inexpensive panels

BFL13 wrote:
"I was able to test each 250w panel. They were producing at 37V and 9.1amps. Seemed very healthy. I'm curious if anyone can comment on the likely output of these to the batteries from the MPPT controller after the controller converts it to be used on my 12v system... Is there a simple equation for this?"
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if the panels are aimed at the sun at high noon, your 500w loses 10% from panel heating, so now at 450w. Line loss from panels to controller using proper wire should be under 3%, so call it 2% off the 450, so now at 439w input watts to controller. Say controller is 95% efficient doing 24-12, so lose 5% of 439 = 22w so output watts = 417w

Amps to battery (assuming no other loads) = output watts / batt v, so
it depends on batt v at the time what amps are. (there might be some more loss on the wires from controller to battery too, but we are pretending that is no loss for this example)

417/13v = 32 amps
417/13.5 = 31 amps
417/14 = 29.8 amps

With 500w of PWM aimed at high sun for full Isc (you get about the same amps to battery as the Isc and no loss for panel heating since Isc does not drop with heat (it goes up a bit)--expect about 31 amps

31 = 31. but as previously mentioned that is with panels aimed at high noon on 21 June in say, Texas.

You need another calculation for say, 9am with flat on roof panels on 21 Sep in, say Seattle, and another for 21 Dec.

All you can do is stick those panels up there and you get what the weatherman will allow that day for your latitude and calendar date. Looks like you got a very good deal on those 250s!

Great info! Taking into account all the potential losses here and there that you mention, the 500w drops down to closer to 417w under "ideal conditions". Well, we know that we will almost NEVER be in "ideal conditions" so the panels will produce less than 417w most of the time. That makes me think that the 30amp MPPT controller is going to work perfectly and it will most likely NOT limit the charging probably ever. I was not "worried" but was just a little bummed thinking that the 30amp controller might actually cut off the maximum amount of amps provided by the panels sometimes, but it sounds like max amps out will likely be somewhere between 25-30amps 99% of the time. Good enough for me! :-)

Too much holiday stuff going on/planning right now to do the install without my wife killing me...lol. So I'll likely tackle installing solar onto my 2 RVs in January (one with a 200w WindyNation sytem which cost ~$280, and the other with a 500w SanTan Solar used panel system which cost me $225). When I do, I will run some tests and collect some data and report back. I'm also going to lay out my 200w WindyNation panels such that I can easily add a 3rd 100w panel if I decide to do so later (...the controller can handle up to 300w).

Testing fun... My battery bank on each RV is ~200AH. I will have an Aili battery monitor on each RV when I do the testing. (I already have it installed on one and love it). I plan on doing the following test: Drain each RV battery to 50% at nighttime. Then 3-4 times throughout the next day I will record the Amps flowing into the battery and the SOC% as measured by the battery monitor. I will do one day of 200w WindyNation PWM vs 250w SanTan MPPT, and another day with 200w WindyNation PWM vs 500w SanTan MPPT. Yes, I'm a bit of a nerd...lol

Yes, I think $35 per 250w panel was a pretty good deal ;)
(But it was only possible because I happened to be visiting someone who lives ~10 miles from SanTan headquarters...otherwise I'm 700 miles away and shipping makes them impractical). But if you go look on your local craigslist you will likely find some very good deals on used 200-400w panels locally that you can pick up. Just be sure to bring a volt/amp meter and test them.

For my case:
- New WindyNation 200w system cost: $1.4/watt
- Used (panel) SanTan Solar 500w system cost: $0.45/watt (68% less)

I'm guessing that with 500w of solar and 200-300AH of battery bank, I'll never have to worry about keeping my batteries fully charged. Now, I'm gonna go and create a new post: How to use MORE battery power when camping!

Chris