My proposed Solar System

I'm starting to plan out our solar system and will be buying the components over the winter.  Our 30' trailer roof (pic attached) has ample open space for 6, possibly 8, RCL-200w Hightec solar panels (pic attached).  If I install 6 panels, I was thinking 2 sets of 3 in series, paralleled to the controller.  If I go with 8 panels; it would be 4 sets of 2 in series paralleled to the controller.  Putting the 8 panels on the roof might be a little tight and I would end up with shading I'm sure.  With 6 panels that would give me 1200 watts.  I currently have 4 100ah Battle Borns and thinking I'll add 2 more this winter.  I read someplace a good rule was to have 1 1/2 to 2 times more solar than your battery bank, 6 panels would give me that if I add the 2 batteries.

I have been reading info on the MPPT controllers and I'm a little confused.  If I go with 6 panels (2 sets of 3) I'll be around 75 volts, 20 amps going into the controller.  Does this mean a 100/30 MPPT controller will work with this setup?  Help here would be appreciated.

My current 400ah system (pic attached) worked very well this summer.  Didn't get out as much as we wanted but on a 5 night/6 day stay in the Adirondacks I only had to run the generator once about 3 hours to charge the batteries on our stay.  Adding 2 more batteries and the solar we should be pretty well set for power I would think.  Our biggest draw is the mini fridge in the outside kitchenette we use for drinks.  I've been looking around for a more efficient beverage fridge for a replacement, any ideas there would be helpful.

Looking forward to everyone's insight and ideas... this is a Great forum!

I'm a fan of some solar "overkill" for those cloudy periods, but based on what you've described, I think 1200 watts is going to be quite a bit of overkill. There's nothing "wrong" with that, it's just that it can be unnecessarily expensive. As a frame of reference, we've got 960 watts of solar, and an 840 AH battery bank, and we haven't had to run our generator in over a year to recharge our batteries.

Our six panels are wired so there are three sets of two panels wired in series. We've got a large (22cf) residential fridge and inverter, so our coach is pretty power hungry compared to most RVs. On sunny days, even in the winter months (the lowest sun angle) we always see more than 50 amps during peak charging periods, and we're always at 100% SOC by 1:00 PM... 3:00 to 3:30 on cloudier days.

FWIW, I would never install a panel knowing that it will be shaded, especially if it's connected in series to other panels. It's just a waste of money. Shading just a few cells is all it takes to reduce a panel's output to zero, as well as any others that are connected to it in series.

When people start talking about installing relatively large arrays, I always recommend that they first install a good battery monitor to measure their actual power consumption. Once they know how much power they actually use, they're better able to "smartly" determine how much solar and battery capacity they're going to need.

Kev

hopefully you will read this : http://www.rvforum.net/SMF_forum/index.php/topic,113514.0.html

a few points to note here..


1. 1200 watts usually requires a higher voltage battery, however in this case with LFP we CAN think differently and forget FLA rules, but at a cost
as we shall see below..

2. 1200 watts of panel into a 12 Volt battery requires a ( 1200 / 12 ) = 100 Amp controller !!!
this is not a good idea and certainly expensive..

3. using a prime number of panels in series is not normal practice for off grid, however if the VOC is in limits then you can do this.

for 6 panels on an RV you only have two practical choices: 3S2P or 2S3P there is the option of 6S, but unless
you boon dock in the desert with no shade, then it's not recommended.

lets look at the panel output...

for 3S2P you will have 3 * 21 vmpp = 63 Volts at a 9.5 A * 2 = 19 Amps
the main issue here is the VOC voltage.. in cold weather the panels in series will max at approx 25 * 3 * 1.25 = 93.75 Volts.. that's uncomfortably
close to 100 Volts of your proposed controller. one really cold desert night with bright morning sun will damage a 100 V controller.

for 2S3P you will have 2 * 21 vmpp = 42 Volts at 9.5 Amps * 3 = 28.5 Amps
and a max VOC of 25 * 2 * 1.25 = 62 Volts. a much better fit for a 100 V controller.

ok, now lets talk MPPT controllers..

1200 watts is well above what the industry recommends for 12 V, also at this amperage ( 1200 W / 12 V ) = 100 Amps you will need
heavy cabling from the controller to the batteries. quite often with lower cost controllers the input terminals are sub standard and cannot
accept the correct size cables so beware...

your controller inputs are :

a) 63 Volts at 19 Amps with 3S2P
b) 42 Volts at 28.5 Amps with 2P3S

and your output is 12 V at 100 Amps max.

you can expect to pay upward of $700 for a 100 A controller. also there is the cabling issue.. for such high
currents you will need heavy cables to minimize losses and that adds to cost and weight.

let's do a simple example using 35 feet from panel to controller and 6 feet from controller to battery bussbar
we are going to design for 3% or better voltage drop in both paths. ill omit the calculations for now..

