full cloud cover, I'm now going to test them when wired in series to produce a nominal 24V input)

 

[AndrewP]

Calstar-without knowing your system and how you were actually measuring things, it's hard to comment.

Were you really charging at 12 volts(ie a dead battery?)? If your battery was nearing full, the charge controller starts to down regulate the amperage to avoid overcharging.

Also-a 12 battery isn't really 12 volts. In charging conditions it's more like 13.5+ volts as it nears full and will rest around 12.8volts.

How long was the cable that ran to the panel-you'll have losses there.

What gauge was the cabling? You'll get voltage losses there too.

What charge controller are you using? There are losses and inefficiencies at every step of the process that will reduce your efficiency.

The Renogy panels are good ones. I can share some real world numbers from my Renogy 100 watt panel and Morningstar Sunsaver 10 PWM controller from yesterday.

My power meter was reading 5.4 amps at 13.7 volts(ie charge nearly completed). Doing the math, that's 74 watts AT THE BATTERY. Not too bad for an inexpensive low power system.

If you are getting only 48 watts in good sun conditions(noon, cloudless), either your math is wrong, or your wiring is too small, or your batteries are actually full already. Tell us more about how you were charging, your wiring, and how you took your measurements.

 

[calstar]

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Coupled to a good solar charge controller placed in clear, high sunlight, a good 100-watt panel will output well over 5-1/2 amps and at least! 16 volts.

Thanks for the info guys. While I can muddle through a lot of adds and specs for panel and charge controller I'd like some recommendations from experienced people who actually have used and like the solar equipment they own. I don't need top of the line but I'd rather spend more now on decent, good quality than have to replace something after buying it only to soon discover it's shortcomings/inefficiencies.

thanks, Brian

 

[jonyjoe101]

With a 100 watt 20 volt panel you wont get the best performance even if you hook it up to a mppt controller. A cheap pwm controller will at most get the amp listed in the back of panel ipmax. If the ipmax is 4.8 amps thats all your going to get. Mppt controllers will extract up to 97 percent of a panels wattage, but it needs 30 volts or higher panel to work properly. It takes the excess voltage and converts it to more amps. A 20 volt panel connected to an mppt will only get 4.8 amps or even less, it just doesnt have the high voltage it needs to work.

You multiply 14.4 volts x amps to get the max watts put out by the panel. Thats the voltage the controller is putting out to charge your battery.

I would get the largest panel that will fit on your roof, i have a 240 watt 36 volt panel on my astrovan connected to a 20 amp ecoworthy mppt controller. Lying flat on the roof I have seen up to 15 amps in summer when the sun is overhead, in the winter I get about 11 amps. Once you experience such high amps, you dont want anything less. Even in cloudy weather i get usable power where a smaller panel would give me nothing. With 240 watts, thats all you need to keep you house battery top off, In 2 years i never have had a reason to use generators or alternators to charge my house battery, its 100 percent solar.

Putting a panel on your camper takes several hours and is something you dont want to do more than once, originally I had a 120 watt panel but was not happy with its output, so i upgraded to the 240 watts and have been very happy with its performance.

A 120 watt 20 volt panel will get you about 6 amps with either pwm or mppt controller.

 

[228B]

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A 20 volt panel connected to an mppt will only get 4.8 amps or even less, it just doesnt have the high voltage it needs to work.

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My experience differs from that posted above. A Renogy 100W monocrystalline panel, where the charge controller is reading it's output at 17-ish volts, ouputs well over 5.5 amps. With two of these panels connected in parallel (staying in "12V" mode), I've seen 11.86 amp to the battery through my Morningstar SunSaver MPPT 15L. That is nearly 12 amps from two 100W panels.
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Another way of doing this is to connect the two panels together in series, creating a 24V (30 to 40 volts in reality) solar input to this controller. It's amperage output is the same; nearly 12 amps, regardless of whether it sees "12V" or "24V" going in. The only advantage I can see from the series connection is that I only need one extension cable from the panels in the sun going to the trailer in the shade.
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I like having two separate panels (each measuring just under 24" X 48") in this modular-input setup because when en route I can then have just one panel deployed to keep charge voltage going to the battery while the Engel MT-45 keeps on hummin'.
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With this particular controller and either one or both of these 100W monocrystalline panels and the proper-size cable! my battery can be correctly charged. It's a great setup.
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[rayra]

the correct answer is 'Never'.

eta what folks are often talking about without explaining is the watt / power equation. Watts = Votls x Amps, and the algebraic permutations of that equation.
Mppt controllers are not magic boxes, they don't 'create' anything. They allow access to a broader flavor of power for greater thoughput to your storage battery.
There is always loss / conversion into heat. Nothing is 100% efficient. Panel efficiency, transmission loss over the wires, conversion loss, etc they all stack / work against the total output.
The ratings of the panels amount to a 'category' or class rating as much as anything. Divide the watt rating by the voltage rating / range of your controller or storage device, to get a rough measure of the amps you'll get. The rest is minor / fractional differences in brand efficiencies. Learn how small those differences are before you throw a ton of money for 'buy once cry once' recommendations. The 80/20 rule applies to solar just as much as anything else.

 

[unseenone]

What tracker did you use?

 

[LeishaShannon]

A 100W rated panel will produce 100W in Standard Test Conditions assuming the manufacturer isn't BSing you . STC are an irradiance of 1,000 W/m², solar spectrum of AM 1.5 and module temperature at 25 °C

STC conditions are rare in the real world but they provide a standard method to compare solar panels.

Environmental factors such as temperature and CEE impact panel output significantly. In cooler climates panels can generate higher than their rated watts and in warmer climates they will generate less.

Correct controller choice will increase the percentage of the solar panel output that is actually used for doing useful stuff like charging your battery.

 

[CaliMobber]

From my experience I seem to get a little more power from the rated panels in good direct sun. most popular panels are under rated a little to offset the ideal conditions. under normal condition I dont see the rated power because Im not cooling the panel and keeping it perpendicular to the sun.

 

[VanIsle_Greg]

Interesting thread.

I was having a "discussion" online with someone about this a little while ago. There is a local store selling panels, their 100 Watt panel is rated on their packaging as delivering 2 amps under ideal conditions. 2 amps? The same company sells a 40 watt panel rated at 3.5 amps? Hmmm.... whats the deal?

I have 2 of their 40 watt panels, and used one of them extensively while dry camping last summer. I had a fully charged battery all summer. Mind you we were only running lights, toilet, water pump and so on. No fridge (LPG).

My buddy bought 2 and runs his trailer including a Dometic 35L fridge off a single Group 24 deep cycle and it is great. Had no problems freezing all of our food in 120* outside temps at NWOR. lol

 

[LeishaShannon]

There is a local store selling panels, their 100 Watt panel is rated on their packaging as delivering 2 amps under ideal conditions. 2 amps? The same company sells a 40 watt panel rated at 3.5 amps? Hmmm.... whats the deal?

The Vmp (volts they throw out at maximum power) might be much higher on the 100W panel. i.e. 100W panel = 50v @ 2 amps , 40W panel = 8.5v @ 3.5A (useless for charging a 12v battery unless you had 2 in series)

 

[VanIsle_Greg]

Looked up the specs again, had them off by a bit... oops.

Power Rating: Maximum rating 40 Watts (under ideal conditions)
Current: 2.3 Amps @ 17.1V

And I have 2 of them I plan on running in series.