Correct angle for Solar Panels

Polaraco

Hi Gang

I know the answer has to be in here some where, but not by this title. I can't find it

What is the best universal angle to mount Solar panels?

I have 8 on a 44* roof to start, But I am starting to make my awning for my south house exposure. I need the angle to make the awning.

I have 6 panels on my 9 X 11 foot roof area, facing due south. I have 2 on a 40* pitch facing due west. I have strong, unshadowed, sun late afternoon to mid evening facing west. The back of my house on the 2nd level is perfect for later afternoon and early evening. I fear if I angle these paneles, I will loose the later sun.

Or should I angle 1 panel and leave the rest flat? ( I can fit 3 landscape and one Portriat facing west, verticle.

In case ot matters, I am in the north east. While the houses here are close together, I get good sun late dawn to evening on my south exposure.

Wish I could find some really inexpensive followers for individual panels

Thanks

BB.

Re: Correct angle for Solar Panels

I would play with PV Watts for your location... And you have to decide if you need Winter/Summer (i.e., off grid power or GT with Time of Use/Summer Peak power pricing) or to maximize year round collection (net metered power).

I have been surprised that for many locations, you can really point the panels quite differently and get only 5-10% loss over "ideal".

By the time you add the Marine Layer (if near ocean) and summer thunder storms (such as Florida)--Which PV Watts takes into account--The optimum orientation may not be facing south and tilted to latitude.

-Bill

niel

Re: Correct angle for Solar Panels

try here,
http://forum.solar-electric.com/showthread.php?15164-KISS-Find-Optimum-Winter-Tilt-Angle-or-Best-Year-Round-Tilt-for-Fixed-Solar-Panel&p=113218#post113218

i'm also moving this thread out of the facts, links, and info sources section and to the general solar section.

Polaraco

Re: Correct angle for Solar Panels

Thanks Guys

Niel that was the thread I was looking for but couldn't remember the title.

I guess I'm kinda stuck with nowhere to go then. I'll just have to mount these and hope for the best.

In the mean time, right now, I am getting 58 amps off the 6 - 248's I have up there. And the sun isn't all the way up yet. I knew I was good on that roof. Too bad it's so small

solarix

Re: Correct angle for Solar Panels

While the standard advice of mounting PV modules at the same fixed angle as your latitude angle for most year-round power is close enough, a math wiz at http://www.macslab.com/optsolar.html has refined it a bit more. If your latitude is between 25° and 50°, use the latitude, times 0.76, plus 3.1 degrees. By the way, when I dug into PVwatts, I found the projections to be a bit off based on the assumptions used for optimum tilt angle.

Lee Dodge

Re: Correct angle for Solar Panels

solarix wrote: »

While the standard advice of mounting PV modules at the same fixed angle as your latitude angle for most year-round power is close enough, a math wiz at http://www.macslab.com/optsolar.html has refined it a bit more. If your latitude is between 25° and 50°, use the latitude, times 0.76, plus 3.1 degrees. By the way, when I dug into PVwatts, I found the projections to be a bit off based on the assumptions used for optimum tilt angle.

You found PVWatts to be off relative to what, the recommendations at the link that you provided? The link that you provided states "Solar panels should always face true south if you are in the northern hemisphere, or true north if you are in the southern hemisphere. True north is not the same as magnetic north." This ignores the fact that PV panels work with higher efficiency when they are at lower temperatures, giving a preference to slightly east of south orientation, particularly in desert climates. In my area (mountains of Colorado), the summer late afternoons are often cloudy, also giving a preference to slightly east of south. So in my area, PVWatts says the best azimuthal angle is 11 degrees east of south, a recommendation that sounds reasonable to me.

PVWatts is not just a table look-up of some canned information, but rather calculates the PV output accounting for temperature variations and solar insolation variations throughout the day based on 20 years or so of average monthly weather data. I would use it as your primary reference source rather than the link that you provided.