Using standard AWG cable resistance for copper:

for 3S2P we have 63 V @ 19 A, we could use a minimum of 10 AWG
for 2S3P we have 42 V @ 28.5 A, we could use a minimum of 8 AWG
for 12 V @ 100 A we could use a minimum of 3 AWG

ok so $700 for a controller is a bit high.. so what other options are there ?

well, in the 3S2P setup we could use two 50 A controllers..
each string is considered as a single array, the controllers would both feed the 12 V busbar.
only thing here is to setup both controllers with the same charging profile.

for 600 W we require 600 / 12 = 50 Amps..

a good 50 A 150 V MPPT can be had for $250 upwards..


battery capacity:

for LFP, use my calculations on the page I linked for lead acid and multiply by 0.75
( for 100 Ah FLA you need 75 Ah LFP )



ok.. what would I do ?

well i would do a power budget and determine my REAL needs first, then design the system around that..

if you intend to stay 12 V then I would suggest to lower the panels to 800 watts, increase capacity to 600 Ah and set the array
in a 2S2P format and use a Victron 150/70 controller. or expect to spend big bucks on a high current controller..

without the 12 V limitation then I would go up to 24 or 48 V..

Kevin Means said:

I'm a fan of some solar "overkill" for those cloudy periods, but based on what you've described, I think 1200 watts is going to be quite a bit of overkill. There's nothing "wrong" with that, it's just that it can be unnecessarily expensive. As a frame of reference, we've got 960 watts of solar, and an 840 AH battery bank, and we haven't had to run our generator in over a year to recharge our batteries.

Our six panels are wired so there are three sets of two panels wired in series. We've got a large (22cf) residential fridge and inverter, so our coach is pretty power hungry compared to most RVs. On sunny days, even in the winter months (the lowest sun angle) we always see more than 50 amps during peak charging periods, and we're always at 100% SOC by 1:00 PM... 3:00 to 3:30 on cloudier days.

FWIW, I would never install a panel knowing that it will be shaded, especially if it's connected in series to other panels. It's just a waste of money. Shading just a few cells is all it takes to reduce a panel's output to zero, as well as any others that are connected to it in series.

When people start talking about installing relatively large arrays, I always recommend that they first install a good battery monitor to measure their actual power consumption. Once they know how much power they actually use, they're better able to "smartly" determine how much solar and battery capacity they're going to need.

Kev

Click to expand...

I mostly agree with what Kevin said and agree 1200 watts is likely overkill in your case and recommend you doing an energy audit on your electrical needs. Like Kevin, I installed 960 watts of solar panels, except ours are 3-high voltage residential panels and I wired them in series to maximize our MPPT controller output.  We too have a residential fridge and fairly high energy demand, though admittedly I suspect not as much as Kev's. For our battery bank I installed 7-100Ah Battle Born lithium batteries and so far our system has proved quite adequate for our energy usage. 

The only exception I would note is "FWIW, I would never install a panel knowing that it will be shaded, especially if it's connected in series to other panels".  This is essentially correct and was prevailing wisdom, but less of an issue with recent solar panels, as many now have diodes within the solar panel circuitry to minimize the effect of shading.  So while shading, even with more modern solar panels will reduce solar output, not nearly as dramatic as older panels without this technology.  Having said this, it is still good advice to mount your panels to eliminate any potential shading or at least minimize the potential of shading from potential shading.

Well... I've been dealing with Vertigo since last Saturday and looking at all the numbers Solarman threw at me, my head is really spinning.. lol.  Figured I'd post a quick reply now and answer a couple questions.  When I feel better I can decipher the numbers.

I have done somewhat of an energy audit on our trips this summer.  As of now we use around 100ah's a day give or take.  Once the solar is installed and my DW gets comfortable with having electric off grid, I can see our usage increasing slightly.  As I mentioned in the OP, we use a mini fridge for beverages and it seems to suck a lot of power, almost 6 amps.  Looking for a replacement for that.

My thinking (right or wrong) on installing 1200 watts.  Right now we live in the NE and most of the places we camp are in the woods so shading is inevitable.  The roof of the trailer has ample open room to install 6 panels with no shading.

Let me ask about shading.  As I said we are in the NE and camp in the woods most of the time.  We try to find sites that have some sun but they aren't optimal for solar by any means.  With all that said, maybe solar is not the best option for us right now?

With retirement just a few years out, my plan was to get our trailer all set up while I was still working.  It would be one financial expense I would't have to plan for once retired.

