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Ask Slashdot: Home Testing For Solar Roof Coverage? 85

Posted by timothy
from the how-long-do-the-ants-survive? dept.
DudeTheMath writes "Here in the Sunshine State (Florida), solar should be a no-brainer. However, large oaks that require permits to trim partially shade my roof. I'd like to (inexpensively) 'pre-qualify' my roof for effective panel area. Googling for 'home solar testing' gets me equipment for checking the efficiency of an existing PV installation. Do any makers know what I can do on my own in terms of placing a few individual cells and, over a year, measuring and recording their output, so I can get an idea whether solar would be cost-effective for me?"
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Ask Slashdot: Home Testing For Solar Roof Coverage?

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  • Use the telephone (Score:5, Informative)

    by tftp (111690) on Thursday March 29, 2012 @05:34PM (#39515813) Homepage

    You don't need to do it yourself. Call a solar installer, and they will come for free and measure everything. They don't need to wait for the whole year because there is only one Sun for all on Earth.

    • Re:Use the telephone (Score:4, Interesting)

      by Moryath (553296) on Thursday March 29, 2012 @05:43PM (#39515891)

      If you want a good back-of-the-envelope measurement and don't trust the solar guys, why not just buy a cheap time-lapse camera, set it to record every half hour or so, and check the solar coverage on the images of some representative days?

      • by iamhassi (659463)

        If you want a good back-of-the-envelope measurement and don't trust the solar guys, why not just buy a cheap time-lapse camera, set it to record every half hour or so, and check the solar coverage on the images of some representative days?

        That was my thinking, except forget the camera, go straight for the PC. Run the electrical cords into the house and hook up a cheap USB voltmeter to your PC. [digital-measure.com] I would contact that website with what you're trying to do and ask if their USB voltmeter would be suitable for your experiment.

        USB multimeter would be a good option too. [google.com]

      • Re:Use the telephone (Score:5, Informative)

        by tftp (111690) on Thursday March 29, 2012 @06:00PM (#39516119) Homepage

        why not just buy a cheap time-lapse camera, set it to record every half hour or so, and check the solar coverage on the images of some representative days?

        My advice is based on the fact that I have a PV system at my house; I had it ordered and installed.

        There are many reasons why your own measurements are pointless:

        • Installers have instruments, specialized software, and the knowledge how to use it.
        • Installers have access to local insolation levels - not from the US department of statistics but from the house next door.
        • Installers are up to date on many laws, from local to federal, that you must be aware of. I got a huge tax credit, for example, that took two years to draw out. It considerably dropped the cost of the system.
        • Installers know of all the typical problems. The trees in the way are probably the most common issue. They know how to deal with that and they can arrange for the permit for you if necessary.
        • Installers also know the size of the system that is optimal for your house. Too small and you won't realize all the benefit. Too large and you will be selling the energy for peanuts (if you can get any money for it at all.)
        • The PV system must be inspected by a building inspector; to pass, it must be done by the code. The cost of doing it right can vary from low to high depending on your specific circumstances. It's part of the equation, and your measurements won't help to estimate it. But the installer will give you a decent estimate.
        • All of the above is free and you get your personal proposal, with spreadsheets, drawings and financials, within a few days.

        If you don't trust one installer, bring another one in and compare the numbers. If the numbers match then perhaps they know what they are doing.

        It would be a waste of time to do the analysis yourself. You won't be even aware of many potential problems that installers know by the heart. Why would anyone want to risk a large amount of money ($20-30K at least?)

        • Re: (Score:2, Insightful)

          by Anonymous Coward

          Because it is fun to learn this stuff...

          • by drinkypoo (153816)

            Because it is fun to learn this stuff...

            Great, do the process yourself and then ask them as many questions as you can to check your work. You'll still learn, and you'll still get a free and accurate estimate.