As others have stated, the effect of azimuthal and tilt angles is not as strong as might be assumed, based on PVWatts calculations for my area and shown in Figures 2-5 at http://www.residentialenergylaboratory.com/comparison_of_pv_systems.html

solarix

Re: Correct angle for Solar Panels

Of course PVwatts takes into account many factors including weather - which is what makes it such a valuable resource, however if you plug in various tilt angles (keeping everything else constant) you will find that it figures that the optimum tilt angle is the latitude angle. Macslab goes into a detailed explanation as to why this is not quite correct.

Polaraco

Re: Correct angle for Solar Panels

I've come to the sad reality there is nothing I can do with the area I have to work with. I guess angle isn't going to matter in my case. Since I have very little real estate I have to just cover everything with panels the best I can. I am sure there will be times when the angles will be right. Particularly in the summer, when I need to make the most. The best I saw out of 2KW worth today was 45 Amps @ 24VDC A little over 50% of capability.

I was interested in this for the awning I am making. I need to come up with a universal angle for the panels.

I looked at several tracking syetems and have a plan of my own, but they are not going to be too practical in an 18' alley.

vtmaps

Re: Correct angle for Solar Panels

Polaraco wrote: »

I have 6 panels on my 9 X 11 foot roof area, facing due south. I have 2 on a 40* pitch facing due west.

Polaraco wrote:

The best I saw out of 2KW worth today was 45 Amps @ 24VDC A little over 50% of capability.

Generally, if you have one controller it is best to have all panels with the same orientation, and same shading (or lack of shading). If you do have different orientation or shading issues within an array, it is best to have multiple parallel strings and make sure that within each string the orientation and shading is the same. A better approach for that situation is to have two (or more) controllers.

There are lots of folks who have multiple arrays with different orientations and the key to their success is that they have a separate controller for each array. For example if I wanted to increase my production I could add a tracker to my array, but with the price of panels so low now it might make sense to have two arrays, one pointing southeast and the other southwest. The larger array would face southeast (bulk) and the smaller array would face southwest (absorb and float). In order for that scheme to work well I would need a separate controller for each array.
--vtMaps

Polaraco

Re: Correct angle for Solar Panels

I agree. My problem is I have limited space and not allot of useable roof space

vtmaps

Re: Correct angle for Solar Panels

Polaraco wrote: »

I agree. My problem is I have limited space and not allot of useable roof space

My point was that you have two arrays with (as far as I know) one controller. The west array may be pulling down the performance of your south array.
How are the two arrays wired? Are the west panels in series with any of the south panels? Depending on circumstances you may get more overall harvest by disconnecting the west panels. --vtMaps

edit: grammar error

Polaraco

Re: Correct angle for Solar Panels

vtmaps wrote: »

My point was that you have two arrays with (as far as I know) one controller. The west array may be pulling down the performance of your south array.
How are the two arrays wired? Are the west panels in series with any of the south panels? Depending on circumstances you may get more overall harvest by disconnecting the west panels. --vtMaps

edit: grammar error

I have everything going to a common bus in my attic and then dropping it all to my utility area in the basement. If I didn't do it that way, I would have more copper than panels. It's 85' run from my point of entry in my attic down to my utility area. I pulled 3/0 welding cable down, where 2/0 would have worked. Granted it's all piped, but pulling too many runs down can be a disaster financially. Now I guess I should have done things differently.

I onlt have a south roof, 9' X 11' and a 7 X 11 West facing that are useable. That was the idea of th awning. I can get more activity from array if I put something on the south side of my house. This time of year, it get's shadowed around 4:00, but in the summer it is hot sun all day into the mid evening. But the East to west angles change obviously. It almost makes sense to have the panels straight out, but that would be bad in the winter with the snow. And look kind of stupid, since it's seen from the street.

My back wall of the house faces due west. My second floor gets plenty of direct sun late in the evenings. I am pulling 50 Amps @ 24VDC now out of 2000watts. That is this time of year. Once the sun moves south some more, I'll get more. It's not all that bad. But given what I have to work with, I won't be able to do much more than cover areas I have that get decent sun and hope for the best.

