Am looking at solar on a small class A. I currently have 3 group 27 batteries (rv/marine) for the house. (prior owners had just installed 2 new, and I had one from a trolling motor setup) they will be replaced in about 2 years with 4 GC-2 batteries. I have run across a 325 watt panel , which is at approximately double the voltage you see on the typical RV panels. Am I correct that a 325 watt panel at the following specs is approximately 4 to 6 times the charging capability as compared to a normal 100 watt panel from renogy or another small panel?REC - TwinPeak2 Mono - 325W Solar PanelDimension (mm) 1675(L) x 997(W) x 38(D)Net weight 18.5kgModule efficiency 19.5%Rated Maximum Power (Pmax) 325WCurrent at Pmax (Imp) 9.56AVoltage at Pmax (Vmp) 34.0VOpen-Circuit Voltage (Voc) 40.3VShort-Circuit Current (Isc) 10.15ANominal Operating Cell Temp. (Tnoct) 44.6°CMaximum system voltage 1000V DCMaximum series fuse rating 25AModel REC325TP2MManufacturer RECI believe it should easily be enough for my planned usage, or, should I buy 2 and also due to the higher voltages, what should I get for a charge controller?

100W vs 325W is 3.25 times the power.You need a MPPT charger for the 325W panel which converts the panel amps and voltage to the battery amps and voltage. For a discharged battery 325W/14.8V = 22A (maximum) charging minus wiring and controller losses.

Doesn’t the old saying “watts is watts” apply? And just under 10 amps max isn’t going to be enough for a four battery 6 volt bank IMO. It is a start however. Don’t understand why panel is rated at 10+ amps. My three 100 watt panels putout 16+ under ideal conditions.

I understand the 100 vs 325 part, but what effect does the voltage difference have? Open of 40v vs 20 on typical 100 watt panels? Is there a doubling there as well?

renogy 100 watt as found here renogyversus rec 325 rec along with the 3.25 times as many watts, what effect does the voltage change have? My first thought was a doubling, but I'm no arcky sparky whiz.Would 2 of these 325 watt panels be grossly overcharging a 4 battery setup that primarily runs 2 cpap machines and an absorbtion reefer?

Voc is no load and is used to help verify that a panel is working OK.Vmp is the voltage at maximum power as is Imp the amps. So 34.0*9.56=325W. That voltage and amps is converted to the lower battery voltage at higher amps for charging. 100W panels are often used with a PWM controller which uses Isc of the panel which will be about 5A+.

slightly different solar qx | Good Sam Community

red31
Explorer
Apr 08, 2021
FWC wrote:
red31 wrote:
controller eff of mppt is a real loss of power via heat sink, esp cheaper models!
mismatched or in low power conditions.

While I can't speak to some of the off brand/cheaper models of MPPT controllers, for the reputable ones (think Victron, Morningstar) the conversion efficiency is really pretty good. In the figure below, the orange line is a Morningstar MPPT controller, they don't say which controller the blue line is. In either case the power lost to heat is much lower than the power gained by MPPT and reduced wire losses.

Victron's whitepaper and the graph 7.2 Absolute performance as a function of temperature (~page 10) convinces me with my various small 100w systems that mppt was a waste of my time locally in the heat. Their little blue shaded area is actually wider than they suggest!

7.2 Absolute performance as a function of temperature
https://www.victronenergy.com/upload/documents/Technical-Information-Which-solar-charge-controller-PWM-or-MPPT.pdf

graph assumes mppt has higher voltage to continue to 'buck'. :E
BFL13
Explorer II
Apr 08, 2021
When I got into solar we were spending summers off-grid out in the open at a "seasonal" site we had. PWM and an "aimed" 130w panel got us the AH needed.

I did my solar mostly ground- mounted in a tilting/twirling contraption I made. jimindenver got me onto the low cost Eco-Worthy 20 amp MPPT with a 250ish watt panel as a combo. Worked great. I now have a Tracer MPPT with another panel on the TC's roof.

