Solar sizing

[wavery]

Bill, from BHA Solar from the above link.

I'm not sure why you would want to pay almost triple for the same item but to each his own..

[regis101]

I didn't say that I was buying from him.  Just asking about the weight of the panels.

Thanks for the heads up when piecing it out.  One can definitely save some coin with a little research.

[wavery]

I didn't say that I was buying from him.  Just asking about the weight of the panels.

Thanks for the heads up when piecing it out.  One can definitely save some coin with a little research.

My 80W panels were 17.5# each. That same eBay seller has 65W panels for $129 each. do you have room for 3 of them?

3x 65W would give you 195W @ $390 + controller & wiring.

http://www.ebay.com/itm/65W-solar-panel-for-12V-DC-high-efficiency-great-price-/320887294075?pt=LH_DefaultDomain_0&hash=item4ab65f887b

[wavery]

For what it's worth, I just sent a message to that eBay seller about his 80W panels. A friend of mine want to put 2 of those on his camper. The seller responded that they will be back in stock in May.

[regis101]

Good info.

I guess that the next thing to do is dig up some info about the charger controllers.  Once the needed amperage is known, the bells and whistles probably determine the price.

[wavery]

Good info.

I guess that the next thing to do is dig up some info about the charger controllers.  Once the needed amperage is known, the bells and whistles probably determine the price.

You can't really go wrong by buying a 20A controller. You can't go too big, you can only go too small. A 20A controller would be fine with a 100W solar panel and give you the flexibility of adding another 100W panel without having to change the controller. You could easily put 3, 80W panels on a 20A controller. My 80W panels are rated @ ~6A each (max output).

There are different types of controllers with the MPPT controllers giving you the best amperage output. PWM controllers do a good job for the price. You will get ~10% more amperage out of an MPPT controller but for the price difference, you might be better off just buying 10-20% bigger panels.

Here's a 20A MPPT controller at a reasonable price:
http://www.ebay.com/itm/MPPT-20A-solar-controller-MPPT-20A-Solar-regulator-controller-MPPT-Power-go-up-/260994949569?pt=LH_DefaultDomain_0&hash=item3cc482e1c1

This is the 20A PWM controller that I have and it works fine:
http://www.ebay.com/itm/20A-12V-24V-PWM-Solar-Street-Light-Panel-Charge-Controller-Regulato-AUTO-SWITCH-/230759422965?pt=LH_DefaultDomain_0&hash=item35ba555bf5

The price difference is only ~$45 but $45 would buy you the next bigger panel too. The thing is, most people that do go with the MPPT controller opt for the more expensive ones that run ~$200..... Like this one:
http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/Morningstar-MPPT-Solar-Controllers/Morningstar-SunSaver-MPPT-15A-Charge-Controller/p6185/

I'm not really sure how well that $72 MPPT controller is. With the PWM controller, quality isn't as critical.

If I had a solar system that was 1000W or bigger, I would certainly go with an MPPT controller. I just don't see the upside with the smaller RV systems.

[techntrek]

... I would certainly go with an MPPT controller. I just don't see the upside with the smaller RV systems.

That is key for any RV system - since space for PV is limited going MPPT is always the best choice, IMO.

[wavery]

... I would certainly go with an MPPT controller. I just don't see the upside with the smaller RV systems.

That is key for any RV system - since space for PV is limited going MPPT is always the best choice, IMO.

That depends on the size of your roof. I could easily add another 80W panel to the rear of my roof. If I wanted, I could add 3 more to the front. I think that a good # of campers have ample space for a number of panels. Now...... if one is struggling to keep the weight down and an additional 18# for another 80W panel is a deal breaker, that's another issue.

$200-$300 for a decent MPPT controller that would increase the output of my 160W system by (maybe) 20W At max output) or spend that same $200 on an additional panel and increase the wattage by 80W at max output........ I'm going for the 80W every time.