Sigh The life of an old suburban town. 140 year old houses with a bunch of additions makes it tough.

Edit
One is a6 panel array and the other (West) is only 2

vtmaps

Re: Correct angle for Solar Panels

Polaraco wrote: »

I have everything going to a common bus in my attic and then dropping it all to my utility area in the basement.

Are you saying that all 8 panels are in parallel? Are they all the same voltage? What voltage? What charge controller do you have?
--vtMaps

Polaraco

Re: Correct angle for Solar Panels

vtmaps wrote: »

Are you saying that all 8 panels are in parallel? Are they all the same voltage? What voltage? What charge controller do you have?
--vtMaps

Yup! All parallel 24V. Just a dump controller for now. The Out Backs seem to be handling it. I haven't found an affordable controller to handle the 7000 watts of solar I have planned. But with the winds the last few days and the solar, I haven't broken 25.7 that I am aware of. My dump is set to engage at 26V

vtmaps

Re: Correct angle for Solar Panels

Polaraco wrote: »

Yup! All parallel 24V. Just a dump controller for now. The Out Backs seem to be handling it. I haven't found an affordable controller to handle the 7000 watts of solar I have planned. But with the winds the last few days and the solar, I haven't broken 25.7 that I am aware of. My dump is set to engage at 26V

I'm confused... you are using an Outback charge controller? I thought all their controllers were MPPT. If you have an MPPT controller put a combiner box in your attic and make 4 strings of two panels each (the west panels will be one string). You will be sending twice the voltage down the line to your basement. --vtMaps

Polaraco

Re: Correct angle for Solar Panels

You lost me. I'm not suffering any voltage problems. It's the same at my home grown combiner as it is at the inverters, the furthest point. Not eve a 10th off. Lots of stairs to get that reading too.) My current is the same on top as it is down stairs too. That read a 10th lower in the basement. Could be anything. I'm running 2000 of wind too. I need to cut a diode in on the negitive line on those. That may make a slight difference. But none of that is my issue. All I want to know is a good universal angle to mount my panels, where I have the opportunity. I'm stuck with the 44* west and 45* South. I can only squeeze one more panel on the roof. Someday, I may put three start up panels on the east roof. They'll be useless after 11:00 in the summer, but get the grid started up in the morning. That's several more hours. Again that will be 42* angle. I have no control over that. I guess I could raise the bottoms some if I needed a shallower angle.

My inverters are Out Backs. Not my controller. I only have a 10,000 watt dump controller for the time being.

Reading back I see what you were saying about the separate orientations. I wish I had the room for the controllers. If I can refer to some pictures I have them posted on another forum, non-solar related.



These are my south panels. I can't even get back far enough to take a picture of them. The West panels went on the roof to the right.



This is a home grown combiner I came up with. I saw no loses from this point to the controller.



If you look in the background, you can see my environment. Here's my wind generators. That's 2 of 4- 500's.

In order to make all this work, I would have to extend all my grounds inside the attic, move all my controllers up there, and then feed back into my bus doen stairs. It sounds easier than it is. I may change that later, in my spare time.

I'm trying. . . . Nothing I can do about the angles here. Well there is if I want a parachute on my roof. As it is, the edge of the mounting rails protrude over the top of the roof. I flashed it at the top, because my strongest winds come from there. But I did come up with some 24V fans I will be running off a separate set of panels for coolung under the panels. Will help with the efficency. I think. . . .

mike95490

Re: Correct angle for Solar Panels

how much shade does that pipe on the left cast on the array, every panel shaded, even a small amount, is mostly useless while a shadow is across it.

Polaraco

Re: Correct angle for Solar Panels

Mike
It doesn't get shaded until very late afternoon. And then it's just a siloette of the pipe. The turbine is cast further to the right right now. You can see they are a small profile except for the tails. By that time the West panels are strongest powered. From there I will be relying on west wall panels. The shadowing will reduce as we get more into spring and summer as the sun will be much higher.

