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gypsydan

Solar Panel Height Above Roof

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I'm putting solar on my 25' class C roof, and have front & rear vent covers sticking 5" above the roof.  The panel will be 305w @ 64" x 39".  I chose this panel for price, 1/2 the cost of three 100w panels.  Because of the rooftop configuration, and for installing a potential 2nd panel, I am having to fit the 1st panel across the roof from side to side, so wind will be hitting the front of the panel.   I'm putting the 1st panel in front of the A/C, and behind the front vent.  The front of the panel will be have a 3" lift, and the rear will have, for drainage, a 4" lift, .   A tilt will probably be put on later.

If I put on a 2nd panel, it will have to go behind the A/C and over the rear vent, which sticks up 5".   To assure there is no shadow from the A/C, and to clear the vent,  I will have to lift this panel at least 4-5" in the front and 5-6" in the rear.  The A/C will disturb the wind hitting the rear panel, so I'm hoping there won't be a wind problem on the rear panel

Does anyone see a potential problem with these heights, especially 5-6", and for this configuration?

 

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Wouldn't the savings for that single panel get eaten up by the high voltage?  Those aren't normally 12v-18v,  so to get all the power from it wouldn't you need an MPPT controller to convert the extra unusable volts to amps?   I was looking at an extremely well rated morning star 45 amp controller for about $170 Morning Star TS-45 45 amp controller  The equivalent MPPT controller is almost triple the cost.  Morning star 45 amp MPPT controller.

I have yet to pull the trigger, but I think I know what I need now.  I like fewer panel concept, but a really good MPPT controller is a bit pricey compared to a PWM which is all I really need.  -Bill

 

Edited by bigbillsd

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Not sure why you need to tilt the panels, will not help your solar production if you cannot park just right because every degree off axis from directly overhead will decrease it a little bit. Panels are normally glass, and water will not hurt them, if in the sun, they will heat up and evaporate any water on them pretty quickly. Mppt is the way to go if you are considering buying a second panel. You can run in series and not have to change your wiring. 300+ watt panels are normally 40 VOC, or more so I would get an mppt controller that can handle at least 90 volts solar input.

Edited by jcussen

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On 4/5/2018 at 11:15 PM, bigbillsd said:

Wouldn't the savings for that single panel get eaten up by the high voltage?  Those aren't normally 12v-18v,  so to get all the power from it wouldn't you need an MPPT controller to convert the extra unusable volts to amps?   I was looking at an extremely well rated morning star 45 amp controller for about $170 Morning Star TS-45 45 amp controller  The equivalent MPPT controller is almost triple the cost.  Morning star 45 amp MPPT controller.

I have yet to pull the trigger, but I think I know what I need now.  I like fewer panel concept, but a really good MPPT controller is a bit pricey compared to a PWM which is all I really need.  -Bill

 

Sometimes price is not the best factor. You might want to research the difference between PWM and MPPT controllers, especially if you use computers and printers. I recommend looking at jackdanmayer.com articles on controllers.

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7 hours ago, jcussen said:

Not sure why you need to tilt the panels, will not help your solar production if you cannot park just right because every degree off axis from directly overhead will decrease it a little bit. Panels are normally glass, and water will not hurt them, if in the sun, they will heat up and evaporate any water on them pretty quickly. Mppt is the way to go if you are considering buying a second panel. You can run in series and not have to change your wiring. 300+ watt panels are normally 40 VOC, or more so I would get an mppt controller that can handle at least 90 volts solar input.

I am buying an mppt controller. The panels will be tilted to help wash the dirt off. If panels stay flat, the morning dew will slowly collect dust and dirt and drastically effect the panel efficiency, so tilting them slightly will make it easier to spray with water to wash them off, without climbing on the roof. Water will evaporate, but the daily accumulation of dust and grime will be left behind. Plus tilting them slightly will help the wind clear them while driving.

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4 hours ago, gypsydan said:

I am buying an mppt controller. The panels will be tilted to help wash the dirt off. If panels stay flat, the morning dew will slowly collect dust and dirt and drastically effect the panel efficiency, so tilting them slightly will make it easier to spray with water to wash them off, without climbing on the roof. Water will evaporate, but the daily accumulation of dust and grime will be left behind. Plus tilting them slightly will help the wind clear them while driving.

Never thought of it that way, Have used solar panels for 25 years, and have always had to use detergent and long handled brush to clean the panels of dirt and pollen. Of course when I really use the panels, is when I am dry camping, and water conservation is a primary concern, so just use a dust mop.

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The height is important determining the construction of the roof is bigger factor.