        • I agree wholeheartedly with this. Call at least 3 solar outfits and have them come out. Be sure to do a little research yourself before and after to see who is being straight with you. I selected 3, two of which came recommended by a local government solar promotion program. One of them I eventually told to eff off as they were dishonest about just about everything including structuring the contract to pocket some of the tax credit money while telling me the "laws had changed so I wouldn't get that much
      • Because it takes a year to get all the representative days. You can't move the sun to the "fall" position unless you wait for fall.

        Just call an installer, they have tools and software packages for this. They'll do this for free.

    • by JimWise (1804930)

      There may be one sun, but there are many other variables involved where I could see a longer or larger sample size being quite useful. Overhanging and changing objects such as tall nearby structures or trees (which also lose leaves for part of the year which greatly changes the amount of sunlight that can reach the roof), seasonal variations in cloud cover, the change in strength and duration of daily sunlight due to the tilt of the earth throughout the year, etc would make me very wary of a single, short

      • by dwywit (1109409)

        My off-grid system was audited (part of the subsidy conditions) for safety and performance. The guy who did the audit very kindly brought out his specialist device for calculating theoretical performance based on environmental conditions. It was a very fancy data logger/field computer, equipped with GPS receiver, and a camera with a 180-degree lens, i.e. it could "see" from the eastern horizon to the western horizon. He placed it dead centre on my PV panels and pressed the "go" button. This was very close

    • by ShakaUVM (157947)

      >You don't need to do it yourself. Call a solar installer, and they will come for free and measure everything. They don't need to wait for the whole year because there is only one Sun for all on Earth.

      Yep, they're required by law (at least here in CA) to do it before they do an installation, in fact. They need to run all the math to find out what the rated capability of the system actually is. They can't just use the nominal rating of the PV panels.

      It's called a solar site survey or obstacle survey, some

  • Sounds like there's a potential for a startup business!
  • There's some online mapping tools out there that will estimate your coverge based on sattelite photos of your home.

    It's worth noting that these "estimates" can range from fairly accurate to way, way off. They generally assume a flat roof, so don't bother if you live in, for example, the Transamerica Pyramid.

  • http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/atlas/

    This will get you maps that will tell you the expected power, accounting for panel angle, cloud cover etc for your area. Then it's just a matter of subtracting your unique situation, shade from trees, angles of collectors, type of collectors etc.

  • You can science the answer. The sun tracks the same path every year without change (in the time frame you're dealing with) so you can determine exactly where it will be and determine where the shadows will fall on your roof.

    • And, before someone derps in with, "But the sun's location changes in winter and summer", I'm aware of that. It's a band. The limits of that band will give the information necessary to determine where the shadows will be throughout the year.

    • by nschubach (922175)

      It actually doesn't track in the same position every day... it's off by a bit every day. Otherwise we would not have winter/summer months. Some parts of the year the sun is closer to the horizon at noon, other parts of the year it's almost straight overhead. (Of course, it depends on how close you are to the equator.)

    • by reason (39714)

      If you have a smartphone, there are apps that will work this out for you. You can sit on your roof where you expect the panels to be, open the app and see, overlaid on the camera view, where the sun's path would be on any given day of the year.

  • by sylvandb (308927) on Thursday March 29, 2012 @05:50PM (#39515981) Homepage Journal

    Well you could do it on your own just as you described. Make it as fancy or as simple as you like. Hook up some cells in various places on your roof to an arduino's analog inputs, write some arduino code to read the output and send it up to the PC, write some code on the PC to read and store the readings. Write some more PC code to analyze the readings, or simply pull them directly into a spreadsheet. Then wait a year or more to get comprehensive readings, and hope your sample is truly representative of what a full install will experience.

    Or you could it on your own in a much shorter time using (expensive) tools like the Solar Pathfinder or cheaper (more cumbersome) home-made equivalents to evaluate your solar potential.

    Or you could call a local installer (or multiple) to come out and do a site survey using similar tools, and to give you a bid. The site survey should include all the info re. solar potential and the bid will let you know the capital cost, both of which you will need to know to make a decision.

