Ask Slashdot: Home Testing For Solar Roof Coverage? 85
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?"
Use the telephone (Score:5, Informative)
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.
Homebrew solar insolation analysis (Score:5, Informative)
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.
That's pretty much my job (Score:5, Informative)
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.
Re:Use the telephone (Score:5, Informative)
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:
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?)
Solar coverage (Score:5, Informative)
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
Re:Use the telephone (Score:4, Informative)
Actually it only takes 6 months to get all the representative days since the solar cycle is effectively symmetrical.
Not on Earth.
http://scienceblogs.com/startswithabang/2009/08/a_question_of_why_the_analemma.php [scienceblogs.com]
-AI