Some manufacture provide drains at the corners of the panel to minimize the water/dirt collecting along the edge.

Sloping the panel is a good idea but try a water spray test before choosing the amount of slope.  A slight increase in the slope can make all the difference depending on what coatings are on the glass.  Rain X may help.

 

 

 

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On 4/7/2018 at 2:54 AM, gypsydan said:

Sometimes price is not the best factor. You might want to research the difference between PWM and MPPT controllers, especially if you use computers and printers. I recommend looking at jackdanmayer.com articles on controllers.

Dan,  I just reread Jacks site, I had read it when I was determining what design to go with, along with Handy Bob's and quite a few others "experts" sites.    I could not find  computers/printers and Pulse Width Modulation discussed.  Since I will use my PSW Magnum to invert the power from the batteries to 120v I am not sure how a solar controller charging the batteries could effect my computers.    And for most folks, cost is an important consideration.  I still don't see a good reason to install high voltage panels on my 40' RV, their larger size makes it more difficult to find an un-shaded spot on the roof, and the need to go with an MPPT to use all their capacity just doesn't seem (to me) to be worth triple the cost of a very good PWM controller.  

If i was doing an off the grid house I would probably use the high voltage panels and then go with an MPPT controller.   

Edited by bigbillsd

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11 hours ago, jcussen said:

Never thought of it that way, Have used solar panels for 25 years, and have always had to use detergent and long handled brush to clean the panels of dirt and pollen. Of course when I really use the panels, is when I am dry camping, and water conservation is a primary concern, so just use a dust mop.

Good points.  I may be over thinking this project.  I am raising the front 3" so I don't have any shadowing from front vent, and then 4" in back to give it the slope and help with ventalation and, of course, the self-washing effect. Also, with the slope there is much less chance fo water pooling on panels when I'm camped in the rain for several days.

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3 hours ago, bigbillsd said:

Dan,  I just reread Jacks site, I had read it when I was determining what design to go with, along with Handy Bob's and quite a few others "experts" sites.    I could not find  computers/printers and Pulse Width Modulation discussed.  Since I will use my PSW Magnum to invert the power from the batteries to 120v I am not sure how a solar controller charging the batteries could effect my computers.    And for most folks, cost is an important consideration.  I still don't see a good reason to install high voltage panels on my 40' RV, their larger size makes it more difficult to find an un-shaded spot on the roof, and the need to go with an MPPT to use all their capacity just doesn't seem (to me) to be worth triple the cost of a very good PWM controller.  

If i was doing and off the grid house I would probably use the high voltage panels and then go with an MPPT controller.   

My choice for an MPPT is due to computers and printers which require a pure sine wave, and do not operate wel  (from what I've been reading) with modified sine waves (usually square) from the PWM controllers. Also, as I udnerstand it, MPPTs are designed to operate with higher voltage panels (37v), like the one I'm planning on using (Canadian Solar CS6K-305), which I've read PWMs have a problem with.  Also, with my footprint I could conceivably only put 3-100 watt panels on, at a cost of close to $700, vs the $298 i will be paying for the 305w panel.

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You need to do some additional research  if you are buying 100 watt panels for that price!

You may also look at some of the square 12 vdc panels for a better fit. They come in sizes larger than 100 watt and smaller than 300 watt and might give you some added flexibility in placement on your roof  

It’s not always wise to buy an panel and make it fit!

I’m a bit confussed on your choice of a MPPT solar controller based on “ the need of a PSW” output. Is your plan not including a PSW inverter?

Edited by DesertMiner

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Some confusion here. Type of charge controller is immaterial to what type of inverter you have. PSW inverter is the best choice for most applications, but not absolutely necessary.

Charge controller feed the battery only, inverter takes battery voltage and turns it into 120 volt.

For solar charge controllers, PWM pretty well limits you to smaller panels, [normally 200 watt or less] which is fine for smaller installations. You can run many smaller panels through a bigger PWM controller but because of the lower voltage and higher amperage, wire size is going to be large. The nice thing about MPPT, is that they can accept a higher voltage, [ as from multiple smaller panels in series] or larger, higher voltage panels. I have 960 watts from three 320 watt panels in series feeding 120 volts to my MPPT controller with small 10 gauge wire. If i tried to run nine 100 watt panels to a PWM controller, it would be a much lower voltage with a much higher amperage and require much bigger wiring. In other words, MPPT controllers allow expansion to almost any type of panel, whereas PWM pretty well limits you to smaller panels. Your choice, one is cheaper, and one allows for expansion.

