Watts and voltage

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PancakeBill

PancakeBill

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Joined
Apr 9, 2005
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I was at an electronics store the other day and they have some surplus sales.  The had 250 watt 30 volt panels, it mentioned 'Canadian' units.  I assume, just the mfg'er, I am new to looking at systems, and already have a great one, but would these work?  Doesn't the controller essentially step down the voltage?  These are very inexpensive!  $99 each. 

However the cheap price if it won't build into a system is worthless. 
 
No, a controller will not usually do anything to volts.  All it does is control the charge rate to the storage bank.  It sounds like what you were looking at is a house panel, not one suitable for an RV
 
Actually, a lot of people are installing residential solar panels on their RVs, and for the most part, they seem to be happy with them. Residential panels output higher voltages, so you'd have to use an MPPT controller vs. a PWM controller. MPPT controllers convert excess voltage to amps. PWM controllers don't. 

Kev
 
Isaac-1 said:
MPPT controllers also cost more and perform better
Click to expand...

quite true, these days there is no reason whatsoever to use a PWM controller unless you have
an extremely low budget. much better electrical and cost efficiency is obtained with higher voltage panels
and a MPPT controller.
 
WHen you divide WATTS by VOLTAGE on a solar panel you get the theoretical maximum amps the panel can put out (It will never deliver)  Canadian or US the math is the same.

Of course depending on the type of controller and a few other variables.. Power delivered to the batteries may be as low as 25 watts (At a guess) or 2 amps. I'd not expect more than about 3.5 tops. but.. You won't know till you meter them. from those panels in full sun.
 
Get them if they can fit on your roof.

An MPPT controller will reduce the voltage to an appropriate one and transform the excess voltage into spare current.  I have my panels wired in series which gives me 40 volts output from my panels with no problems using an MPPT controller.
 
Any solar is usually good solar but you might consider your end goal. An mppt will realistically give you about 10 % better performance. Some will claim 30% which is more likely only when the batteries are extremely low and in very cold temperatures which are conditions many RVers avoid.

24 panels are often cheaper per watt but may have limitations on physical placement. My layout worked better with two strips of 12 volt panels from front to back on each side with room to walk. I couldn't have gotten as much wattage up there with 24s. Nothing wrong with pwm controller if you are staying under 600 watts or so. It's best if you look at your end goal and how much power you will need then figure out where you can place it.

With any solar being good solar a person can start out with just about anything and it will work for them. Others will want a controller with temperature compensation, voltage adjustment and equalization. Good mpot controllers cost more but at the same time panel wattage is pretty cheap. If space is available adding more panels and less costly pwm is just fine.

So that panel may be a great price but will it work if you want to expand? You never hear anyone say they bought too much but it's very common to hear that they want more.
 
PancakeBill said:
I was at an electronics store the other day and they have some surplus sales.  The had 250 watt 30 volt panels, it mentioned 'Canadian' units.  I assume, just the mfg'er, I am new to looking at systems, and already have a great one, but would these work?  Doesn't the controller essentially step down the voltage?  These are very inexpensive!  $99 each. 

However the cheap price if it won't build into a system is worthless.
Click to expand...
$99 each for a 250watt panel is an excellent price if they are new panels.  Yes, an MPPT controller will step down the voltage.  Actually the MPPT takes the higher voltage and converts to extra power at the 12V needed by the battery. 

At that price I would buy as many of them as I could fit on my roof and still be able to walk around to clean the panels (dirt really reduces the amount of power output). 

When I install panels on my RV's I scrounge up some cardboard and tape together a template the same size as the panels and position the template where I will install the panels.  Really helps to have a visual of where the panels will be. 

I have a pair of 325watt panels on my RV and 400AH of lithium battery.  Last fall I moved my solar, battery, inverter system from my 29' MH to my 36' MH.  I now have room for another 325watt panel if I decide to add one. 
 