    As part of your analysis, consider whether you can take advantage of net metering and time of day billing to minimize the amount of solar needed to offset your bill. Consider whether you can carry over excess solar credit from month to month (annual zeroing) or if your utility does monthly zeroing, or if (very rare) you can get paid cash at retail prices for your excess production. Consider whether you are doing this for economic, environmental or "just for fun" reasons (or a combination).

    • by iamhassi (659463)

      Well you could do it on your own just as you described. Make it as fancy or as simple as you like. .........Or you could call a local installer (or multiple) to come out and do a site survey using similar tools, and to give you a bid.

      See, his question is confusing, because he wants to know "what I can do on my own in terms of placing a few individual cells and.....measuring and recording their output...." but then didn't think to google "USB voltmeter" [google.com] which, clearly, would be the simplest method of "measuring and recording" the output from the solar cells.

      So my question is this: how is he going to have the knowledge to install solar panels correctly but not have the knowledge to google "USB voltmeter"?

      I feel like I just gave a 3

    • by cbass377 (198431)

      Hobbyboards sells 1-wire Solar Radiation Detector. If you are interested

      http://www.hobby-boards.com/store/products/Solar-Radiation-Detector.html [hobby-boards.com]

      No relationship with the seller, but if they sell it, so do others.

  • by Hyperion X (120480) on Thursday March 29, 2012 @05:50PM (#39515991)

    Before I put solar panels on my roof, I built a system with a camera placed vertically over a reflective sphere (one of those cheap garden decorations), and then took photos from each corner of my roof. I then manually aligned each photo to north based on a compass in the photo and trimmed it to a square centered on the sphere. A script computed the path of the sun transformed onto the surface of the sphere, and drew a line over the photo for each month, with crossing lines for hours in solar time, and a point plotted for the position of the sun at the time the picture was taken. The point lined up "close enough" with the sun in the photo for me to assume that the lines were accurate. Any segment of a month line that was across sky would signify time where the panels would be active. and line crossing trees would be time lost to shade, enough to get a rough estimate of how well the panels would work.

    Then I called a solar installer, who came out for a free quote with a handled tool that took a single photo, autodetected the position, orientation, and where the photo was sky vs. trees, and spit out the percentage of total incoming solar energy that would be absorbed at that point. I recommend doing it that way.

    • by Anonymous Coward

      So how accurate was the estimate in the end?

  • by AaronLS (1804210) on Thursday March 29, 2012 @05:52PM (#39516015)

    You can use on of these and spot check in a grid like pattern. When you place it on a particular spot, the reflection will show you trees overlayed with a grid which indicates the times of the year which that tree would shade that location. This can help guide your panel placement trimming of trees, and tell you how much of the year that each spot would be unshaded.

    If you went about in a grid like pattern you could enter the results into a spreadsheet and do a weighted average.

    http://www.solarpathfinder.com/ [solarpathfinder.com]

    Disclaimer: I do not work for nor am affiliated with this company.

    • by AaronLS (1804210)

      They also have some analysis software to help do the math for you and account for other factors like panel orientation. I haven't read up on the various versions of the software though, to understand what features are provided.

    • by demonbug (309515)

      You can use on of these and spot check in a grid like pattern. When you place it on a particular spot, the reflection will show you trees overlayed with a grid which indicates the times of the year which that tree would shade that location. This can help guide your panel placement trimming of trees, and tell you how much of the year that each spot would be unshaded.

      If you went about in a grid like pattern you could enter the results into a spreadsheet and do a weighted average.

      http://www.solarpathfinder.com/ [solarpathfinder.com]

      Disclaimer: I do not work for nor am affiliated with this company.

      Gah, couldn't remember what the damn things are called. Similar devices have been used for years for determining insolation of streams that are pretty simple. Basically a transparent hemisphere with a grid underneath, you look at the reflection of the sky in the hemisphere and record how much and which portions of the sky are covered. Then you can use charts (or I'm sure there are online tools somewhere) to calculate insolation for your location (important in stream ecology because insolation has a major ef

  • by BlackPignouf (1017012) on Thursday March 29, 2012 @05:53PM (#39516031)

    That's pretty much my job.
    You want something like this:
    http://www.solmetric.com/buy210.html [solmetric.com]
    If you don't have access to it :
    Do you have any picture? Did you model your roof+trees it in Sketchup? Could you sketch an elevation profile of your horizon with (azimuth,elevation) coordinates?
    I usually ask $$$ for design/monitoring of big projects, but I have many scripts to get a complete report with rough estimates in a few minutes for smaller projects.

