Solar Panel Price Comparisons

[valterra]

I've read on here before that Solar Panels (PV) are "Current Supply Devices."  From what I understand, a 3A, 25 volt module will put out 3A into at 24V, and 3A at 12V (nominal).  

I've experienced that with my Harbor Freight "45W" panels.  45W divided by their 23.57V is 1.91A, which is more than I've ever seen with those panels into my 12v system.

1.91A multiplied back out at MY system voltage (let's say 13.8vdc) would be 26W of actual usable power.  So looking at a store website and comparing Dollars-per-watt seems futile to me.  A 100W, 16V panel typically puts out more current than a 100W, 25V panel, for example.

So would it be better to judge them based upon Dollars-per-Amp?  I was surprised by some of the results when doing the math this way.  

What do you guys think?

[DamonHD]

If you use MPPT then you can really get something like the Wp advertised.

For simple systems I think that you have to accept system loses of 25%+ into batteries, including voltage mismatch, but with the benefit in your case of charging at lowish light levels.

Rgds

Damon

[electronbaby]

not really. The current pricing $/watt makes a lot of sense. You just have to remember that whatever module you buy, it will be a rare occurance to see the nameplate rating from it (you usually will see 75% of it if you are roof mounted, and more if you are pole or tilt array ground mount). It is possible, but rarely happens. Testing is standard nowadays at 1000w/m^2 so the performance of most modules on the market will be the same. Amorphous is a little different animal as you have found out. Although all modules are rated very similarly, amorphous has slightly different temperature coefficients than other types of modules, and there fore a little different power curve across similar ambient temps. Low light performance is usually better with amorphous modules.

-----"A 100W, 16V panel typically puts out more current than a 100W, 25V panel, for example."

This is not true. We should be talking about watts here. not current.  The voltage and current will be inversely proportional so if the wattage stays the same (cant remember the last time my 100 watt module became a 150 watt), the voltage and current can fluctuate, but will always add up to equal the wattage. Basic Ohms Law stuff.

say we have a 100w module. It can only do 100w @1000 w/m^2.

Now say, the solar insolation is only measured at 800 w/m^2      This means we can only generate 80w with our module.

If I used that module on a 12v battery, I would see 6.66A of current. If I used the same module on a 18v battery (not very common) then the current would be 4.44A.  Either way, its the same wattage with the same module, with the same sunlight intensity.

Hope I didnt confuse you.

[tecker]

Solar panels as a whole loose power under direct sunlight or proximity convection( roofing etc). They loose potential with Internal expansion  . Good cooling is an issue when your setting up your site . Amorphous panels rarely match manufacture specs  as you've seen . An Mppt won't be effective with less than 10 amps so don't bother .There's an impedance factor that comes into play where the photon burst internal to the panel is blocked by the junction ( at it's rated voltage) where electrons get stacked up  in the substrate so an Mppt has a tendency to cap up the panel and the current just is wasted .

 I spread the amorphous panels through out  a 1500 ah bank an allow the 4 130s to move to other loads The bank comes to charge voltage fast . The amorphous  go to maintenance when everything is topped off.

 I used low speed pulsing with good success . and looked at Mppts . I am currently seeing good results with a 40 amp pwm ( a lot less internal heating ) frequency range 200 hz to 10k . It looks like there will be some impedance matching possibilities with this method and simple zener and thermistor tracking for feed back loops I can switch on  air handlers during the day and catch up the battery bank before the heat of the day . Some really tasty options with inexpensive parts .  If you want to play with one there's one in most battery drills it's not the optimum frequency range but you can see some improvement with low power charging .

I have an 80 volt bank of car batts that  I switch charge and use for running 100 v power tools..These were some of my heavy hitters that didn't work well with inverters . Charging is good switching in 8 pwms on the 4 130s in between uses .

[valterra]

"A 100W, 16V panel typically puts out more current than a 100W, 25V panel, for example."

"This is not true. We should be talking about watts here. not current"

I went to a solar site and double-checked this.  When comparing two 165W panels, for example, the 25v one put out LESS current than the 35v one.  

So my original supposition was wrong.

There is just so much variance in price between even similarly-rated panels.  It's hard to figure out (for me) which is really a good deal and which is paying too much.

[TAH]

a couple of points

First Maybe my setup is not the norm but if it is within a couple hours of noon and the sky is clear I never see less than 90% of the rated output of my fixed plane roof mounted panels. 75% would suck.

Second. Ohms law for wattage won't work on a panel if you have a direct load. Panels have a max current out even if shorted. A 3 AMP panel will not produce more current with a load that is rated for a lower voltage. Unless you 80 watt output was a panel with a 6.6 amp output you won't get 80 watts at 12 volts.

[ghurd]

People should think in "Rated Amps" at peak power.  "Imp" or "Ipm".

I have a feeling the 1.91A means 1 HF PV is not working.

G-

[Bruce S]

You beat me to it

each panel at peak out put should be reading above an Amp . I would split them and go checking .

Welcome back !!

Bruce S