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Ask Slashdot: Home Testing For Solar Roof Coverage?
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?"
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.
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.
There's no -1 for "I don't get it."
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.
"File to fit, pound to insert, paint to match" - Aircraft Maintenance 101
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.
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).
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.
-- Colin Cross
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/
Disclaimer: I do not work for nor am affiliated with this company.
That's pretty much my job.
You want something like this:
http://www.solmetric.com/buy210.html
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.
This is what you want. http://www.solarpathfinder.com/ Its been around since the 70's to do solar site surveys.
Assume there is no shadow on the roof and poison the trees.
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. The VantagePro2 is probably overkill for your purposes, but it's a good thread to chase down for options.
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 than you have roof space.
Now, that will be the size of the array at optimum efficiency. you'll have to subtract out all the efficiency losses for incorrect angle and shade, and add extra panels in order to get you back up to your needs.
I'm assuming you need certified installers in Floria because of the hurricanes, plus you'll need to get an electrician to install the inverter and breaker boxes, and probably a second electrician from the electric company to wire it up to the grid. If Floria has installation rebates, you'll need to do everything up and up in order to get your rebate.
So, first thing first: find a way to reduce your power draw by at least 1/3. it'll start saving you money now, plus you'll need a smaller array if you do commit to installation. it's much cheaper to start with efficient appliances and adequate insulation than it is to overcome it with more solar panels. Panels are getting cheaper, but labor costs are at least half of the costs. and while equipment costs go down, labor costs are flat or rising. If you build a house, build it with solar in mind and you'll save a ton of money up front, and you'll break even much much sooner.
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 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).
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 also occur.
The latitude where you live would also affect those calculations significantly, and would also affect the ideal angle for mounting the solar panels.
I have seen a few websites and solar books which briefly describe how to calculate some of those angles, but I am not an expert. I just have somewhat of an interest in the subject. One such somewhat similar calculation that I have seen described in a couple of solar books, is how to size a roof overhang, so that the windows are totally in the shade during the hot summer, but receiving full sunlight during the cold winter. As I recall, the homes latitude was also taken into consideration.
If you're going to do some measurements yourself with some solar cells (using an Arduino / USB multimeter / whatever) then make sure you measure the short-circuit current of the solar cell, not it's open-circuit voltage. To a good approximation, Current is proportional to light level; Voltage is more proportional to temperature than anything else, and doesn't really give you any useful power production information.
You seriously need a permit to prune or cut down a tree on your own property? Is Florida still part of the USA?!
The Christian Right is Neither (Christian nor right). See: Matthew 23, Matthew 25, Ezekiel 16:48-50
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 strong is your energy lobby to get any feed in rates cut or removed?
Shade can be worked around with a well designed system. Understand what the size of the system can produce, what your utility connection fee is, how much
you get when you export and your usage over the year. Use your solar during the day as produced vs keep most things off so you export all you can.
A good free community site for user stats is pvoutput.org - a few US systems are listed.
Domestic spying is now "Benign Information Gathering"
Silicon panels are strings of cells. If you shade one of them it will turn into a resistor and heat up because of the current generate from the others in the string. This will destroy your panels and can even cause them to burn up.
It is no problem if the panels are not hooked up, but once they are in the sun and current starts to flow through them things get serious.
The only option is NO shade ever.
I am an expert and soon to be professional installer.
Do it a little bit at a time, unless you've been a jerk to your neighbors no one is going to report you.
"Who is the Journal of Quantum Physics going to believe?" --Stephen Hawking
Google RETScreen. It's free software developed by the Canadian government that allows you to calculate energy based on your location, and the specifics of the installation you're contemplating.
You can model your house in sketchup and geotag the model with the houses's gps coordinates. Sketchup then has a feature that simulates the path of the sun. There is a video howto somewhere on the web.
I would use a 3D modeling tool like Sketchup (free). You can position/orient your house exactly using Google Earth. It has accurate shade modeling. There is an inexpensive plugin ($130) for solar power estimation - http://skelion.net/index.htm?key=33. I would do this even before I called in an installer. I do this for all home improvement projects. It cuts down on rework and you can easily explore lots of options.
I bought an inexpensive solar panel kit from Harbor Freight in December 2011($149 w/ coupon) connected it to several gel batteries, and take data daily on its performance. The big picture: I see a 6x reduction in power output between total sun and shade. Every minute of sunlight contributes to your ROI. Depending on your situation, you may be able to keep efficiency relatively high by using one microinverter per panel, rather than a conventional single-inverter system.
besides using google maps to get an overview the first thing theyll do is sit on your roof with one of these. jist google it. its a little plastic device that will help u calculate sun insolation over the year.