[regis101]

Got this hit using a search engine.  Dunno about the jist of the thread but the summary is decent info with added links.  I sped read it the first time.  Going to return with more intent.  Thus the quest continues.  And again, thanks for the replies on this thread
http://www.solarpaneltalk.com/showthread.php?4109-Oh-the-Mystery-of-PWM-vs-MPPT-Controllers

[regis101]

Sizing of the controller is baking my noodle.  I would first think that the controller amperage rating should be larger than the panel(s) capacity.  This link sheds some clarity.  If for nothing else, being able to increase number of panels in the future.
It's also important, as shown, that the panel(s) voltage output must be taken into account when buying the controller.
I've seen kits that when doing the math, watts / 12v, the amperage at the output side of the controller is higher than the rating of the included controller. What the? i.e. 280w kit with a 20a controller.  The math is 23.3 amps at 12v.  Maybe I'm missing something.  Half of the problem is that I'm an electrician by trade so load calc's are common.

Sorry,  I'm obsessed, for now.

http://www.sunsoglobal.com/faq/How%20to%20Size%20a%20Solar%20Charge%20Controllers.pdf

[wavery]

Solar panels are rated at maximum output. The thing that one must remember is that the votage must be a minimum of ~17V in order to have enough "Pressure" to force the amperage into the 12V battery. A 30V solar panel hooked up to a 12V battery (through a controller) will only show up as 12-14.8V. At 14.8V, the controller will shut the charge off.

As an example...... if you take a 20V cordless drill battery and hook it up to your camper's 12V battery, the 20V battery will poor amps into your 12V battery until the 2 batteries even out (I don't recommend doing that).

Another example is..... some wind generators (which put out a lot of amps) are made from 24-36V, 4 magnet motors. They are commonly used on sailboats without issue.

As for you math for the controller........ It's fairly accurate but I would recommend a 30A controller with 280W of panel. As you well know, the formula is V x A = W..... Your battery voltage will seldom be 12V. When hooked up to the solar panels (Of decent size) you'll seldom see lower that 13.8V..... 280W divided by 13.8v = 20.3A. The chances of 280W panels actually achieving maximum output during practical use is slim to none. 250W would be about the best that one might expect in practical use....... which would be 18.1A @ 13.8V.

[regis101]

Here's a copy/paste from an email response to my inquiry about the controller seeming to be below the panel out when doing simple math.  And to think I thought I was on a roll.

*Thanks for your interest in our products. When sizing a charge controller,
you will need to use the Voc (open-circuit voltage), and the Isc (Short
circuit amperage) ratings that you will find listed on the spec sheet for
the solar panel, or else they are oftentimes written on the back of the
panel.

The solar panels we use in the 280w package have an Isc rating of 8.68 amps.
Multiplied by two, equals 17.36 amps, so you need a charge controller rated
for at least 18 amps.]Thanks for your interest in our products. When sizing a charge controller,
you will need to use the Voc (open-circuit voltage), and the Isc (Short
circuit amperage) ratings that you will find listed on the spec sheet for
the solar panel, or else they are oftentimes written on the back of the
panel.

The solar panels we use in the 280w package have an Isc rating of 8.68 amps.
Multiplied by two, equals 17.36 amps, so you need a charge controller rated
for at least 18 amps.*

[teejaywhy]

A new question on the related topic:

What, if any, isolation is needed for the solar charge controller when the trailer is connected to shore power and the converter is outputting 12V power ??

[regis101]

I don't think I would have the solar hooked up if on shore power. 

[techntrek]

regis - as I mentioned before, your best bet is to drop in on the Wind Sun forum and start firing off our questions.  The guys there will walk you through your system sizing from start to finish.

teejaywhy - no problem at all, you can have multiple charge controllers (AC or PV driven) connected at one time.  You just need to be careful about not exceeding the battery's maximum charge rate per the manufacturer's specs.  Usually only a problem with a heavily depleted battery.  When the battery is above 80% charge it won't accept many amps so you don't have to worry about that problem.