bob day

I found the site you used to produce the diagrams/formulae and have bookmarked it, but have too much to do to worry about it now. webpage

It may be that these are formulae given to field installers that helps them maximize systems, without needing to do the (likely) much more complex computations, a shorthand  of sorts . I say this because the voltage increase of the lower volt panel  (in my little experiment) was not proportionate to the increased size and voltage and total watts of the larger panel. To preserve all the watts (or most of them), the voltage increase should have gone closer to 30.


It's all just theoretical at this point, my system is already up with newer  matching panels and outproducing the old system, and our discussion has convinced me to match voltages as close as possible in parallel, and amps as close as possible in series as I go forward setting up outlying systems.

Oh, it has also reinforced my idea of many smaller independent controllers and battery banks

speaking of which, I just heard the water filter in my little koi aquarium come on, an independent system I set up with a good controller yesterday. That means the battery voltage is already up to 12.7. Seems like it will be a good day :-)

that actually makes more sense to me as well,  

the good news is with two identical  panels so far hooked up (i have seven in all) it's looking like I'll have  plenty of extra power--in fact I'm already thinking it's time to double my storage

So you're saying if I hook up in parallel, two panels of different voltage without a controller,  and read the open circuit voltage, I won't get some voltage in between the high and low values of the panels?

The 26v I mentioned was the voltage output from two panels in parallel, and more or less a direct data driven response to the formula put forward earlier in this thread that the lowest voltage in the circuit was the determinant voltage of the pv supply.

the power point modulated by the controller  was likely around 16-18 v, but I didn't directly record that.

today with a 20v panel and a 36v panel hooked in parallel , the controller read about 26 volts overall.

You have given me enough information/ insight to start to chart a course. The main drawback was just getting plywood down on the roof so I could mount some panels (beyond the five small ones up there now.

Toward the end of today I was covering up the roof and disconnecting them, but I also had moved  several of the other more powerful but mismatched panels up there, so now the roof is quite a mix of stuff, most of it hooked to nothing. So tomorrow early I go ahead and load the 7 newest matched panels still on the ground onto the roof and take down all the others. This will be my primary house supply--1500 watts in all.

The others will get used on secondary systems, and i do like the idea of multiple systems, so maybe I'll have a second system  set up with a dedicated battery bank for a refrigerator- freezer (I'm using a propane one now, spending 10-20$ each month (not counting travel time and car wear and tear). Maybe a third system for the fish tank filter, etc etc.

I have several smaller controllers on order--I usually only buy mppt , but the chinese seem to use those initials loosely, and even though I'm also spending 67$ on a new 80A controller,  maybe I need to consider them all mostly pwm's, and think about a really well engineered  one for the central system that won't waste so many watts.

I actually run one system without a controller at all, and have found two 35 watt (ancient) panels put just enough juice into the 12v pump battery to keep it charged and running. Hooking a controller in the circuit seemed to interfere with the charging, although those were all pwms I tried, 10 plus years ago, maybe the newer pwms are better.

I always hook my panels in parallel anyway, but it's good to have that validation. I would think though that the actual system voltage would be somewhere higher than the lowest voltage in the system.

I get how the minimum amps holds back the overall amps in series.

I was looking at my controller today, hooked to older 20v panels, charging 12 v system, newer panels are 36 volt, so I will probably move the 20v panels to other places for pumps and such. and keep the newer 36 v panels on the roof. that will mean matched panels that are meant fore a 24v system, and I was really just curious if  anyone knew if a controller could step the voltage down and amps up that far. Maybe it depends on the controller, mine are not the cheapest , but far from the most expensive.

Remember those guys are into forges, which can go way beyond the temps rocket stoves and batch burners play with.  I think 1500 (F) is a nice high end mid range for us, while they run to 3000 (and beyond)

I got several solar panels (about 2300 watts) for 500$