inexpensive panels

Wanted to post a follow-up on this "inexpensive panels" thread...

I finally got around to installing the 500w system on my RV with two 250w panels. I'm happy to say that the SanTan used solar panels (~10yr old) are working beautifully and cranking out plenty of power.

Here's a good link to the install project:
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/30217540.cfm

I'll be adding a 3rd lead acid battery soon making my battery bank 300AH. Then, as a test I'll drain the batteries at night to 50% (-150AH) and I'll let you know how much the 500w system gets them back up to by the end of the next sunny day.

I liked this setup so much, I've decided to ditch the brand new 200w Windy Nation syetem and just install an identical 500w system (two 250w SantTan panels and a 30amp MPPT controller) onto my 2nd RV. If anyone lives in the CA Bay Area and wants a good deal on a brand new 200w Windy Nation system, feel free to send me a private message.

Happy Camping!
Chris

Hi Chris,

It is easy to use the "excess" energy, if you have an electric/gas water heater, and 2000 watt inverter. Simply turn on the water heater, after the batteries are full.

To turn the watt hours and watts in my post to amps and amp-hours, simply divide by 14.8 volts.

My guess is that with 500 watts, the solar harvest will max out about 135 amp-hours.

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?"
----------
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

"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?"
----------
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!

SJ-Chris 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?

-Chris

Hi Chris,

I would definitely go with 500 watt systems.

I'll be interested in your real life results.


High end MPPT charge controllers "steal" about 5% of the output.

500 watts X .95 = 475 watts X 5 hours = 2375 watt-hours (in perfect solar conditions).

Batteries "steal" 10%. I.E. use 100 amp-hours and return 110 amp-hours.

2375 X .9 ~= 2137 watt-hours sent to the battery.

Depending on the type of battery (a single 100 amp-hour) acceptance rates are, at 85% state of charge 12.5 amps

12.5 x 12 volts = 150 watts for each 100 amp-hour battery.

In SiO2 that is doubled to about 300 watts.

In Li the battery management systems typically limit charging to 1C or about 1200 watts.

So the answer to your question really depends on the type and number of battery in use.

Forget used panels.
forget cheap panels and controllers.
Then you just have to buy more of them.

You won't regret going with 500 watts. For those days that are mostly cloudy you want to get charged as quick as possible.

Thanks all for the MPPT vs PWM explanations...I understand now. These ~37v panels will need the MPPT controller to be properly utilized.

Well...I found myself in Arizona within 10 miles of SanTan Solar so I couldn't resist going over to check them out. I had anticipated purchasing 2 of their 250w solar kits each which come with a 20a MPPT controller. Each kit cost $165. Seemed like a "reasonable" way to add solar onto my 2nd RV. I've already got a 200w WindyNation PWM kit (~$280) that I am putting onto my other RV (I have two) once I get some free time. (I planned on buying an "extra" 250w solar kit because I'm expecting to add a third RV in a couple of months.)

I want to (eventually) run a test to compare the output of the 200w PMW WindyNation system to the 250w MPPT SanTan Solar system. I'll let you know once I do that and post the results here (might not be for a while).

Well, as luck would have it, just the other day they lowered the price on the 250w panels from $50 to $35....they are practically giving them away...lol. So I could have bought their complete 250w solar system for $150 total each. They have a nice showroom and were very helpful. Because the panels were so cheap, I asked them what would be the easiest way to upgrade the system to be able to handle 2 panels (500w). Well, for just $10 more, they had a 30a MPPT controller. Might not be completely large enough to handle all 500w, but for just a few dollars more I could go with 500w of panels. Total cost for each "500w" solar system was just $225. Considerably less than the 200w WindyNation system I recently bought. This system will likely be a total overkill for my needs, but because it was so cheap I couldn't resist. :)

I was able to test each freestanding 250w panel. They were producing at 37V and 9.1amps as measured on a volt/amp meter. Seemed very healthy. They have a VMP of 30.3v and a max current rating of 8.27amps. If I understand correctly, the MPPT controller will covert that down to (for example) 14v with a current of 17.9amps (per panel). (30.3/14.0 x 8.27). Does this sound about right (under an ideal scenario)? Assuming this is correct, then in real life application the 30amp MPPT controller should work okay (during peak sunlight, the panels might be outputting slightly more than 30amps so there would be some inefficiency, but I can live with that because everything was so inexpensive).