  • by Anonymous Coward
    Assume there is no shadow on the roof and poison the trees.
    • by AaronLS (1804210)

      You have to be really careful with some of that stuff, as it is extremely potent and indiscriminate. Some of those tree poisons will kill plants 30ft. away that you didn't intend.

      It is also uncontrolled and unpredictable as to which way the wind will be blowing the day that tree decides to come down...

      • by Surt (22457)

        Well once the tree is dead getting the permit for a controlled removal should be easy.

        • by AaronLS (1804210)

          IDK if you are serious and stupid or just joking? Chopping a tree down is usually fatal... the poison is redundant. Maybe a few sprouts may come back, but are easily handled with hand clippers.

          • by Surt (22457)

            The point was that he was in a permit-requiring area. Getting permits to work on live trees is a hassle, but for dead ones its expedited because of the fall danger.

  • Something like a Davis VantagePro2 (http://www.davisnet.com/weather/products/weather_product.asp?pnum=06162) weather station has a small panel that measures UV and solar radiation in addition to weather measurements. I think you can also get additional sensors to add to the station and you can get a dongle that lets you speak with the wireless panel via usb-serial. Then, you can write an easy script to log your measurements (the protocol is well defined) and get a real idea of what your roof is capable of.
  • I took a solar installer class, and learned that it's usually not cost effective to retrofit a house. Few houses have the proper size roof at the right angle to maximize efficiency. If there are trees in the way you can't cut down, just forget it.

    but lets assume you want to find out anyway. well, you first need to look at how much power you use, because it will impact the size of installation you'd need. add up everything that draws electricity in the house. You'll probably find you need a much bigger array

    • by drinkypoo (153816)

      I took a solar installer class, and learned that it's usually not cost effective to retrofit a house.

      I took a solar industry class and in my area of Northern California (pretty far north, but not by the border or anything) the estimate is maybe 15% of roofs in the entire county are suitable between the issues of facing, occlusion, access, and physical integrity. In Texas it's more. In Oregon it's less. In Florida, fucking move.

  • Perhaps it's just me, but I'd be too worried about having a leaky roof some years down the line. Poking lots of mounting holes in a roof can't be good for it. Even if the installer uses some "leak-proof sealing system", how do you know that all holes are properly sealed, even assuming that, um, "low-cost labor" isn't being used to do the installation? (Possibly worse still, some solar power systems are rented -- what happens to the roof when the system is uninstalled??)

    Many years ago, I installed a satel

    • by Arker (91948)

      Perhaps it's just me, but I'd be too worried about having a leaky roof some years down the line. Poking lots of mounting holes in a roof can't be good for it. Even if the installer uses some "leak-proof sealing system", how do you know that all holes are properly sealed, even assuming that, um, "low-cost labor" isn't being used to do the installation?

      How do you know that about your roof already? I mean they are legitimate concerns, yes, but a lot of times I have seen solar installers who were professional

      • by darrylo (97569)

        How do you know that about your roof already? I mean they are legitimate concerns, yes, but a lot of times I have seen solar installers who were professional about their job actually find and fix shoddy workmanship by the homebuilder so ymmv.

        Yup, no argument there. It's just a concern, and one possibly where I'm the only one that cares about it. :)

    • by drinkypoo (153816)

      Poking lots of mounting holes in a roof can't be good for it.

