Questions about solar power - Page 2 - Fiberglass RV


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Old 03-05-2014, 11:52 AM   #15
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We have two trailers, a Scamp 5er and a Surfside. We've had the Scamp for seven years now, and installed a 50w roof-mount panel on it shortly after we bought it. It wasn't quite enough for our needs, so we have since installed a second 55w panel. That does it pretty well for us. The panels cost us around $400, I think. Prices have come way down.

We bought the 100w panel we installed on our Surfside from SolarBlvd for $125. Haven't had the trailer out yet, as we're still in the middle of our refit, but I did test the panel in the sun and SolarBlvd has a good reputation.

How much panel do you need? Depends a lot on how much power you use and what size battery you have. For us, 50w was just below our threshold and 100w is major overkill. We probably need about 60w or 75w of panel to meet our needs.

Something we've included in our Surfside plans is a secondary, portable panel we can set out in the sun for times when our trailer is parked in the shade.
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Old 03-05-2014, 01:04 PM   #16
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Originally Posted by MCDenny View Post
Now with the new fridge we enjoyed boondocking so much I'm researching solar panels to see how expensive and practical it would be to extend our off-grid time.

I've determined I'd like to get around 300 to 400 watt-hours of battery charge per day. We are in Florida at about 28 degrees N latitude and the 'full sun' period here is about 6 hours per day.

I would like to learn more about

1) I think a portable unit that folds in half for storage would be the best for us as we try to park the trailer in the shade. We could put the panels in the sun and adjust the position as the sun moves across the sky. Does this mean I can expect more juice than the insulation tables suggest? It seems like they are based on flat or at least stationary panels.

2) my first guess is that I need about 100 watts of capacity. I see kits on offer for between $200 to $600 for this much capacity but I don't understand what the difference is. For instance

this 90 Watt Portable Solar System - Professional Series | Off The Grid RV SolarOff The Grid RV Solar at $625

This 90 Watt Portable Solar System - Economy Series | Off The Grid RV SolarOff The Grid RV Solar at $425

And this 100WATTS Portable Folding Poly Solar Panel 100W 12V RV Boat Battery Charger Kit | eBay at $200

3) I consume between 500 and 600 watt hours per day and have enough battery for 4ish days with no recharging. I'd like to double or triple that. Does a 100 watt panel sound like about the right size?

Thanks for your help.

First off a 100Watt panel will probably do everything you need. We use a 65 Watt and it's more than adequate to keep two separate batteries charged and only charging the house battery about every 3rd day or less depending one the weather. I would recommend that all lights be changed to LED to reduce battery consumption.
I would also recommend that for solar panel sizing and calculations you use amps and amp-hours rather watt hours. Watts are great way to calculate when using different voltages, i.e. a 1000 watt heater draws 8.3 amps at 120 volts and 83.3 amps at 12 Volts. Your battery is rated in amp hours, the battery is what you're concern about. If you've been talking to a solar "expert" he starts talking in watt-hours, don't walk away, run. He's probably doesn't know what he's talking about and trying to flim-flam you.
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Old 03-05-2014, 04:19 PM   #17
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Originally Posted by Byron Kinnaman View Post
First off a 100Watt panel will probably do everything you need. We use a 65 Watt and it's more than adequate to keep two separate batteries charged and only charging the house battery about every 3rd day or less depending one the weather. I would recommend that all lights be changed to LED to reduce battery consumption.
I would also recommend that for solar panel sizing and calculations you use amps and amp-hours rather watt hours. Watts are great way to calculate when using different voltages, i.e. a 1000 watt heater draws 8.3 amps at 120 volts and 83.3 amps at 12 Volts. Your battery is rated in amp hours, the battery is what you're concern about. If you've been talking to a solar "expert" he starts talking in watt-hours, don't walk away, run. He's probably doesn't know what he's talking about and trying to flim-flam you.

Thanks Byron,

All the lights are LED, phone and computer charging can be mostly done in the car so not much inverter use.

Since putting up this post a couple of days ago I've been doing my own research and learned I can expect the equivalent of five 'full' sun hours each day here in S Fl in January. I have a 220ah battery bank use about 40ah/day so can go 3ish days at 50% discharge. I'd like to at least double that so at a minimum my solar system would need to harvest 120ah over six days. In theory with perfect cloudless sky that would be 5 hrs x 6 days = 30 hrs. 4 amps per hour x 30 hrs = 120 ah.

4 a x 12 v = 48 watts. 100 watts is double my theoretical minimum . Is this an OK safety factor to allow for clouds and shade. We will have a portable, probably folding, panel so shading shouldn't be too bad.

A question about solar panel ratings: typically a 100 w panel is rated at 17 volts output. A controller would knock that down to 13.6v if the battery is nearly full and 14.5 if they are somewhat discharged. Figuring worst case do I derate the panel by 1-13.6/17 or 20% to allow for a PWM controller's "waste". If I got an MPPT controller could I count of the full panel output?

Thanks for your help.
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Old 03-05-2014, 04:40 PM   #18
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The higher the voltage of the panel, the more amps you'll actually get into the battery. Panels rated higher (>17V, therefore more costly) will be a better deal than an expensive MPPT for a small system.
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Old 03-05-2014, 04:50 PM   #19
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Originally Posted by MCDenny View Post
Thanks Byron,

All the lights are LED, phone and computer charging can be mostly done in the car so not much inverter use.

