Originally Posted by Capt Gill
Which vendors? Solar panel or controller? All of my solar panels (4 different brands) all used Vmp for their output calculations, not Voc. Of all of the brands of solar panels I've looked up, all of them have used Vmp, not Voc, to calculate wattage. The main thing I use Voc for is to ensure I don't exceed the max input voltage of the solar controller, either in series or parallel.
Can you give a specific example of this? My 20A solar controller has a max PV wattage of 300w, which comes out to 20 A output @ 15v, not 14.2v.
Your confusion is well warranted, not only to my statement, but to the wealth of info and misinfo. in this thread, also because of the many variables involved
in determining the actual amount of power getting to the reservoir, your battery(s). So, as to not cause any further confusion I'll say that my point was to not take the mfg's power rating on a solar panel as what you'll actually get. The best, easiest to understand (in my opinion,given the subject) info on solar power is found in Nigel Calder's book, Boat Owners Mech. and Electrical Manual
(3rd ed. or later
Some anecdotal info re. my first statement on power ratings: I inquired to a few venders for more spec's and info on how they determined their power ratings, I got only one response that was not spin, Go Power! Solar Flex™, an upstanding company I believe. Below is the email conversation:
From: Jack via Go Power! [email@example.com]
Subject: Form submission from: Contact Go Power!
First Name: Jack
Company: Skill Marine
Please explain your power ratings, e.g., the below is rated @ 100 watts and
current rating is 5.62 amps, since most batteries won't safely take a
charging voltage much over 14 volts I don't understand how you get a 100 watt
rating (5.62 x 14 = 78.68 watts).
Solar cell type Monocrystalline
Output power 100 W
Rated current 5.62 A
The open circuit voltage (Voc) of the solar panels are much greater than 14V. This is the unloaded voltage the solar panel will produce when measured under the test conditions they are put under.
For example, the 100 flexible solar panel you're referencing in your email has an open circuit voltage of roughly 18V. The panel also has a short circuit current rating (no load) of ~ 5.6A. The product of these two give you the 100W rating.
The controller can charge your batteries as high as 14.8V if you have flooded batteries. If not, it would be as high as 14.4V and lower depending on how full they are.
Please feel free to contact us if you have any additional questions or concerns.
Carmanah Customer Support
Toll-free (US & Canada): 1.877.722.8877
Thanks, for the speedy response. Yes, I see how you arrived at that rating and will venture to say that if you changed the method to reflect useable power it would put you at a competitive disadvantage as they all use the same method. Useable power, which is the figure one needs to compute for realistic charging assets, e.g., my system uses AGM
batts.so, 14.4 v is the highest voltage acceptable which gives approx. 81 watts useable. Is this not correct ? Stating up front the amperage and/or the power at the most common voltages would be useful for deciding the # of panels needed.
"The panel also has a short circuit current rating (no load) of ~ 5.6A
" I believe that would be max load as the leads are shorted.
You pretty much have it,
The best way to look at it would be to look at the current the panels produce. This is the real world number you can see under ideal conditions (which typically you won't always see all day/every day).
This way you can have an idea how much charging you can expect from the solar system - especially if you start adding multiple panels.
You can also do comparisons between different panels to see the current they can produce.
Carmanah Customer Support
Toll-free (US & Canada): 1.877.722.8877
= END of EMAIL=
There you have it or at least what I have, I hope this helps a bit.[/QUOTE]
I was trying to be polite. I'm afraid you are mistaken. The method used to measure and publish rated power output has been posted twice above.
All solar panels are rated exactly as noelex and I posted earlier. They are subjected to a flash in a controlled lab environment
, called STC and all measurements are derived from that.
When any company mfrs solar panels, they group their cells by efficiency and build sets of panels using these tightly grouped cells. The best cells might all go into their 300 w panels. The next group make up their 295 w panels, next = 290 w panels, and so forth. The panels are all identical size and construction, it's just that the cells
are better in the 300 w panel vs the 295 w panel due to production tolerances.
When you buy a solar panel, it will produce 3 to 5% MORE than rated output for a few weeks, and slowly drop as it ages with power output guarantees at future dates, like 10 yrs and 20 yrs, for example.
The rated power output of a solar panel is always
based upon Rated Voltage (Vmp) x Rated Current (Imp) = Rated Power (Pmax)
This example is from the Sunpower 327 w panel:
Rated Voltage (Vmp) = 54.7V (obviously NOT battery charging voltage!)
Rated Current (Imp) = 5.98A
Rated Power = 54.7 x 5.98 = 327.106w
Ratings are so tightly controlled that their next rated panel is 315 w with:
Vmp = 54.7V
Imp = 5.76A
Rated Power = 54.7 x 5.76 = 315.072w.
specs for solar panel output are published by the mfr, not
solar panel mfrs rate their panels in exactly the same method, using the exact same specs: Vmp and Imp.
Your confusion came in when you tried to figure out how 14 volts entered into the picture. It doesn't. Solar panels don't charge your batteries.
They produce current at a higher voltage to the solar charge controller
, which converts that higher voltage current down to (nominally) 14.8V at a higher current to the batteries.
In the case of the Sunpower 327w panel, if it's connected to a good MPPT controller, in perfect conditions you'll see close to 21-22A going into the batteries, much
higher than the current posted right on the back of the panel.
54.7V @ 5.98A going into the solar controller @ 96% efficiency gets converted down to 14.8V @ 21.21A. The voltage is lower, but the current goes up, drastically in this example.
Now a little word about Go Power! Out of over a dozen RV solar system installs I've done, 3 were to rip out the Go Power system and install a system that actually works. All 3 owners complained that their Go Power solar system never produced rated output and gradually dropped lower and lower over a very short time. Go Power doesn't mfr anything
, they buy no-name products and slap their sticker on it, then jack up the price
. Their solar controller isn't even an MPPT controller. All 3 of the controllers I pulled out were flat black panels with a very cheap
PWM (like $25 unit from China) mounted to the back of the panel, with a single
line battery voltage display on the front.
In one case, the solar panel was shot, virtually no output. I replaced it with a single
230w panel and a 20A MPPT controller and they were getting far more power than they ever saw, even when the system was new.
The other 2 cases, I tested solar panel output, it wasn't in spec, but not too bad. The owners wanted to keep the panel, add another and upgrade to the 20A MPPT controller. Again, they saw more than double the power output than they ever saw new.
I've never seen anything like that with any other brand - never saw a failed solar panel (yet) and certainly not such low output from a solar controller. 2 other issues about those controllers - they only allow 2 absorption voltages - 14.1 or 14.4 - both too low for many brands of batteries. Even their equalization
voltage is too low, 14.8v. My normal charge voltage is 15.3v, Go Power isn't even close! At 14.4v, most batteries are only 80% charged, thus the owners complaining of low battery performance.
The other issue was false readings on the front panel display. I understand that a small amount of error can occur from production tolerances, but in all 3 cases, the battery voltage was reading 1.5V-2V HIGHER than actual battery voltage, current readings were also higher than actual, with actual current being almost nil.
Go Power is the only
brand I've warned people off from. I'd recommend taking a calibrated (or accurate, if not calibrated) DMM and taking your own measurements of voltage and current in peak sun and compare them to the displayed values. If they're anywhere close, you're very fortunate. I've ony seen 3 Go Power systems, and that was only because I was asked to figure out why they produced almost nothing and replace them with something that produced rated output.