SOLAR - Switch from PWM to MPPT, series vs parallel matter?

When you have 12v panels and PWM, the watts don't matter. All that matters is the Isc of each panel, which you add up when in parallel.
All the talk of "lost wattage" using PWM is totally bogus since the watts don't count anyway.

It is also wrong to use the Vmp and Imp of a panel when using PWM--they have no meaning. Just use the Isc.

So the OP can get 16.8 amps as it is now. What he would get with MPPT is a huge variable starting with the battery voltage variable.

With MPPT, amps to the battery would now be OUTPUT POWER (watts) divided by battery voltage. As the battery voltage goes up the amps decline. With PWM you just keep on getting nearly the same amps until near the end of the recharge (see the IV curve for the panel for details)

In practical terms it really is mostly a wash between the two set-ups

Strange that they say they're 140 W panels, when 8.00 A x 17.0 V = 136 watts.

But, looking at the numbers, compare that 136 W to what one panel would be putting out charging the battery with PWM. 8.4 A x 13.8 V = 116 W would be a reasonable average working number (12V panels generally put out the short circuit current until you get to around 15V). Making the unreasonable assumption that MPPT is 100% efficient, you'd gain 20 W x 2 panels = 40 W total.

But, for the $200 and time spent upgrading to MPPT, you could easily add another 100 W (80 W effective) or more on the roof, if you have space and controller capacity.

edit:redid the math slightly

Thanks BFL13. Good input and makes feel less insecure about my less-than-optimized solar setup. :-)

I compared PWM with MPPT in the past, and found they come out fairly close because with MPPT you have the buck converter in it, which uses the input POWER which is reduced by the panel heating up in the sun which reduces the voltage component in the power.

The PWM controller just passes through the AMPs which are not reduced by panel heating (they go up a little in fact)

You can still get a little more from the MPPT at times, but it is trivial for RV camping.

This summer I had a different solar set-up and got to play with three 100w panels and an MPPT controller. I tried series vs parallel with them and got the same amps either way when there was no shade on any panel. I was not able to run a comparison of series vs parallel in low light. In low light there are hardly any amps anyway, so there is not much to be gained with a small array from that I figure, even if it is better in series then.

BTW, I got this 20a controller when it looked like I would have two 100w panels, but then I got a third so I should have bought the 30a version for an extra 20 bucks. Mistake!!! :(

The 20a version is not limited to 20a as such but has a 260w input limit, which has the same effect. So my 300w array was acting as a 260w array. But now I got to play the "over-panelling" card when the array woukld only be collecting under 260w anyway, and that was when the third panel got to do some good. I should still have chosen the 30a version though.

http://www.wegosolar.com/products.php?product=EPS%252dMPPT%252d20-EPSolar-20-Amp-12%7B47%7D24V-MPPT-Solar-Charge-Controller

You did comparisons for MPPT vs PWM, or series vs parallel with MPPT?
My estimate would be I could maybe get an additional 5 amps out of my setup if batteries were low and the sun was just right with MPPT vs my PWM. Not really enough to make me want to do it right now. But it also seems like an easy way to optimize my existing system.

Note that with two 12v panels in series you have to add their Voc to the controller which has a Voc limit. So yours would be 42 Voc.

The older Eco-Worthy 20a controllers work great but have a Voc limit of only 42. Newer ones are 50 but they have the same model number and appearance. Other more expensive MPPT controllers can be 60 or 100 Voc limit or more. Read the specs.

I have tried two 12v panels in parallel and in series on an MPPT controller (Tracer) and it makes no difference in amps to the battery in good light. Supposedly in low light you get more when in series because of the bigger voltage spread between array and battery. No real way to tell if you are getting that or not while camping.

If you get shade on one panel in parallel you still get the amps from the other panel, but in series it kills both panels. Depends on where you are parked.

You will get a trivial improvement in amps to the battery by swapping your existing set-up for an MPPT set-up. I took lots of comparison measurements and posted the results last year and the year before. But if you don't believe that, it is your money! :)