Since putting up this post a couple of days ago I've been doing my own research and learned I can expect the equivalent of five 'full' sun hours each day here in S Fl in January. I have a 220ah battery bank use about 40ah/day so can go 3ish days at 50% discharge. I'd like to at least double that so at a minimum my solar system would need to harvest 120ah over six days. In theory with perfect cloudless sky that would be 5 hrs x 6 days = 30 hrs. 4 amps per hour x 30 hrs = 120 ah.

4 a x 12 v = 48 watts. 100 watts is double my theoretical minimum . Is this an OK safety factor to allow for clouds and shade. We will have a portable, probably folding, panel so shading shouldn't be too bad.

A question about solar panel ratings: typically a 100 w panel is rated at 17 volts output. A controller would knock that down to 13.6v if the battery is nearly full and 14.5 if they are somewhat discharged. Figuring worst case do I derate the panel by 1-13.6/17 or 20% to allow for a PWM controller's "waste". If I got an MPPT controller could I count of the full panel output?

Thanks for your help.

Look at "short circuit" current for maximum current you can expect. It should listed in the specifications. Higher voltage does not necessarily equate to higher current. Actually it the opposite for equal wattage ratings.
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Old 03-05-2014, 05:50 PM   #20
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Questions about solar power

But how does the controller reduce the panel's 17v to the 13.6v float charge the battery needs. Don't PWM units turn off the panel output( I know, 1000s of times per second) so the average voltage over time is 13.6? That would mean the panel is turned off 20% of the time the sun is shining.

And I thought MPPT controllers had some way of getting all the power and converting it to the lower voltage.

Please enlighten me. I'm just trying to learn this stuff.
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Old 03-05-2014, 05:56 PM   #21
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It has been my understanding that a PWM controller discards the additional voltage, whereas a MPPT controller will utilize the higher voltage to push more current (with some internal losses due to power consumption of the MPPT circuitry). Which would seem to suggest that a 21V, 3A input would be turned into something like a (roughly) 14V, 4A output. Yes, no? Discussion?
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Old 03-05-2014, 06:11 PM   #22
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A charge controller prevents the solar panel overcharging the battery. The MPPT advantage doesn't go into effect until the batteries are so low that more amps can be forced in at a higher voltage. To get that small increase in wattage, you pay a lot of money.

You'll also have a lot of loss the farther from the battery all your equipment is - that's one of the reasons I was arguing about putting the batteries on the wheels and extending the cables. The voltage drop over the system would be considerable.

I'm still planning to put a couple of panels on the roof of the Snoozy, connected to a PWM under the bed and very close to the batteries - using heavy wire for less resistance. Haven't figured out the size of the system, yet, but I'm loving your calculations of use:-) They're real world help for my system, so Thanks!
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Old 03-05-2014, 07:13 PM   #23
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You'll also have a lot of loss the farther from the battery all your equipment is - that's one of the reasons I was arguing about putting the batteries on the wheels and extending the cables. The voltage drop over the system would be considerable.
I would be surprised if anyone here was to argue with you over the fact that its best to put a solar charge controller as close to the battery as possible (mine is actually in a water proof box connected directly to the battery box) and most installs I have seen here have been done with the solar controller as close to the batteries as possible.

I believe the suggested made to you & which you argued against was in reference to your concern for the location of the "electronics" as you referred to them with no mention of it actually being a solar controllers location you where concerned about. As such the assumption was made that the "electronics" you were concerned about was your power converter. On many trailers the converter is more often than not located at the opposite end of the trailer from the batteries thus the reason the suggestion was made.
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Old 03-05-2014, 07:39 PM   #24
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I think I also mentioned electrical "stuff" :-)

In the Snoozy, converter (and stuff) are located under the bed in front. With the batteries on the tongue and the other electrical operations under the bed, solar's a relatively easy add. When I was at the factory, Richard and I discussed a way of glassing in the cable run through some sort of conduit so that it would be an integral part of the trailer and would take a linear route to the controller, making it a shorter run.

As I've said, though, I'm open to suggestions.

PS "Argue" was used in the sense of debating, not fighting.
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Old 03-05-2014, 08:44 PM   #25
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I finally found an understandable discussion of PWM vs MPPT controllers. It's a white paper written by Morningstar, leading maker of both types. Spoiler alert: It says PWM is recommended where power is low <200 watts and temperature is warm.

http://www.sunwize.com/documents/Mor...r-jf_12-12.pdf
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Old 03-05-2014, 09:00 PM   #26
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I just added a review of Solar Blvd's 200 watt folding panel at my website.
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Old 03-05-2014, 09:06 PM   #27
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In as much we are talking a small trailer unless you park the panels a long way from the trailer, line loss while always an issue will not be major.. You do need to be primarily concerned about cable size from the panel to the charge controller
We have the folding 100 Watt from Solar Blvd and use the stock cheap controller and connecters that came with it.. Works well for our purposes, maybe not so for others.

Now for our house system. Different story.. Mppt controller and wiring sized specifically to distance from panel to charge controller.. Only a couple of feet from batteries and inverter.
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Old 03-05-2014, 09:20 PM   #28
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I just added a review of Solar Blvd's 200 watt folding panel at my website.

Thanks Jon. (And thanks for posting a link to my blog. I get lots of visitors coming form Lakeshore images)

You have a 160w panel but measured 8.5 amps at, I assume, 12.7 volts for a fully charged battery. That's 108 watts. What's up with that?
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