Edited by jcussen

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On ‎4‎/‎5‎/‎2018 at 3:43 PM, gypsydan said:

I'm putting solar on my 25' class C roof, and have front & rear vent covers sticking 5" above the roof. If I put on a 2nd panel, it will have to go behind the A/C and over the rear vent, which sticks up 5".   To assure there is no shadow from the A/C, and to clear the vent,  I will have to lift this panel at least 4-5" in the front and 5-6" in the rear.  The A/C will disturb the wind hitting the rear panel, so I'm hoping there won't be a wind problem on the rear panel. Does anyone see a potential problem with these heights, especially 5-6", and for this configuration?

The A/C will be a good wind break from that direction and substantial brackets & attachment points will battle the strong cross winds that you'll encounter. To fully open the vent this panel will need to tilt so overbuild this feature as well, and be sure to make use of the roof studs.

 

9 hours ago, DesertMiner said:

You may also look at some of the square 12 v panels for a better fit. They come in sizes larger than 100 watt and smaller than 300 watt and might give you some added flexibility in placement on your roof  

I've seen various sizes in square & rectangle, with the various wattages, as I try to make use of available space while still leaving me access for inspection/maintenance, and will have at least two charge controllers... one for the 295w that fits nicely in its spot & one for the remaining panels that I'll use to fill. 

The most difficult portion, the part giving the most anxiety, is the roof attachment points and their ability to withstand tornado alley, or at the very least the West Texas Winds.

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Dan, to answer your direct question, no there is no issue with raising the panel in the fashion you describe. AS LONG AS it is properly secured. The only issue you "might" have is getting a branch under it. Should not be a big problem, but something to be aware of.

As to MPPT or PWM, don't be confused with PSW vs PWM they are unrelated acronyms.....the solar controller does not matter vis the electrical characteristics of your AC power. The solar controller is DC - power only. As others have stated. 

The choice of PWM or MPPT gets down to personal preference as much as anything, on a very small array. Unless you have distance issues with wiring. You will squeeze a little more power out with the MPPT, but frankly, on a small system, most people will not notice it. 

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26 minutes ago, Jack Mayer said:

Dan, to answer your direct question, no there is no issue with raising the panel in the fashion you describe. AS LONG AS it is properly secured. The only issue you "might" have is getting a branch under it. Should not be a big problem, but something to be aware of.

As to MPPT or PWM, don't be confused with PSW vs PWM they are unrelated acronyms.....the solar controller does not matter vis the electrical characteristics of your AC power. The solar controller is DC - power only. As others have stated. 

The choice of PWM or MPPT gets down to personal preference as much as anything, on a very small array. Unless you have distance issues with wiring. You will squeeze a little more power out with the MPPT, but frankly, on a small system, most people will not notice it. 

Jack,  Dan has a 305 watt panel which probably 60 or 72 cells.with an output voltage of 35 to 45+ volts, do they make a PWM controller that will take that input voltage and output 12 volts?

 

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Just now, jcussen said:

Jack,  Dan has a 305 watt panel which probably 60 or 72 cells.with an output voltage of 35 to 45+ volts, do they make a PWM controller that will take that input voltage and output 12 volts?

 

Nope, so he is committed to MPPT. But others reading this are not.

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Yes, I have been readin gthis thread and finding it very enlightening.  Everyone has provided excellant suggestions.  What I think I've learned:

#1 - Because of the 305w panel voltage (39 Voc, 32 Vmp) I do need MPPT, especially if I later add another panel. 

#2 - I still need to research which 40+ amp controller to get, Victron or Morningstar, as ease of use and panel readability are important.

#3 - Because of the panel size (65x39), and because of "Tornado Alley" type winds in OK and TX (which I had not thought about),  I have to make sure I attach to the aluminum framing under the roof, rather than just dicor and multiple screws into the thin plywood sheeting under the rubber roof.

#4 - A 2" & 3" rise with a 1" slope over 39" will not be a problem, but I have to do more research if I want to have 4" & 5" rise on a 2nd panel.

It was easy to become confused over the various acronyms and what they related to (e.g., mppt, pwm).  Reading and trying to digest all the great material on the net (Jack Myer, Handy Bob, etc.) further added to the steep learning curve, but it was needed to give me a background to finally start putting things in perspective. 

Thank you all for insights, as I'm feeling satisfied as to my path forward.  I know this thread will be of use to others in the future. 

