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Matching solar panels and batteries: what's watt?
Yahoo Message Number: 99799
Several recent references to the "1 watt, 1 ah" rule of thumb raises a question (at least in my tiny mind).  1.  There is the 1/1 rule above.
2.  I have also read in this forum (Andy Baird) and learned from AM
 Solar that to compare the output from solar panels, one must multiply watts X volts--not just look at wattage.  So:  Example A.  (BP-85s) 4 panels @ 85 watts each (18 volts) = 340 watts.  Example B. (AM-100s) 3 panels @ 100 watts each (22 volts) = 300 watts.  Which set outputs more power to the batteries?  If the rule of thumb is correct, the answer is "A".  If Andy and AM Solar are correct, the answer is "B".
Which is it?  My impression is that the rule-of-thumb is misleading--that the correct answer is "B".  What say you?  --Ted
Re: Matching solar panels and batteries: what's watt?
Reply #1 –
Yahoo Message Number: 99821
Quote
From: rapgroupltd
 Several recent references to the "1 watt, 1 ah" rule of thumb raises a question (at least in my tiny mind). 1. There is the 1/1 rule above.
2. I have also read in this forum (Andy Baird) and learned from AM
 Solar that to compare the output from solar panels, one must multiply watts X volts--not just look at wattage. So: Example A. (BP-85s) 4 panels @ 85 watts each (18 volts) = 340 watts. Example B. (AM-100s) 3 panels @ 100 watts each (22 volts) = 300 watts. Which set outputs more power to the batteries? If the rule of thumb is correct, the answer is "A". If Andy and AM Solar are correct, the answer is "B".
Which is it? My impression is that the rule-of-thumb is misleading--that the correct answer is "B". What say you? --Ted

A watt is a unit of electrical power. The formula is P=IE or (P being power in Watts = current in amps X voltage or volts).
 So, all else being equal (and it may not be) the 340 watts of BP panels can deliver more power to the batteries than the 300 watts of AM.
 But things are a bit more complicated than that, though this wasn't always the case. Before the advent of the MPPT (maximum power point tracking) charge controller, the advantage clearly went to the 18 volt array when charging a 12 volt battery bank. This is because a solar panel is basically a current device and is rated with a current output. If you hook an 18 volt solar panel to a 12 volt battery, the battery will accept a charge up to the PV panels maximum current output. But the panel voltage output will in any case be "clamped" to the battery voltage, in this case 12 volts (nominal), so that the extra 6 volts of PV voltage is essentially wasted (remember that panel wattage is rated at the higher voltage given. i.e in your example 340 watts = 18.9 amps X 18 volts). So if the panel voltage is clamped to 12 volts when using an old fashioned charge controller, then the max power delivered would be approximately 227 watts = 18.9 amps X 12 volts)
 Along came the MPPT charge controller. The MPPT has a DC voltage inverter to take the incoming 18 volts from the PV panels and change that to AC. The AC is then rectified back to DC at the correct and optimal voltage for charging the batteries. In this case, the extra PV voltage is "traded" or converted to more available charge current, so the extra 6 volts from our previous example is not wasted. In fact, with a MPPT charge controller you often get more current (amps) from charge controller than the current capacity of your PV array.
 If the MPPT charge controller is designed to accept even higher voltage, say 22 volts rather than 18, then there is an advantage in using a higher voltage PV panel array. This is so because the higher voltage array will have less "line loss" or voltage drop (wasted power) on the wires connecting the PV array and the charge controller. This can be significant if installed wire size is small, PV current is high, or the wiring distance is long.