      On one hand, you're poking holes in the roof. On the other hand, the roof already has a shitload of holes in it. On the gripping hand, the solar panels help protect the roof by being in between it and stuff that would damage it, like driving rain. You want a competent installer with references.

      years ago, I installed a satellite dish on a roof, and sealed the mounting plate and all of the bolts using UV-resistant caulking. Years later, when I replaced it, I was amazed at how corroded the bolts were (they were supposed to be galvanized, but apparently weren't). Somehow, and I don't know how, water was getting to the bolts, and down the holes in the roof (the caulking appeared to be in great shape).

      First mistake, using galvanized. Use stainless. It's more expensive, but it's worth it. When you're talking about four bolts or something, who cares? Second, you've got three dissimilar metals involved, the bracket, t

  • Solar is barely cost effective with all the rebates and incentives, if you've got significant shade, you'll be... less than cost effective.

    Also, try to think about how much that oak is going to grow, how much you're going to need to trim it, how much that costs, how much extra heat load you're getting from trimming the tree....

    I live in Florida, I'd love to go solar, but I love my trees more and as long as they shade my home, solar is out for me.

  • The sun is actually higher in the sky during the summer than during the winter, so that what gets shaded by the trees, would vary depending upon the time of the year. For example, it is possible to design the size the roof overhang, combined with the size of the home’s windows, such sunlight enters the windows during the cold winter, but not during the hot summer. For solar panels on the roof (or ground or wherever), a similar seasonal change of the location of the shadow from the oak trees, would als

  • You seriously need a permit to prune or cut down a tree on your own property? Is Florida still part of the USA?!

    • by Anonymous Coward

      Large oaks. That translates to "Heirloom" or "Heritage" trees which are protected in large parts of Florida, mostly to keep wankers who want to splat out a subdivision from clear-cutting it and putting postage-stamp homes on it.

      When your state is among the flattest in the union, you really can't afford to take chances with parts of it being washed away.

      Couldn't tell you the specific laws, but basically, if they were removing anything but dead or diseased parts, they'd have to clear it with an arborist.

    • by compro01 (777531)

      Where does it say it's on his property? Some oaks can get pretty damn big and easily shade a single-story house from outside one's property in a city.

    • by sylvandb (308927)

      I doubt Florida has tree regulations that affect every tree anywhere on private property.

      Lots of cities in lots of states have regulations affecting trees on private property. The most frequently controlled are trees along the public street and sometimes any tree in the front of the house.

      Innumerable housing developments have covenants or deed restrictions that put requirements on trees. Anywhere from how many you can or cannot have, to what type, to what size, to requiring professional arborists to care

    • by dammy (131759)

      Depends on the location and type of tree. I had friends who were fined by FL DEP for taking down some trees on their 3 acre lot that their house is on after the trees were dead or badly damaged after two hurricanes hit the area. Taking down exotic trees, not a problem.

  • As people have noted, get some quotes from people who install in your area, city.
    They will have the feed in, costs and can chart your past yearly usage and system sizes on a laptop/tablet. Factor in local government inspections/regulation costs, upgrades to your home power system, new meter.
    Does you state and federal gov give good cash back for setting up the system, does your utility give you cash or some $0.00 credit only?
    Will your state or federal gov reduce or stop any solar feed in payments? How str
  • Do it a little bit at a time, unless you've been a jerk to your neighbors no one is going to report you.

  • Solar coverage (Score:5, Informative)

    by JWSmythe (446288) <<moc.ehtymswj> <ta> <ehtymswj>> on Thursday March 29, 2012 @09:23PM (#39518075) Homepage Journal

    Some people posted good links. Here's some more information.

    I live in Florida too (Tampa/St. Pete area), and have looked into it. We're finally making starting ourselves. I've lived in this general area of Florida most of my life. I assume you have too, but in case you haven't, I'll mention some obvious weather patterns.

    One of the things you'll want to consider is, how much of the roof is shadowed and when. Look around online for information on peak solar exposure. There's a 5 to 6 hour window of the day that you get enough sunlight to make the panels practical. Ideally, you want the panels on the south and west facing side of your roof. Although we are farther South than most of the country, the North side of your roof is useless, unless you mount the panels very high and angled to the SSW. You don't really want to mount them high because of winds.