I am planning on turning the towers to reduce the shadowing. During the peak times I am generating 675 watts net right now. I still have another 250o to go. The next set will make the difference I am looking for

vtmaps

Re: Correct angle for Solar Panels

One more thought... you have a fire hazard in the way your home-made combiner is set up. With 8 parallel strings you should have fuses or circuit breakers in each string. If you have a fault in one panel, the other 7 panels could push enough current through it to cause a fire on your roof. If you use polarized DC breakers in each string, the minus (=load) side of each breaker should be connected to the plus side of each string (because in a fault the string becomes the load). This is another reason to use an MPPT controller... with fewer (but higher voltage) strings, the expense of the controller is offset by the savings in the circuit breakers and the wire gauge from combiner to controller.
--vtMaps

niel

Re: Correct angle for Solar Panels

vtmaps,
it is hard to see them in his photo, but he does have fuses there.

vtmaps

Re: Correct angle for Solar Panels

niel wrote: »

vtmaps,
it is hard to see them in his photo, but he does have fuses there.

Whew! what a relief --vtMaps

Polaraco

Re: Correct angle for Solar Panels

They are fused. Wouldn't have it any other way. You can't see them because they're red against the black. Look closer

Polaraco

Re: Correct angle for Solar Panels

niel wrote: »

vtmaps,
it is hard to see them in his photo, but he does have fuses there.

Yeah I mean really. The Buss Fuse box is laying right there. LOL Just kidding

That little fuse block is rated for 150 amps with 10 fuses on it. Makes a neat package

ggunn

Re: Correct angle for Solar Panels

solarix wrote: »

Of course PVwatts takes into account many factors including weather - which is what makes it such a valuable resource, however if you plug in various tilt angles (keeping everything else constant) you will find that it figures that the optimum tilt angle is the latitude angle. Macslab goes into a detailed explanation as to why this is not quite correct.

One thing about PVWatts is that it stipulates a 10% margin of error. It's kind of silly to (as we all do from time to time) quote a PVWatts prediction of something like 2499846 kWh per year with a 10% MOE.

A while back I ran a system simulation through PVWatts v1 and then ran the same system through v2 with the same derate factor, and the system I was simulating was very close to a data collection center. Since v2 interpolates data based on distance from collection centers and v1 just uses the data from whatever collection point you specify, I expected them to be very close, but they disagreed by quite a bit (~8%, IIRC). I contacted NREL and asked them which set of data I should consider to be the more accurate, and their position was that since there was less than their stipulated 10% MOE difference between the two data sets, I could take my pick.

If you run PVWatts simulations comparing the effects of minor changes in azimuth and/or tilt, you should take any differences in predicted power production with a grain of salt.

Lee Dodge

Re: Correct angle for Solar Panels

ggunn wrote: »

One thing about PVWatts is that it stipulates a 10% margin of error. It's kind of silly to (as we all do from time to time) quote a PVWatts prediction of something like 2499846 kWh per year with a 10% MOE.

...snip...

If you run PVWatts simulations comparing the effects of minor changes in azimuth and/or tilt, you should take any differences in predicted power production with a grain of salt.

Let's consider the sources of error and/or uncertainty in PVWatts and its use for looking at the effect of azimuth and/or tilt angles. There are large uncertainties in solar insolation, up to 40% on a monthly basis and 20% on a yearly basis, http://www.daimc.com/Insights/Publications/ases_paper.pdf, variations in local weather conditions within a zip code, etc., that are all rolled into the overall quoted plus/minus 10% accuracy. When it comes to computing the effects of azimuth and tilt, are you going to tell me that there is a 10% uncertainty factor in computing (1-sin(theta)) and the reflectivity of glass versus angle???!!! There may be an uncertainty in measuring theta, but if I am performing a parametric study for the effects of angle on power at fixed solar conditions, I can do a whole lot better than plus/minus 10%. Since PVWatts takes into account temperature variations and diurnal variations in cloud formation, it is an excellent tool for optimizing azimuthal angles for maximum average energy generation.