By the way, I don't understand AM Solar saying to compare panels by multiplying watts X volts. As mentioned earlier, the formulas are

P = IE thus E= P/I I= P/E
 Never heard of a formula that used watts X volts. Is this like the "new math"? (g)

bumper
bumper
"Yonder" '05 MB
"WLDBLU" glider trailer
Re: Matching solar panels and batteries: what's watt?
Reply #2 –
Yahoo Message Number: 99823
Quote

 From: rapgroupltd
 Several recent references to the "1 watt, 1 ah" rule of thumb raises a question (at least in my tiny mind). 1. There is the 1/1 rule above.
2. I have also read in this forum (Andy Baird) and learned from AM
 Solar that to compare the output from solar panels, one must multiply watts X volts--not just look at wattage. So: Example A. (BP-85s) 4 panels @ 85 watts each (18 volts) = 340 watts. Example B. (AM-100s) 3 panels @ 100 watts each (22 volts) = 300 watts. Which set outputs more power to the batteries? If the rule of thumb is correct, the answer is "A". If Andy and AM Solar are correct, the answer is "B".
Which is it? My impression is that the rule-of-thumb is misleading--that the correct answer is "B". What say you? --Ted

A watt is a unit of electrical power. The formula is P=IE or (P
being power

Quote
in Watts = current in amps X voltage or volts).

So, all else being equal (and it may not be) the 340 watts of BP
panels can
 
Quote
deliver more power to the batteries than the 300 watts of AM.

But things are a bit more complicated than that, though this wasn't always the case. Before the advent of the MPPT (maximum power point tracking) charge controller, the advantage clearly went to the 18 volt array when charging a 12 volt battery bank. This is because a solar panel is basically a current device and is rated with a current output. If you hook an
18 volt
 
Quote
solar panel to a 12 volt battery, the battery will accept a charge
up to the
 
Quote
PV panels maximum current output. But the panel voltage output will
in any
 
Quote
case be "clamped" to the battery voltage, in this case 12 volts
(nominal),
 
Quote
so that the extra 6 volts of PV voltage is essentially wasted
(remember that
 
Quote
panel wattage is rated at the higher voltage given. i.e in your
example 340
 
Quote
watts = 18.9 amps X 18 volts). So if the panel voltage is clamped to 12 volts when using an old fashioned charge controller, then the max power delivered would be approximately 227 watts = 18.9 amps X 12 volts)

Along came the MPPT charge controller. The MPPT has a DC voltage
inverter to
 
Quote
take the incoming 18 volts from the PV panels and change that to AC.
The AC
 
Quote
is then rectified back to DC at the correct and optimal voltage for charging the batteries. In this case, the extra PV voltage is "traded" or converted to more available charge current, so the extra 6 volts from our previous example is not wasted. In fact, with a MPPT charge controller you
often get
 
Quote
more current (amps) from charge controller than the current capacity
of your

Quote
PV array.

If the MPPT charge controller is designed to accept even higher
voltage, say
 
Quote
22 volts rather than 18, then there is an advantage in using a higher voltage PV panel array. This is so because the higher voltage array will have less "line loss" or voltage drop (wasted power) on the wires connecting the PV array and the charge controller. This can be significant if installed wire size is small, PV current is high, or the wiring distance is long.

By the way, I don't understand AM Solar saying to compare panels by multiplying watts X volts. As mentioned earlier, the formulas are

P = IE thus E= P/I I= P/E
 Never heard of a formula that used watts X volts. Is this like the "new math"? (g)