    Morning fog and haze partially obscures the sun at sunrise, but typically burns off by 10am to 11am. That happens more in the winter, but we also have shorter days, so it has more of an effect.

    Summer thunderstorms start building up cloud cover around 1pm to 5pm, so even though we have longer days, they'll be cut short many days of the summer.

    Look at how your roof is shaded, Just check out your roof in the morning, afternoon, and evening. If that part of the roof is shaded for any part of the mid-day, from about 10am to 5pm, you probably don't want to use it. If you don't have a good candidate area in that period, you may want to consider panels mounted in your back yard. Check your local zoning to see if that is acceptable.

    Thunderstorms are accompanied by heavy winds, rain, and hail. The winds can frequently have gusts that are the same as Category 1 hurricanes, and tornadoes spawned by them that are harsher than any hurricane. You may have a plan in place to pull the panels down for a hurricane, but a summer thunderstorm can build up and hit with very little notice.

    Then we have tropical depressions, storms, and hurricanes. Your panels will experience heavy winds at some point in their lives. Make *very* sure they are well secured. If the wind hits just right, they can rip right off the roof. A friend of mine lost his solar pool panels a few years ago during a Category 2 hurricane. He found their remains about a half mile away.

    Make *very* sure that you have the panels grounded properly, and the power system set up for lightning surge suppression. Something similar to what the power companies provide is a good idea.

    Have the appropriate plans in place to provide for electricity in days without enough sun. I'm sure you've seen the skies stay black for days during a hurricane, or even just a very stormy summer. You'll probably want to reinforce your solar with a generator and tie it to the power grid. The boxes to do that can be pricy, but you'll want it.

    I believe Florida law currently states that the power company will pay you wholesale rates for feeding back into the power grid. They charge you retail rates when you draw back from it. Depending on who your local power provider is, they may charge differently for day and night. Wholesale is usually about 10% of the retail rate. You do have to request a special power meter to make it work properly. They will work with you and your electrician to get that in for you. In some areas, it's free. Others charge a nominal fee.

    And finally, just about everywhere in Florida that I've been, zoning is strict, and will be arbitrarily enforced. Make sure you have the proper permits, and licensed people doing the work as applicable (i.e., a licensed electrician does at least the final connections). Some HOA's have specific restrictions. For example, where my mom lives, she isn't allowed to make any external changes to the house. She can't put

  • I am in foggy El Granada, California and dealing with shade from several 200+ foot tall Eucalyptus trees that I can not directly cut. If it is at all safe, you can get up on your roof and do some natural science astronomy.

    I made a point of working out a safe ladder location because I also have a FTA satellite TV 30" dish, plus gutters, a chimney that needs work, and solar water heater panels I hope to re-install in an earthquake safe configuration. On the side, I plan to do a fun corrugated tubing air heat

  • by Anonymous Coward

    For baseline calculations use the NREL calculator. This is a link for version 1. They have another version but I don't like it so much.

    http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/

    For shading factors, roof orientation, etc. here is a link with lots of options most of which work pretty well.

    http://www.builditsolar.com/References/SunChartRS.htm

    As for cost effectiveness, if you are willing to do installation yourself you can save over 50% of the cost of an installation. Based on the bids I was gett

  • However, large oaks that require permits to trim partially shade my roof.

    And people ask me why on Earth I chose to move to the middle of nowhere...

    / Finishes sharpening the chainsaw blade and heads back out to drop another three cord
    // To burn in my totally unregulated outdoor wood-fired boiler next winter
    /// Actually 11:30pm local, so this will have to wait until morning - But no local noise ordinances would actually prevent me from doing it right now
    //// Enjoy your HOA.
  • Get an installer to bring out a Solar Pathfinder and get an estimate.

    http://www.solarpathfinder.com/?id=mxhu3Rj5 [solarpathfinder.com]

    The specially designed dome and template inside the dome let them take a picture of the reflection from your skyline and trees on the dome, their software analyzes the picture and tells you what your solar potential is.

    I saw it on This New House and have no affiliation with the company.

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