How do you find out what wattage a solar panel is good for?

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FireFighter38

FireFighter38

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Joined
Aug 14, 2007
Posts
83
Location
Victoria, BC Canada
Hello all,

I recently purchased a Surveyor 5th Wheel and had them include a 120 watt solar panel (installed) as part of the deal.  I was recently doing some research to learn more about solar and looked up the brand they had installed which, written on the controller, is a Go-Power model from Carmanah (Canadian made).  Looking at their website, they only offer a 50, 80, and 110 watt system, no 120 watt system as is written on my paperwork from the dealership.  Also, in comparing the panel I have to the picture on their website, it exactly matches the 80 watt panel, not the 110 watt.  When I measured it though, it didn't match the dimensions of either panel.  This leaves me to wonder if they installed a different brand panel than what the brand is of the controller or did they simply install an 80 watt panel hoping I wouldn't be any wiser. 

My quesiton is... is there any way to measure the wattage of a panel to see if it really is an 80 watt or 110/120 watt??  I was unable to find any writing on the panel itself and I really don't want to remove it from the roof to look under it to see if the writing is there as the mounts are all nicely sealed to the rubber roof. 

I appreciate any help I can get.  Thanks!

Corey
 
Corey,
Testing the true output of a solar panel is almost impossible. It would require an absolutely clear day at the equator, your solar panel being exactly perpendicular to the sun's rays, and a very low resistance load shunting an ammeter, and an accurate voltmeter. In short, it's not practical, and the difference between a 110 and 120 is only about 9%. That said, if the dimensions and design aren't even close, you have no way of knowing what they installed. Manufacturers will make changes to equipment and their spec's, and the websites don't always reflect them on a timely basis. I would suggest you present them with the info from the Carmanah website, and have them prove to you that they installed an equivalent or better panel.
 
Thanks Karl for your input.  I don't care whether its a 110 or 120 watt necessarily, just want to ensure that it is not an 80 watt.  I think I will do exactly as you say and just simply present them with the info and see what they have to say.  If I'm still not convinced by their answer I could probably take my trailer to where the solar panel was made as I found out its actually made right here in the city I live in.  Thanks again for your reply!!
 
As you have probably observed, a 110 watt Go-Power is about 35% larger than an 80 watt model, so even if your dimensions aren't exact it should pretty well tell you which one you have. And the brand name won't make much difference either. There is no magic to solar cells and there are only a few companies making the innards anyway, so the difference in brands is largely the packaging of the panel and the controlling electronics.  It takes X amount of square inches of solar array to produce W watts regardless of who made the panel, so if yours measures close to the specs for an 80 watt panel, in my opinion that's is undoubtedly what you have.
 
Thanks Gary, thats the same thinking I was doing.  Mine looks exact to the picture of the 80 watt model with the number of cells on it but it is larger than the 80 watt model, but not as big as the 110 watt model and nowhere close to the same amount of cells pictured.  I am definitely going to talk to the dealership about this.  I think they tried to sneak something by me here.  Thanks for your input!
 
FireFighter38 said:
I am definitely going to talk to the dealership about this.  I think they tried to sneak something by me here.
Click to expand...

What?  NO!  ;)

Yes unfortunately RV dealers will do this, just about any chance they get.  Luckily you are an educated consumer and it looks like you are going to challenge them on their word, which many people don't do.  If you have the solar product in writing that they were supposed to give you, then you should be golden.  Let us know how it works out!
 
Oh I made sure I had it in writing for sure.  Being that I live on Vancouver Island and I purchased the trailer in Abottsford (mainland) I will be stopping in there in May when we go to the mainland to travel.  I don't want to pay the ferry twice by making a special trip!!  I'm cheap that way, lol.  I will let you know what comes of this in a month or so :)
 
I have been reading this thread off and on and Think I have the answer to the question in the subject

Murphy's laws for niave young engineers say

"Take the factory specs and divide by 2,  Take the salesman's claims and divide by 4"

So, when it comes to solar panels... I'd say take the label, and divide by 10
 
Actually the only way to be sure is to measure voltage and current on a very sunny day with the panel properly "Aimed"

My statement above (any statement involving Murphy's Laws) is something of a joke. but there is also some accuracy.

About the only thing I'd never expect is to see 10 amps at 12 volts coming off a 120 watt panel.  I've never seen a panel that put out half it's rated wattage.  Not to say it can not happen. (I've never measured in full sun in say Az only Mi) but I think the folks who rate 'em do tend to be a bit... Optimistic.
 
Actually, the output of a solar panel will be significantly higher than that. It must be, or else your batteries would never be charged. Here is a pdf document for a Mitsubishi 110 watt panel. You'll see that the maximum power voltage (17.1V) times the maximum power current (6.43) almost exactly equals 110 watts (109.953). A charge controller is required to regulate the charging voltage and current to the batteries. 
 
Thanks for sending that Karl.  Maybe I have a Mitsubishi panel because looking at the dimensions for their 110W, mine measures up to that.  Maybe my dealership installed a different brand controller than the panel is?  I'll give them the benefit of the doubt and hope they are reputable but I am still going to check it out with them in mid-May when I take my trailer in for some warranty work.
 
Solar panels are much better at voltage than at current, so in full sun they tend to put out really high voltage (16-19 DCV), which yields a nice wattage figure even though the amperage is modest. Of course, the batteries can't tolerate that voltage for very long, but in most regions the sun doesn't shines that brightly, at the correct angle, for very long either!  Thus the peak wattage is not very meaningful. It does, however, indicate that you still get decent output even when the panel is operating at the more likely 50-70% of peak.
 
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