bumper
No, Bumper.  Unfortunately, it is "Ted's math".  (I told you I have a tiny mind.)  Forget the "watts X volts" nonsense (mine) and return to the basic question, given an MPPT controller (22B) and two 6 volt 300 ah AGM batteries (Interstate).  I am confused because I keep hearing that wattage rating is not the whole story--you need to look at voltage as well, but I am gathering from what you are saying that that is not the case?  If two panels are rated 85 and 100, 4 85s are better than 3 100s, even if the voltage of the 85s is 18 and is 22 for the 100s?  That's really the question I am asking.  Perhaps what I am not understanding is that the footprint of the panel per watt is smaller with the 100s than with the 85s.  That is to say, I have more power with 4 85s than with 3 100s, but if the goal is to get the most wattage I can for a given number of square feet of roof, I am better off with the higher voltage 100s than the 85s.  Would that be right? Obviously, I have done a great disservice to everyone by expressing this problem totally incorrectly.  For that I apologize and greatly appreciate your corrections.  My goal is to seek illumination, not obfuscation, but I have failed miserably.  I guess the lesson here is to just ask the questions in plain English and leave the math to those who know what they are talking about.  Ted
Re: Matching solar panels and batteries: what's watt?
Reply #3 –
Yahoo Message Number: 99824
Quote
Several recent references to the "1 watt, 1 ah" rule of thumb raises a question (at least in my tiny mind).  1.  There is the 1/1 rule above.
2.  I have also read in this forum (Andy Baird) and learned from AM
 Solar that to compare the output from solar panels, one must multiply watts X volts--not just look at wattage.  So:  Example A.  (BP-85s) 4 panels @ 85 watts each (18 volts) = 340 watts.  Example B. (AM-100s) 3 panels @ 100 watts each (22 volts) = 300 watts.  Which set outputs more power to the batteries?  If the rule of thumb is correct, the answer is "A".  If Andy and AM Solar are correct, the answer is "B".
Which is it?  My impression is that the rule-of-thumb is misleading--that the correct answer is "B".  What say you?  --Ted
I think you must know the Total Energy.  That equation is E=mc2.  Then apply Hurpoculus' theorem of electromagnetic forces on moving objects with rubber wheels.  Just my opinion.
Dave
Re: Matching solar panels and batteries: what's watt?
Reply #4 –
Yahoo Message Number: 99829
"I... learned from AM Solar that to compare the output from solar panels, one must multiply watts X volts--not just look at wattage."

Ted, I'm guessing that the problem here is just a little confusion over terminology. What AM Solar probably said was that to compare output from panels, one must multiply AMPS times volts--not just look at amperage.* Amps times volts give you watts, and watts are the true measure of a panel's power.

Now, as Bumper pointed out, higher-voltage panels do have a theoretical advantage when the connecting wires are relatively small and long. But assuming your wiring is properly sized, 340 watts of panels will beat 300 watts pretty much every time. :-)
 * This is why I wish that the Heliotrope solar charge controllers read out in watts instead of amps, and that batteries were rated in watt-hours instead of amp-hours--it would eliminate a whole bunch of common misunderstandings!

Andy Baird http://www.andybaird.com/travels/
Andy Baird
2021 Ford Ranger towing 2019 Airstream 19CB
Previously: 1985 LD Twin/King "Gertie"; 2003 LD Midbath "Skylark"
Re: Matching solar panels and batteries: what's watt?
Reply #5 –
Yahoo Message Number: 99831
Quote
......
understanding is that the footprint of the panel per watt is smaller with the 100s than with the 85s. ....
Oddly enough this is not true.  When I compare my BP85 solar panel data sheet (the old LD panel) with the AM Solar AM100's specs, I get see that the AM solar panel has 12.5 sq inches per actual watt (i.e. the 97.6 Watts the panel is actually rated at) vs. 11.82 watts per sq inches for the BB 85.

Quote
.... That is to say, I have more power with 4 85s than with 3 100s, but if the goal is to get the most wattage I can for a given number of square feet of roof, I am better off with the higher voltage 100s than the 85s. Would that be right? ...
This is one of those complex problems that "depends"; depends on the weather, the temperature, perhaps the phase of the moon -|;-).  The big variable that few know or talk about is panel temperature.  The BP85 panel has an 18 volt max power rating at 25 deg C.  The reason this is so much higher than the battery's ~14 Volts is that that the solar panel's optimum voltage falls 0.08 volts per degree C above that temperature.  So, when the panel is at 75 deg C, which it could on a hot windless day in mid summer's direct sunlight, the max power voltage is down to 14 volts.  This means that it will not put the full 4.5 amps it is capable into a fairly well charged battery.
 The AM solar AM100 panel more cells so its max power voltage falls at 0.098 volts per degree C.  So, at 75 degrees C, its max power voltage will have fallen from 21.5 Volts to 16.6.  It will easily output its
4.5 amps of max power current into the battery (if directly connected)

Then there is the mystery of the MPPT solar panel controllers.  The big thing to know is that they are only 90% to 95% efficient.  Therefore, when the voltage difference between the battery and the maximum power voltage of the solar panel get close together you can actually get less current out of a MPPT controller than if you hooked up the panel to battery directly.  I believe AM Solar's controller senses this condition and goes from an active mode to a passive mode, directly making the connection.