The angle at which sunlight hits a solar panel, and how much a panel is shaded, will have a significant impact on a panel's output. How much so depends on several things. The number of cells that are shaded, the severity of the shading (thin whispy high clouds vs a thick overcast and everything in between) thick tree cover vs. a few branches... will all have an effect on a panel's output. Mono panels supposedly do better than poly panels in the shade, but not by much.

As a frame of reference, last week, we were camping in the Arizona desert, and our flag pole was up. At about 9:00 AM, the breeze was causing the flag's shadow to intermittently shade one corner of one panel. As it did, you could see the difference in charging on the battery monitor. It varied by about 3 amps - from about 6 to 9 amps. Of course, there isn't much charging going on that early, but I just wanted to emphasize the impact of shading.

The time of year and how far north/south you are will also have a strong affect on solar output.

Kev

Kathy & Bill said:

I have done somewhat of an energy audit on our trips this summer.  As of now we use around 100ah's a day give or take.

Let me ask about shading.  As I said we are in the NE and camp in the woods most of the time.  We try to find sites that have some sun but they aren't optimal for solar by any means.  With all that said, maybe solar is not the best option for us right now?

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100 Ah 12 V = 1200 W/hr . as you say, you may need a little more so round up to 1500 W/hrs. here is your shopping list...

you will need 800 Watts of panel, approx 600 A/hr of battery ( LFP ) and an Outback Flexmax 80

for shading then 2S2P is the way to go. additionally a portable panel set of 200/400 Watts would be a good help to mitigate shading.

on your controller/charger, set bulk/absorption at 14.4 V and float at 13.6 V. disable equalization.

don't forget the generator for backup..

Kathy & Bill said:

I'm starting to plan out our solar system and will be buying the components over the winter.  Our 30' trailer roof (pic attached) has ample open space for 6, possibly 8, RCL-200w Hightec solar panels (pic attached).  If I install 6 panels, I was thinking 2 sets of 3 in series, paralleled to the controller.  If I go with 8 panels; it would be 4 sets of 2 in series paralleled to the controller.  Putting the 8 panels on the roof might be a little tight and I would end up with shading I'm sure.  With 6 panels that would give me 1200 watts.

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I may take some flak on this, but I am of the persuasion that as far as panels go, put as many on the roof as you can fit.  Panels are relatively cheap in the overall cost consideration, and their price keeps going down.  4 panels vs 6 panels?  2 more are not going to cost a lot more, but will give you a healthy 'reserve' for rainy or cloudy days.  I've never heard anyone complain of having too much solar. 

Also, if you are buying residential 72 cell panels, their output voltage (around 40 v) is high enough that you don't have to series connect any panels at all, greatly reducing the shading problem.  If a panel is shaded, only THAT panel is removed from the array.  If it is in series with another, you lose ALL the panels in that string. In an all parallel setup, you will need heavier wire for the 'drop' from the roof to the controller, but wire is cheap.

Here is a link to our 1230 watt setup, and I don't regret going for the 6 panels.  Even after using a lot of power overnight, including a hot air corn popper, and then an electric toaster and waffle iron in the morning, our system is fully charged by noon the next day in Arizona in winter.  And I even get a useful charge on overcast and rainy days.  I like not having to worry about whether the power will run out.  While many considered the 6 panels overkill, I had room for all six, and they were only a part of the total cost.

http://www.rvforum.net/SMF_forum/index.php/topic,106669.msg1003893.html#msg1003893

Just one man's opinion. 


Frank.

I completely agree with what Frank said. Just remember to size your battery bank accordingly so you can store all the power they generate. If I could have fit six larger panels on our roof without incurring major shading problems, I'd have done it, but it just wasn't possible. As it is, we have to tilt our smaller (160 watt) panels to get enough power out of them to fully recharge our batteries.

Kev

Lots of good information and I appreciate it all.  At this point I'm just not sure if solar is the best option for us right now.  Shading from obstructions on the roof I think I can minimize except for low angle sun, not much charging then anyway.  The camp sites we usually choose here in the NE are in the woods where shading is inevitable.  It is going to take a little work to convince my DW to start choosing more open sites right now.  Maybe once we start Snow Birding or Full-timing out west and in the south, our site selection will adapt with the change in geographical locations.  We both are looking forward to exploring many of the boon-docking locations in the west.  Solar at that point will definitely make sense to enhance our experience.

Again I thank everyone for the ideas, suggestions and knowledge this forum has brought to me.  This is truly an informative group.

My solution for shading was to make two of my panels portable. They are attached to the side of my coach and can be either be tilted up in place or removed and set out in the sun using extension cables.

...on a 5 night/6 day stay in the Adirondacks I only had to run the generator once about 3 hours to charge the batteries on our stay.