John From Detroit said:
WHen you divide WATTS by VOLTAGE on a solar panel you get the theoretical maximum amps the panel can put out (It will never deliver)  Canadian or US the math is the same.

Of course depending on the type of controller and a few other variables.. Power delivered to the batteries may be as low as 25 watts (At a guess) or 2 amps. I'd not expect more than about 3.5 tops. but.. You won't know till you meter them. from those panels in full sun.
Click to expand...
I don't know where you are getting these numbers. The panel mentioned by the OP is 250 watts. My experience with panels have shone that at times they will actually put out more than the rating. This of course is under ideal conditions. The mentioned panels undoubtedly are 24 volt nominal. 30 volts would be Vmp (Volts Maximum Power) which would mean they could pump out 8.3 amps. These panels would require an MPPT controller.
 
Not for me, just passing along some info,  Terry B put up a great solar system on our coach.  But at 99/ panel seems like a pretty good price.
 
Kevin Means said:
Actually, a lot of people are installing residential solar panels on their RVs, and for the most part, they seem to be happy with them. Residential panels output higher voltages, so you'd have to use an MPPT controller vs. a PWM controller. MPPT controllers convert excess voltage to amps. PWM controllers don't. 

Kev
Click to expand...


Yep!  Sure happy with ours.
 
An MPPT controller is designed to use higher voltages and step those down by converting excess voltage into current.  30 volt panel is fine with an MPPT controller.

My system pushes 42 volts into my MPPT controller all day long as long sun is out.

My next solar system will probably be 63 volts going into my MPPT controler. 

 
Drewd said:
Get them if they can fit on your roof.

An MPPT controller will reduce the voltage to an appropriate one and transform the excess voltage into spare current.  I have my panels wired in series which gives me 40 volts output from my panels with no problems using an MPPT controller.
Click to expand...


36 cell panels are common for RV use as they put out about 18v. 72 cell panels are more common for residential use, but they are physically big.


What many don't know is that residential panels can be built from 6 in silicon ingots, or 5 in silicon ingots. The 72 cell panels made from the smaller ingot are quite usable on an RV roof. I have them on mine. :)  Wattage is of necessity lower, as they have less square inches total. However, they put out the same 40 volt voltage. This works really well with an mppt controller, as you do not have to put any panels in series.
 
There's also a place in Phoenix that's selling surplus residential panels at even lower prices:

https://santansolar.com/products

At first glance it looks like a great deal, except you'll have to buy a seperate MPPT controller to bring their outputs down to 12 volts, or give up 1/2 or more of their rated power.

You can mix 30 volt panels with 18 volt (12 volt) panels in an existing system but the higher voltage panels will de-rate themselves unless they have their own MPPT controller.  The panels won't be harmed, but paralleling mixed voltage panels will drag the higher voltage ones down to the lower voltage, reducing their power output.

250 watts at 30 volts is 8.3 amps.  If your system is operating at a nominal 14 volts output, 14 volts x 8.3 amps = 116 watts.  That's what your 250 watt panel will put out if it's placed into a 12 volt system without a MPPT

And the MPPT has to be controlling only the higher voltage panels by themselves.  Otherwise you'll run into the same problem at 18 -20 volts instead of 14 volts.
 
Well folks a THANK YOU

Thanks to this thread I did a bit of research on MPPT controllers and now have a BASIC understanding of how they work.. I know about PWM's and simple on/off types but now I understand MPPT as well.

Thanks.
 
John From Detroit said:
Well folks a THANK YOU

Thanks to this thread I did a bit of research on MPPT controllers and now have a BASIC understanding of how they work.. I know about PWM's and simple on/off types but now I understand MPPT as well.

Thanks.
Click to expand...


Yeah, it was an education for me as well. It is amazing what they do with electronics and semiconductors these days. :)
 
Frank B said:
Yeah, it was an education for me as well. It is amazing what they do with electronics and semiconductors these days. :)
Click to expand...


imagine the fun I have getting to play with 3000 W single strings and 600 Volt controllers !!
 
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