The MPPT controller really shines in mid winter when the panels are cold as ice.  With cold, the panel's maximum power voltage goes above the 25 degree C value and the MPPT controller will utilize that fact to create well more battery charging current than the panels put out.
 In direct answer to your question, you probably would get somewhat more summer time charging current out of the AM Solar's panels.  However, in the summer, one normally has more charging current than can be utilized by the battery bank anyway.  In winter, the "a watt is a watt" line probably is pretty much true.  Your four 85 watt panels will outwork tree "100 watt" panels ... assuming that a MPPT controller is used with both and that the controller's efficiency is the same at the two different panel voltages.  Based only on AM Solar's unwillingness to talk about efficiencies and their insistence on the merit of the high voltage panel I have always assumed that their controller might be less efficient at lower input voltages.
 With solar systems, all answers are iffy and muddy.  Sorry about that. - |;-)Linley
Re: Matching solar panels and batteries: what's watt?
Reply #6 –
Yahoo Message Number: 99833
Quote
From: gertnmo
 To: lifewithalazydazerv@yahoogroups.com
 Sent: Monday, March 02, 2009 9:00 PM
 Subject: [LD] Re: Matching solar panels and batteries: what's watt?

"I think you must know the Total Energy. That equation is E=mc2. Then apply Hurpoculus' theorem of electromagnetic forces on moving objects with rubber wheels. Just my opinion.
Dave"
 If the moving object is a LD, and the rubber wheels are over 5 years old, then approaching the speed of light is strongly inadvisable if you have not first considered the vector differential of the photon angle of incidence. .
. but you knew that.
 To Sam's question as to which panel is better in terms of roof area vs panel power (wattage). That depends on the panel size, the layout you choose for the available space, and keeping the panels away from shade producing items on the roof like the air conditioner. A shadow on a crystalline PV panel cell will pretty much take that panel out of action.
 Most crystalline panels have something like 15 to 17 % effeciency IIRC. The 22 volt panels will not likely be much different in effeciency than the 18 volt.
 The panels should be physically sized to fit your roof layout. Choice of voltage will depend on the acceptable voltage input range of the charge controller. The higher voltage panels (more cells in series) allow for a somewhat more effecient installation due to lower wiring loss etc.

bumper
bumper
"Yonder" '05 MB
"WLDBLU" glider trailer
Re: Matching solar panels and batteries: what's watt?
Reply #7 –
Yahoo Message Number: 99835
Quote from: Linley"

"Based only on AM Solar's unwillingness to talk about efficiencies and their insistence on the merit of the high voltage panel I have always assumed that their controller might be less efficient at lower input voltages.
 With solar systems, all answers are iffy and muddy. Sorry about that. - |;-)Linley"
 Linley, is there any independent organization that does head-to-head comparison between various systems (PV panel / controller combination?)  How did you guys make your decisions when you decided to go solar?

Sam
Re: Matching solar panels and batteries: what's watt?
Reply #8 –
Yahoo Message Number: 99844
Quote
... is there any independent organization that does head-to-head comparison between various systems (PV panel / controller combination?)  How did you guys make your decisions when you decided to go solar?
As far as I know, the only organization that shows head to head comparisons is AM Solar.  Greg does this in the hour long infomertial presentation he makes at various RV get togethers.  He shows the the input volts/amps and the output volts/amps of various controllers.
However, without a camera to record the values shown (I always forget) one cannot get enough data to go beyond the conclusions Greg makes in his talk.  But, let me be clear. I think Greg does an excellent job with his products and his services.  They consistently are given rave reviews by everyone (including me) who comes near his operation.
However, EVERYTHING has limitations and I am just the eternal engineer who looks for the trouble spots in order to understand them in an attempt to arrange reality so they don't matter.