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
Serious? Seriousness is well above my pay grade.
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 heater that warms the under-floor. I figured the pay off for the latter project and that caused me to decide to do it as a science project, with permitting as an after thought.
I bought a 10 watt solar panel from Halted ($80) and let me tell you; solar panel output really drops if even a portion of the panel is shaded. Because of the shade from these trees, I believe I will be better off to settle for using my existing solar water heater panels for thermal heating rather than solar panels with 20% of the generating units limping along partially shaded.
If your roof is flat enough to set up a photo tripod, you can get a pretty good idea of the annual sun path with some simple astronomy gadgets. I used a Brunton pocket transit, but you can get the same result with a magnetic compass, a protractor, a stick that attaches to a photo tripod and a weighted string on one end of the stick, plus knowing your latitude and magnetic declension (your local difference between geographic north and magnetic north). With these things you can find due south, and tilt the stick to model winter and summer extreme points of the sun's path. See Wikipedia Axial tilt for a starting picture and discussion.
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 getting, I decided to do most of the work myself and saved over $60,000.
Since you are in Florida, the best option for your purchase of parts is right in your back yard. I purchased panels for 98 cents a watt from them. They are difficult to deal with but the hassle is worth it. Be ready to bring a cashier's check for your pick up.
http://www.sunelec.com/
However, large oaks that require permits to trim partially shade my roof.
// 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.
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
Use PVWatts (version 2 is just an interpolation of version 1's data) to find your peak sun hours for each month for a given tilt. These peak sun hours are based on real data collected - in other words they do take into account clouds and such.
Once you have this data, if you are able to use a solar path finder, you can calculate the percentage loss for any given month due to shading. The only problem is a solar path finder can be expensive.
You could always try to make your own:
http://rimstar.org/renewnrg/solar_site_survey_DIY_shade_finder_tool.htm
Living in Florida, a sizeable chunk of your electric bill is from the air conditioner. Having a large oak near your house that shades it greatly lowers the amount of a/c needed. My guess is that you will not get enough electricity from your roof collectors to offset the additional a/c needed.
Hello, the problem with shades is they can affect the whole grid (technically each "string" separately) in your installation.
For the purpose of testing shadows, it is not enough to test each single position, let me explain why :
Particularly with mono and poly crystalline modules, when a cell produces less than the others (because it is covered by a shadow) it will degrade the performance of the whole panel, similarly when a panel produces less than the others (connected in series with it, aka "in the same string") it will degrade the performance of the others.
To limit this issue panels have one or more (normally 3) bypass diodes that allow the exclusion of array of cells eventually affected by shadows.
It helps a bit, but does not fix the problem.
There is a technical solution now used in these situations : micro inverters.
Basically you have a tiny inverter per each panel and this allows to optimize strings with panels having a different output.
Moreover, if shadows are frequent on your roof, you should probably consider using amorphous (or thin film) panels.
They have a lower peak efficiency (less W/sq foot), but they lose less energy when the sun is not perfectly aligned with them or when they are covered by shadows.
They are normally cheaper too, but overall, in the year, they might give you more power than the theoretically more efficient crystalline ones (mono or poly).
Conclusion : If you test your roof with a crystalline technology embedded in small devices (i.e. with solarimeters) you may get misleading results.
I would go for amorphous panels and micro inverters.
I would guess the large trees are providing more energy savings due to lower roof tempatures vs. any payback you would get on the solar panels in the foreseeable future. Keep the shade trees and lower your roof tempatures and A/C (electricity) needs.
Put a lawn chair and a year's worth of beer on the roof. Check your tan in a year.
Get an installer to bring out a Solar Pathfinder and get an estimate.
http://www.solarpathfinder.com/?id=mxhu3Rj5
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.
So I'm in one of the blue states in which the utilities have to buy the renewable energy credits I get by generating my solar power.
One of the options was a buy or a lease. In the leasing case they installer gets the energy credits, I just get a lower electrical bill.
I had various installers come out and since the same production numbers were used in either the lease or buy scenarios I was
confident that they would stand behind the numbers because I had nothing to lose in the leasing case....
This may be of some use to the original poster: http://www.srectrade.com/florida_srec.php /Ed
PS: My electric bill was $6.43 last month...
PPS: See http://lightfarmroad.blogspot.com