The reality is that I probably only need 200-250w of solar power. I have currently just ~200AH battery bank in each of my RVs (could pretty easily expand up to 400AH if I felt the need). I kinda just bought the extra panels because a)They were only $35 each, and b)I was in my RV so I could easily transport them back to NorCal for future use. Not sure yet if I'll put up 250w or 500w. When I do and have some results I'll let you know.

-Chris

SJ-Chris wrote:
How about this charge controller?
https://www.amazon.com/gp/aw/d/B089SH3W85/ref=sspa_mw_detail_6?ie=UTF8&psc=1&pd_rd_i=B089SH3W85p13NParams#aw-udpv3-customer-reviews_feature_div

$49.90 with free shipping
60a PWM
12/24/36v battery system
States up to 720w if using a 12v system.
Maximum input voltage from the panel: 92v according to the seller.

If these specs are true, then it seems like you could buy two 250w panels for $50x2= $100 and this charge controller ($50) and all the misc other items (~$60) and have a 500w solar system for ~$210.
Note: This requires picking up the panels as shipping makes them too pricy. (I happen to be visiting near where their business is located in Arizona right now.... :-)

What am I missing here? Thoughts?
Chris

To be very clear.

PWM controller while it CAN take a higher voltage in, it WILL need a HIGHER VOLTAGE BATTERY BANK to work correctly.

In other words, PWM 30V panel = 24V battery bank voltage.

It PWM WILL NOT WORK with 30V panel to 12V battery bank.

NO PWM controller have any means to decrease the 30V output of the panel to a usable 12V charging voltage.

To use ANY 30V panel with a 12V battery bank you NEED to go with MPPT.

IF you do not want to buy MPPT but want to buy and use a 30V panel voltage then you MUST convert to 24V battery bank.. But then you now have the rub of all of those 12V appliances, right?

Well then you will NOW have to spend more money on a DC-DC converter to bring the 24V battery bank voltage down to a usable 12V..

More money, more things to go wrong, burn up, smoke and now you have a very non-conforming, non-standard "one off" system that most places will never want to touch if it goes badly wrong.

Go that route, you better read up on it and understand everything 100% and you better buy some spare critical parts since you will not be pulling into a Walmart to find parts for it.

If you WANT those low cost $50 30V panels, BUY THE MPPT CONTROLLER, just realize your "bargain" is not longer as much of a bargain because you MUST spend more for the controller ($100-$200) or configure a 24V battery bank and HIGH POWER DC-DC converter ($$$$).

Example a no name 60A 24V to 12V buck converter can cost $120 HERE..

But wait, theres more..

With a lot of buck converters you can run into issues, they tend to not be "isolated" output which now you must pay close attention to whether it is regulating the negative ground or is is regulating the positive voltage..

And not to mention that is only 60A and your batteries CAN supply a lot more than that..

For more insult to injury, if you want a inverter to make some 120V, well that 60A converter isn't going to cut it if you have more than 500W-600W non inductive load.. Yep, you now will have to spend more money on a 24V input inverter..

Be cheap if you wish, that is your prerogative, but personally, SKIP those $50 24V panels if you do not want to spend money on the MPPT controller.

"What am I missing here? Thoughts?
Chris"

You seem to have a mental block about 24-12 vs 12-12 or 24-24. You must understand that an MPPT controller has a buck converter and a PWM does not.

So an MPPT can do 24-12, 12-12, or 24-24, BUT
A PWM can only d0 12-12 or 24-24

Next you must understand that a "12" means it has a Voc around 22v and a "24" has a Voc around 37v. A "12" will be at or under 190w, while a "24v" will be 190w or above usually.

So be looking at the whole set of panel specs, not just the watts, so you can see right away if it is a "12' or a "24"