How did I go solar?  I backed into it.  I ordered two panels on my coach when it was built in 2001 because given LD's compact construction, adding it later would result in an ad hoc installation.
I knew they used BP85 panels (very premimum) and a Heliotrope controller which is a sophisticated on/off controller (manufactured by AM Solar).  Since then, I have purchased two more BP85 panels from AM Solar and cut them into the LD's wiring.
 I haven't moved to a MPPT controller yet mostly because in the winter it is too cold and dark here to think of camping without electricity and in the summer the four panels produce well more than enough electricity to cover our needs.  I have pondered if I am interested enough in AM Solar's MPPT controller to install one just to learn about it.  I have studied the patent Greg licensed to create his controller (6,111,391, see at http://www.uspto.gov/patft/index.html) and understand how it works but the final result depends on exactly how it was implemented.  I may someday.

Linley
Re: Matching solar panels and batteries: what's watt?
Reply #9 –
Yahoo Message Number: 99896
Quote from: mynetsig

How did you guys make your decisions when you decided to go solar?

I'll tell you how I proceeded in November when I started on this, without really knowing anything, after I entered an RV for the first time when I started testing old RVs with the idea of outfitting an RV for independent living.
 I bought a 24' 83 Ford E350 (460ci engine) Fleetwood Jamboree Rallye (rear lounge) for $3500 which looked scruffy on the outside but like new inside. And no leaks.
 I figured the first solar system would be an experiment and might not last so I went cheap and got the HF 45watt setup for $200. It barely would charge the battery, so I got another HF and I bought an MPPT, a BZ MPPT500 for about $200 on eBay. I could see that a good MPPT is required.
 The house battery in the RV was dead so I bought a new one at WalMart for $75 and it seemed to work OK for a few weeks, then died. I figured that the 25 yr old converter is poor so I got a PD9245 on eBay for about $140, but have yet to install it, altho I breadboarded it for testing.
 My plan is to get an AMSolar AM100 and Heliotrope MPPT to add to the roof. the AM100 is pretty compact and will fit nicely alongside the ancient AC on the roof. I figure to use the BZ for the HF PVs and the Helio for the AM100, I guess I can parallel the MPPTs into the battery. Then, if I need it I'll get more AM100s later.
 I plan to change the single 12v Walmart batt to a pair of Trojan T105s. I think they'll fir in where the single 12v is, tho I may have to shoehorn it a bit.

I may build a new battery box under the rear overhang for 4 T105s.
 I've had to feel my way thru most of this stuff, and it's taken more time and money than I thought. I found that the RV.NET "tech" forum is very useful.

John
Re: Matching solar panels and batteries: what's watt?
Reply #10 –
Yahoo Message Number: 99908
Quote
Several recent references to the "1 watt, 1 ah" rule of thumb raises a question (at least in my tiny mind).  1.  There is the 1/1 rule above.
My $02. Rules like the above may apply if you are full-timing boondocked in the SW deserts. Otherwise, you need to adjust according to your actual desired uses. In our case, we travel mostly off season, often in inclement weather (which we LIKE), and sometimes where solar time is poor even in good weather due to obstructions and the low angle of the sun to the horizon. In such cases, electrical charge storage is more important than solar charging capability. If you are in bad weather and solar charging is nil, it is a waste of money if your roof is paneled in PV - you'd be better off with a good converter, a generator, or simply moving on every few days for a recharge from the alternator.
 In our case, we have a pair of 53W panels, no MPPT controller, and about 210 A-hr of storage, no generator. Most places we hike or other outdoor activities which occupy a couple of days per site, max about 4, then move on. We are pretty conservative users, without being ascetic, so if we have good solar siting and cooperative weather, we can camp pretty much indefinitely, as long as we can get water and dump waste.

Steve
2004 FL
2013 Honda Fit
Re: Matching solar panels and batteries: what's watt?
Reply #11 –
Yahoo Message Number: 99938
"As far as I know, the only organization that shows head to head comparisons is AM Solar. Greg does this in the hour long infomertial presentation he makes at various RV get togethers. He shows the the input volts/amps and the output volts/amps of various controllers..."

Linley,
 Thanks for the info.  I'm sure Greg does a good job selling/installing solar systems.  Still he owns or works for AM Solar, so I would take his numbers with a grain of salt.

Sam
 
Re: Matching solar panels and batteries: what's watt?
Reply #12 –
Yahoo Message Number: 99944
Bumper;

as an addendum to the 'relativity' consideration, if an LD could travel at the "speed of light" would a solar panel function?

Dave