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Given that performance, I don't see why you would bother with solar at all. More batteries would extend the time even further.  Or you might add one solar panel and a simple controller just to get a little boost when the sun cooperates on your site.  If you pick up 20-30 AH from solar here and there, you may be good for the 5 days without any genset use at all.    Your preferred camping region and site choice isn't conducive to a large investment in a solar charging system.

Gary RV_Wizard said:

More batteries would extend the time even further... Your preferred camping region and site choice isn't conducive to a large investment in a solar charging system.

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That's what I'm starting to think Gary, at least at this point.  Once we both are retired in 3 1/2 years and head West or South for the winters, I'm thinking we would make use of the solar then.  And who knows, by that time we might be looking to trade our current camper anyway.

Looking into other options and I ran across these China panels today on E-Bay.

https://www.ebay.com/itm/200W-Monocrystalline-Solar-Panel-Waterproof-Solar-Panel-for-Car-Boat-Home-Charge/163994121548?hash=item262ecfb54c:g:dpMAAOSwUQBd-YuT


I wasn't really considering flexible panels but for the price and weight of these it might be hard to pass them by.  One question I have is if (4) of these could be put in series?  Maximum system voltage is 700V DC, so I'm thinking you can.  I would make a simple rack/stand that I could place appropriately on the camp site to get the best sun.  Any incite would be appreciated...

I started off with the portable panels that you keep moving as the sun moves. Gets old really fast.


Frank.

There are flexible panels, and there are flexible panels. Some are better than others. However, if my panels were going to be moved around a lot, I.e. stowing and unpacking them, and keeping them pointed at the sun, I'd want them to be rigid. The regular flexing of all the small internal electrical connections could cause problems.

Like Frank said, when you set panels up on the ground, you end up having to babysit them to keep them pointed at the sun, and so they don't grow legs and walk away. Ends up being kind of a PITA, but lots of people do it nonetheless.

Kev

Frank B said:

I started off with the portable panels that you keep moving as the sun moves. Gets old really fast.

Frank.

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Kevin Means said:

There are flexible panels, and there are flexible panels. Some are better than others. However, if my panels were going to be moved around a lot, I.e. stowing and unpacking them, and keeping them pointed at the sun, I'd want them to be rigid. The regular flexing of all the small internal electrical connections could cause problems.

Like Frank said, when you set panels up on the ground, you end up having to babysit them to keep them pointed at the sun, and so they don't grow legs and walk away. Ends up being kind of a PITA, but lots of people do it nonetheless.

Kev

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Frank, Kev...
I would concur with you both on moving the panels around.  I have an 80 watt GoPower suitcase panel I used and yes, gets old fast and a PITA.  I guess my thinking was; set them up in the best place possible and let them be.  It might be better than the spot the camper is parked if the panels were on the roof.

A question on the MPPT controller.  800 watts of solar on a 12 volt system, equals 66.666 amps if I have that correct.  So.. if I had 4-200 watt panels connected in series to a 100/50 controller, what would be the drawback?  Could the controller be damaged?  Would it work okay but max output would be 50 amps?

You will never get the full rated output from those panels. You will be fortunate to get half. I have 1260 watts of panels on our trailer, I only managed to get 50 amps once when I was testing it.

Kathy & Bill said:

Frank, Kev...
I would concur with you both on moving the panels around.  I have an 80 watt GoPower suitcase panel I used and yes, gets old fast and a PITA.  I guess my thinking was; set them up in the best place possible and let them be.  It might be better than the spot the camper is parked if the panels were on the roof.

A question on the MPPT controller.  800 watts of solar on a 12 volt system, equals 66.666 amps if I have that correct.  So.. if I had 4-200 watt panels connected in series to a 100/50 controller, what would be the drawback?  Could the controller be damaged?  Would it work okay but max output would be 50 amps?

Click to expand...

1. yes, 800 / 12 = 66 amps, you would need a 60 amp controller.

2. if you connect four of those panels pictured in your earlier post in series then the Vmax would be 24.4 Volts * 4 = 97.6 Volts at room temperature. panels
have a much higher Voc when cold, a safe margin is 25% so we now have 97.4 Volts * 1.25 = 121.75 Volts. you will destroy a 100 V controller on a cool day.
you will need a 150 Volt controller to be safe.

3. A 50 Amp controller will clip at it's max output so will define the maximum wattage available. in this case 12 Volts * 50 Amps = 600 Watts.

4. the main drawback of a series connected array is shading. if there is a chance of shading then it may be better to have a 2S2P array.
with 2S2P you will have a Voc of 24.4 * 2 = 49 Volts and a 100 Volt controller..

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