I was surprised I didn't get as many amps as I thought I would would with the MPPT, but I measured along the whole path and learned where the various losses were. Learned how to do that here mostly ISTR. Once I understood how it really works I was happy with the amps

We still go back there but not for such long stretches and don't have our own site, so now the panels are on the roof. Still get bags of AH per day, hardly ever need the gen in the good weather May to Oct.

Take solar for granted now, lost the urge to measure everything as when a newbie.
CA_Traveler
Explorer III
Apr 08, 2021
When I was considering solar my 750W MPPT setup was only $100 more than PWM. Considering the larger PWM panel footprint, installation complexity, increased daily usable power it was a no brainer. Plus shade tolerance of my 3x serial panels is much better than parallel 12V panels.

It's much the same 7 years later with cheaper panels and more MPPT selection.

There are of course options and installation that favor a variety of equipment.
CA_Traveler
Explorer III
Apr 08, 2021
BFL13 wrote:
The knock that PWM wastes power has no meaning because the panel goes to battery voltage, well below whatever panel voltage is at the time. Watts are of no interest--it is all about the amps.
It's certaintly relevant when comparing 3x100W vs 300W. Both panels panels lose about the same amount of power.

Power loss is power loss REGARDLESS of how the power is used.

PWM power loss is even more relevant at lower battery voltages. For example at 13.4V float PWM can only supply Isc. So when the house load exceeds Isc the extra amps come from the battery while MPPT can supply more than Isc amps, reducing the battery draw down.
BFL13
Explorer II
Apr 08, 2021
My Tracer MPPT shows efficiency for 12v RV system with 17v, 34v, and 68v panels.

The curves vary up and down as the panel watts goes up with higher watt charging power being less efficient. Anyway, very roughly:

-17v (what we usually call "12-12") = 95%
-34v ("24-12") = 93.5%
-68v = 90%
FWC
Explorer
Apr 08, 2021
red31 wrote:
controller eff of mppt is a real loss of power via heat sink, esp cheaper models!
mismatched or in low power conditions.

While I can't speak to some of the off brand/cheaper models of MPPT controllers, for the reputable ones (think Victron, Morningstar) the conversion efficiency is really pretty good. In the figure below, the orange line is a Morningstar MPPT controller, they don't say which controller the blue line is. In either case the power lost to heat is much lower than the power gained by MPPT and reduced wire losses.
2oldman
Explorer II
Apr 08, 2021
BFL13 wrote:
I agree with over-panelling a bit with MPPT to get a bigger AH haul over the whole day
There's very little downside to over-panelling.
red31
Explorer
Apr 08, 2021
controller eff of mppt is a real loss of power via heat sink, esp cheaper models!
mismatched or in low power conditions.
BFL13
Explorer II
Apr 08, 2021
"Below 25C panel power increases while Isc declines favoring MPPT."

That 25C is panel temp, not ambient though.

The knock that PWM wastes power has no meaning because the panel goes to battery voltage, well below whatever panel voltage is at the time. Watts are of no interest--it is all about the amps.

You do get more amps using MPPT, especially in the shoulder hours, but by high noon the MPPT is only a little ahead on amps. I agree with over-panelling a bit with MPPT to get a bigger AH haul over the whole day
CA_Traveler
Explorer III
Apr 08, 2021
Reality check for PWM usage: About 10% of the panel watts are also lost because the higher voltage is NOT used. The linked panel above has Isc of 5.72A and with a battery charging voltage of 14.4V the power loss is also about 10%.

Below 25C panel power increases while Isc declines favoring MPPT.

Truthfully nothing beats installing an extra panel and just enjoying the camping experience without stressing over these power and voltage variations.

3 other factors that I considered and still apply today:

1. Panel cost per watt is much less for 24V panels (both 60 and 72 cell) than 12V panels.

2. For equivalent panel watts the square footage of 24V panels is less that 12V panels.

3. A larger panel array of 12V panels didn't fit as well on my roof.