Polaraco

Re: Correct angle for Solar Panels

Gee Whiz
You need to have a PHD to figure this out.

According to the article, there is no such thing as a universal pitch. Sounds like Flat is the solution, something I was trying to avoid.

My wife nixed the awning, so I have to go to plan B. Flat

BB.

Re: Correct angle for Solar Panels

Flat has its own issues... With a 5-10 degree tilt, the panels are more or less self cleaning. Flat, you will be water/soap/squeegee wiping them and need access to the panels (walkway?).

-Bill

Polaraco

Re: Correct angle for Solar Panels

BB. wrote: »

Flat has its own issues... With a 5-10 degree tilt, the panels are more or less self cleaning. Flat, you will be water/soap/squeegee wiping them and need access to the panels (walkway?).

-Bill

Yeah OK I figured. I have a 20 X 20' flat roof. I'm good there. I don't want to put any ballast on my roof though. It can handle it.

Here's the next dumb question. WHICH WAY SHOULD I PITCH THEM?

BB.

Re: Correct angle for Solar Panels

Assuming the install is somewhere near Temecula/Southern California, lift the North End (i.e., point south) all things being equal.

If this is a Grid Tied system with Time of Use Net Metering (i.e., summer afternoons you pay/get paid 2-3x the off peak rates), pointing south west is not bad.

If you do not have any Time of Use billing or just want maximum output, pointing South East to collect more sun in the cooler mornings is not bad either.

I.e., don't get all wrapped around the axle for the "optimum" direction of pointing. Anything in the South West to South East range should be fine (although, closer to South would be my choice unless you have something special like AM shading). You can confirm with PV Watts that you do not lose much output overall if not South Pointing.

-Bill

Polaraco

Re: Correct angle for Solar Panels

Thanks Bill

East is out, unless I can find someone to put 500 watts up for me. My roof is a tough one.
What you said is what I did. So far, as you see in the pictures, I did go with a south coverage.

I wish I had 20 more feet between the houses. I could do something on the ground. Maybe even some tracking. But, ***Sigh*** I live on a postage stamp. My options are as I said before. What is done, and I have real good sun later in the day on my west walls. I can fit 1000 on the upper level and another 750 on the lower. I have a 20 foot garage roof I will grid tie eventually. Maybe the back of my garage too. I would do a separate grid tie there to save on wire. I would have to come up with a way of shutting it down if my Generac is running. Then I have a bunch of energy running wild.

BTW, I live in norther New Jersey. That complicates things.

ggunn

Re: Correct angle for Solar Panels

Lee Dodge wrote: »

Let's consider the sources of error and/or uncertainty in PVWatts and its use for looking at the effect of azimuth and/or tilt angles. There are large uncertainties in solar insolation, up to 40% on a monthly basis and 20% on a yearly basis, http://www.daimc.com/Insights/Publications/ases_paper.pdf, variations in local weather conditions within a zip code, etc., that are all rolled into the overall quoted plus/minus 10% accuracy. When it comes to computing the effects of azimuth and tilt, are you going to tell me that there is a 10% uncertainty factor in computing (1-sin(theta)) and the reflectivity of glass versus angle???!!! There may be an uncertainty in measuring theta, but if I am performing a parametric study for the effects of angle on power at fixed solar conditions, I can do a whole lot better than plus/minus 10%. Since PVWatts takes into account temperature variations and diurnal variations in cloud formation, it is an excellent tool for optimizing azimuthal angles for maximum average energy generation.

I am not telling you anything other than that NREL says that their production projections from PVWatts contain a 10% margin of error. What I get from this is that the variance in PV production from localized environmental factors swamps the variance from minor variations in tilt and/or azimuth. Most rooftop residential systems are constrained in tilt and azimuth by the existing structure, anyway; altering the tilt of an array by a few degrees from that of the underlying roof would most likely not increase production sufficiently to justify the added racking expense.