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If I might make a suggestion with your panel mounts. Using a full length "L" chanel bar to mount to the roof would allow for easier installation into your roof rafters since they may not align well with the mounting positions on the panels themselves. Further, you might consider independent mounts on the panels and then connecting those mounts using an independent riser bar between the panel mounts and roof "L" chanels. You could use those plastic knobed bolts with captive nuts on both the panel and lower chanel connections. If you drilled additional holes along the "L" chanel that would allow you to dynamically change the angle/position of the riser bars for various heights/tilt angle... not to mention, make it a snap to move the panel entirely for roof maintanence.

You may find that travelling with your panels horizontal to be less stressful on the mounts and... also on the panel frame as well, which, aside from the roof itself would be my main area of concern... then raising/tilting them once parked.

Having an independent riser between the roof and panel mounts leaves a great deal of flexibility for trial and error at minimal expense and effort.

Just an idea.

Edited by Yarome

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If the roof really is paper thin then you need surface area for structural mounting. As Yarome states above using an L angle is an easy way to obtain this surface area to spread the load over. Then attach the panel to that. How do you know if you need this? If when walking on the roof you feel like you are going to fall through if you don't step on the rafters then you need it. If you can reliably walk on the roof between rafters with just a little give then you probably don't. But it never hurts.  

As far as the 1"tilt, there is no problem with that at all. Don't worry about it. 

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I have three, soon to be four, 39 x 64 panels on my Class C Roof and FWIW here are my inputs.

 1) I prefer a slight tilt higher to the rear so when driving down the road the wind is pushing somewhat downwards versus trying to get underneath the front of and lifting my panels off the roof. I don't usually drive much over 5 MPH when in reverse lol and also believe a slight tilt is better for surface drainage.

2) Its my opinion the Class C (or most any) RV roof is strongest for mounting right at the top edge where the vertical sidewalls meet the flat horizontal roof. For the majority (roof versus sides) of the load and stress I used brass hinges attached down into the sidewalls and then attached to aluminum angle iron which are attached to the panels. Mine can be lifted for washing the roof and panels as needed.

3) I mounted per the above because I don't like piercing the roof any more then necessary or placing weight or wind loads on it believing the sidewalls  to be stronger.

4) Sure, shading concerns are important and I did the best I could given the available roof area, but parallel wiring helps in such a case.

 

John T

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On 4/9/2018 at 1:07 PM, gypsydan said:

#3 - Because of the panel size (65x39), and because of "Tornado Alley" type winds in OK and TX (which I had not thought about),  I have to make sure I attach to the aluminum framing under the roof, rather than just dicor and multiple screws into the thin plywood sheeting under the rubber roof.

You might use a variation on my PV panel install. First, I screwed down 2" wide aluminum strips to my roof. Then I attached 2" L brackets to these strips and my PV panels. 

My panels are only about 2" from the roof, but if I wanted to jack them up to 5" I'd probably just use wider L brackets.

So with my system, if you get a strong enough wind or an errant tree branch, then a good section of the roof would come off with the panels.

solar_06_pv_on_rails.jpg

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On 4/7/2018 at 7:44 PM, gypsydan said:

My choice for an MPPT is due to computers and printers which require a pure sine wave, and do not operate wel  (from what I've been reading) with modified sine waves (usually square) from the PWM controllers. Also, as I udnerstand it, MPPTs are designed to operate with higher voltage panels (37v), like the one I'm planning on using (Canadian Solar CS6K-305), which I've read PWMs have a problem with.  Also, with my footprint I could conceivably only put 3-100 watt panels on, at a cost of close to $700, vs the $298 i will be paying for the 305w panel.

Dan,  I put these panels on my amazon wishlist a while back so I can find them when I am ready to pull the trigger.  Two of them get you 316 watts, 12 volt allowing a PWM controller.    Unless you are powering your computer directly from the pwm controller the power will either come from the battery or an inverter.  Most folks would either be plugging their laptop into a 12v power connector or 120v outlet.   If you have the PSW interter then you are already golden without the expensive MPPT controller.  I personally power my computers  (SageTV server and a couple laptops in the MH) from 120v PSW power (or shore power if available)   I am currently dry camping on the Beach north of San Diego.    Based on this second week of dry camping I'm confident I understand how much power I use from my batteries based on info gleaned from my Trimetric Battery Monitor so I can size my own solar installation.    Just the part of figuring out how much I will get from panels lying at a slight angle on my roof.   That data is difficult to pinpoint due to so many variables,  it will be mostly trial and error.  Probably buy two of those 158 watt panels and what the actually produce then adding more if need be or just run the genny to make up the charge during the evening.  

 

-Bill

PS.  I ran the infrastructure for large datacenters full of eCommerce systems for many years.  

 

 

 

 

 

2  each 158 watt 12v panels for around $365

Edited by bigbillsd

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